Incorporate signia as @tldraw/state (#1620)

It tried to get out but we're dragging it back in. 

This PR brings [signia](https://github.com/tldraw/signia) back into
tldraw as @tldraw/state.

### Change Type

- [x] major

---------

Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>

apps/docs/content/docs/editor.mdx

@@ -26,11 +26,10 @@ For example, the store contains a `page` record for each page in the current doc
 
 The editor also exposes many _computed_ values which are derived from other records in the store. For example, `editor.selectedIds` is a computed property that will return the editor's current selected shape ids for its current page.
 
-You can use these properties directly or you can use them in [signia](https://github.com/tldraw/signia) signals.
+You can use these properties directly or you can use them in signals.
 
 ```tsx
-import { track } from "@tldraw/signia"
-import { useEditor } from "@tldraw/tldraw"
+import { track, useEditor } from "@tldraw/tldraw"
 
 export const SelectedIdsCount = track(() => {
   const editor = useEditor()
@@ -151,8 +150,7 @@ Note that the paths you pass to `isIn` or `isInAny` can be the full path or a pa
 > If all you're interested in is the state below `root`, there is a convenience property, `editor.currentToolId`, that can help with the editor's currently selected tool.
 
 ```tsx
-import { track } from "@tldraw/signia"
-import { useEditor } from "@tldraw/tldraw"
+import { track, useEditor } from "@tldraw/tldraw"
 
 export const CreatingBubbleToolUi = track(() => {
   const editor = useEditor()

apps/docs/content/docs/installation.mdx

@@ -10,12 +10,12 @@ At the moment the `@tldraw/tldraw` package is in alpha. We also ship a canary ve
 
 ## Alpha
 
-First, install the `@tldraw/tldraw` package using `@alpha` for the latest.  The package also has peer dependencies on `signia` and `signia-react` which you will need to install at the same time.
+First, install the `@tldraw/tldraw` package using `@alpha` for the latest.
 
 ```bash
-yarn add @tldraw/tldraw@alpha signia signia-react
+yarn add @tldraw/tldraw@alpha
 # or
-npm install @tldraw/tldraw@alpha signia signia-react
+npm install @tldraw/tldraw@alpha
 ```
 
 ## Canary
@@ -23,7 +23,7 @@ npm install @tldraw/tldraw@alpha signia signia-react
 To get the very latest version, use the [latest canary release](https://www.npmjs.com/package/@tldraw/tldraw?activeTab=versions). Docs for the very latest version are also available at [canary.tldraw.dev](https://canary.tldraw.dev).
 
 ```bash
-yarn add @tldraw/tldraw@canary signia signia-react
+yarn add @tldraw/tldraw@canary
 # or
-npm install @tldraw/tldraw@canary signia signia-react
+npm install @tldraw/tldraw@canary
 ```

apps/examples/package.json

@@ -37,6 +37,7 @@
 		"@babel/plugin-proposal-decorators": "^7.21.0",
 		"@playwright/test": "^1.34.3",
 		"@tldraw/assets": "workspace:*",
+		"@tldraw/state": "workspace:*",
 		"@tldraw/tldraw": "workspace:*",
 		"@tldraw/utils": "workspace:*",
 		"@tldraw/validate": "workspace:*",
@@ -45,8 +46,6 @@
 		"react": "^18.2.0",
 		"react-dom": "^18.2.0",
 		"react-router-dom": "^6.9.0",
-		"signia": "0.1.4",
-		"signia-react": "0.1.4",
 		"vite": "^4.3.4",
 		"y-websocket": "^1.5.0",
 		"yjs": "^13.6.2"

apps/examples/src/16-custom-styles/FilterStyleUi.tsx

@@ -1,5 +1,5 @@
+import { track } from '@tldraw/state'
 import { useEditor } from '@tldraw/tldraw'
-import { track } from 'signia-react'
 import { MyFilterStyle } from './CardShape'
 
 export const FilterStyleUi = track(function FilterStyleUi() {

apps/examples/src/4-custom-ui/CustomUiExample.tsx

@@ -1,7 +1,7 @@
+import { track } from '@tldraw/state'
 import { Canvas, TldrawEditor, defaultShapes, defaultTools, useEditor } from '@tldraw/tldraw'
 import '@tldraw/tldraw/tldraw.css'
 import { useEffect } from 'react'
-import { track } from 'signia-react'
 import './custom-ui.css'
 
 export default function CustomUiExample() {

apps/examples/src/yjs/YjsExample.tsx

@@ -1,6 +1,6 @@
+import { track } from '@tldraw/state'
 import { Tldraw, useEditor } from '@tldraw/tldraw'
 import '@tldraw/tldraw/tldraw.css'
-import { track } from 'signia-react'
 import { useYjsStore } from './useYjsStore'
 
 const HOST_URL =

apps/examples/src/yjs/useYjsStore.ts

@@ -1,3 +1,4 @@
+import { computed, react, transact } from '@tldraw/state'
 import {
 	DocumentRecordType,
 	InstancePresenceRecordType,
@@ -13,7 +14,6 @@ import {
 	getUserPreferences,
 } from '@tldraw/tldraw'
 import { useEffect, useMemo, useState } from 'react'
-import { computed, react, transact } from 'signia'
 import { WebsocketProvider } from 'y-websocket'
 import * as Y from 'yjs'
 

packages/editor/api-report.md

@@ -6,10 +6,12 @@
 
 /// <reference types="react" />
 
-import { Atom } from 'signia';
+import { Atom } from '@tldraw/state';
+import { atom } from '@tldraw/state';
 import { Box2d } from '@tldraw/primitives';
 import { Box2dModel } from '@tldraw/tlschema';
-import { Computed } from 'signia';
+import { Computed } from '@tldraw/state';
+import { computed } from '@tldraw/state';
 import { ComputedCache } from '@tldraw/store';
 import { CubicSpline2d } from '@tldraw/primitives';
 import { defineMigrations } from '@tldraw/store';
@@ -23,6 +25,7 @@ import { Matrix2d } from '@tldraw/primitives';
 import { Matrix2dModel } from '@tldraw/primitives';
 import { Migrations } from '@tldraw/store';
 import { Polyline2d } from '@tldraw/primitives';
+import { react } from '@tldraw/state';
 import { default as React_2 } from 'react';
 import * as React_3 from 'react';
 import { RecursivePartial } from '@tldraw/utils';
@@ -32,7 +35,7 @@ import { SelectionEdge } from '@tldraw/primitives';
 import { SelectionHandle } from '@tldraw/primitives';
 import { SerializedSchema } from '@tldraw/store';
 import { ShapeProps } from '@tldraw/tlschema';
-import { Signal } from 'signia';
+import { Signal } from '@tldraw/state';
 import { StoreSchema } from '@tldraw/store';
 import { StoreSnapshot } from '@tldraw/store';
 import { StrokePoint } from '@tldraw/primitives';
@@ -78,10 +81,16 @@ import { TLTextShape } from '@tldraw/tlschema';
 import { TLUnknownShape } from '@tldraw/tlschema';
 import { TLVideoAsset } from '@tldraw/tlschema';
 import { TLVideoShape } from '@tldraw/tlschema';
+import { track } from '@tldraw/state';
 import { UnknownRecord } from '@tldraw/store';
+import { useComputed } from '@tldraw/state';
+import { useQuickReactor } from '@tldraw/state';
+import { useReactor } from '@tldraw/state';
+import { useValue } from '@tldraw/state';
 import { Vec2d } from '@tldraw/primitives';
 import { Vec2dModel } from '@tldraw/tlschema';
 import { VecLike } from '@tldraw/primitives';
+import { whyAmIRunning } from '@tldraw/state';
 
 // @public (undocumented)
 export const ACCEPTED_ASSET_TYPE: string;
@@ -163,6 +172,8 @@ export class ArrowShapeUtil extends ShapeUtil<TLArrowShape> {
     static type: "arrow";
 }
 
+export { atom }
+
 // @public (undocumented)
 export abstract class BaseBoxShapeTool extends StateNode {
     // (undocumented)
@@ -225,6 +236,8 @@ export const Canvas: React_2.MemoExoticComponent<() => JSX.Element>;
 // @public (undocumented)
 export const checkFlag: (flag: (() => boolean) | boolean | undefined) => boolean | undefined;
 
+export { computed }
+
 // @public
 export function containBoxSize(originalSize: BoxWidthHeight, containBoxSize: BoxWidthHeight): BoxWidthHeight;
 
@@ -1764,6 +1777,8 @@ export class PlopManager {
 // @public
 export function preventDefault(event: Event | React_2.BaseSyntheticEvent): void;
 
+export { react }
+
 // @public (undocumented)
 export class ReadonlySharedStyleMap {
     // (undocumented)
@@ -2705,6 +2720,8 @@ export type TLWheelEventInfo = TLBaseEventInfo & {
     delta: Vec2dModel;
 };
 
+export { track }
+
 // @public (undocumented)
 export const truncateStringWithEllipsis: (str: string, maxLength: number) => string;
 
@@ -2717,6 +2734,8 @@ export type UiEventType = 'click' | 'keyboard' | 'pinch' | 'pointer' | 'wheel' |
 // @public
 export function uniqueId(): string;
 
+export { useComputed }
+
 // @public (undocumented)
 export function useContainer(): HTMLDivElement;
 
@@ -2738,18 +2757,18 @@ export function usePrefersReducedMotion(): boolean;
 // @internal (undocumented)
 export function usePresence(userId: string): null | TLInstancePresence;
 
-// @public (undocumented)
-export function useQuickReactor(name: string, reactFn: () => void, deps?: any[]): void;
+export { useQuickReactor }
 
 // @internal (undocumented)
 export const USER_COLORS: readonly ["#FF802B", "#EC5E41", "#F2555A", "#F04F88", "#E34BA9", "#BD54C6", "#9D5BD2", "#7B66DC", "#02B1CC", "#11B3A3", "#39B178", "#55B467"];
 
-// @public (undocumented)
-export function useReactor(name: string, reactFn: () => void, deps?: any[] | undefined): void;
+export { useReactor }
 
 // @public (undocumented)
 export function useTLStore(opts: TLStoreOptions): TLStore;
 
+export { useValue }
+
 // @public (undocumented)
 export const VideoShape: TLShapeInfo<TLVideoShape>;
 
@@ -2789,6 +2808,8 @@ export class WeakMapCache<T extends object, K> {
     set(item: T, value: K): void;
 }
 
+export { whyAmIRunning }
+
 // @internal (undocumented)
 export const ZOOMS: number[];
 

packages/editor/package.json

@@ -47,6 +47,7 @@
 	"dependencies": {
 		"@tldraw/indices": "workspace:*",
 		"@tldraw/primitives": "workspace:*",
+		"@tldraw/state": "workspace:*",
 		"@tldraw/store": "workspace:*",
 		"@tldraw/tlschema": "workspace:*",
 		"@tldraw/utils": "workspace:*",
@@ -66,9 +67,7 @@
 	},
 	"peerDependencies": {
 		"react": "^18",
-		"react-dom": "^18",
-		"signia": "*",
-		"signia-react": "*"
+		"react-dom": "^18"
 	},
 	"devDependencies": {
 		"@peculiar/webcrypto": "^1.4.0",

packages/editor/src/index.ts

@@ -3,6 +3,17 @@
 
 // eslint-disable-next-line local/no-export-star
 export * from '@tldraw/indices'
+export {
+	atom,
+	computed,
+	react,
+	track,
+	useComputed,
+	useQuickReactor,
+	useReactor,
+	useValue,
+	whyAmIRunning,
+} from '@tldraw/state'
 export { defineMigrations } from '@tldraw/store'
 // eslint-disable-next-line local/no-export-star
 export * from '@tldraw/tlschema'
@@ -184,8 +195,6 @@ export type { TLEditorComponents } from './lib/hooks/useEditorComponents'
 export { useLocalStore } from './lib/hooks/useLocalStore'
 export { usePeerIds } from './lib/hooks/usePeerIds'
 export { usePresence } from './lib/hooks/usePresence'
-export { useQuickReactor } from './lib/hooks/useQuickReactor'
-export { useReactor } from './lib/hooks/useReactor'
 export { useTLStore } from './lib/hooks/useTLStore'
 export {
 	ReadonlySharedStyleMap,

packages/editor/src/lib/components/Canvas.tsx

@@ -1,9 +1,8 @@
 import { Matrix2d, toDomPrecision } from '@tldraw/primitives'
+import { react, track, useQuickReactor, useValue } from '@tldraw/state'
 import { TLHandle, TLShapeId } from '@tldraw/tlschema'
 import { dedupe, modulate } from '@tldraw/utils'
 import React from 'react'
-import { react } from 'signia'
-import { track, useValue } from 'signia-react'
 import { useCanvasEvents } from '../hooks/useCanvasEvents'
 import { useCoarsePointer } from '../hooks/useCoarsePointer'
 import { useDocumentEvents } from '../hooks/useDocumentEvents'
@@ -13,7 +12,6 @@ import { useFixSafariDoubleTapZoomPencilEvents } from '../hooks/useFixSafariDoub
 import { useGestureEvents } from '../hooks/useGestureEvents'
 import { useHandleEvents } from '../hooks/useHandleEvents'
 import { usePattern } from '../hooks/usePattern'
-import { useQuickReactor } from '../hooks/useQuickReactor'
 import { useScreenBounds } from '../hooks/useScreenBounds'
 import { debugFlags } from '../utils/debug-flags'
 import { LiveCollaborators } from './LiveCollaborators'

packages/editor/src/lib/components/DefaultErrorFallback.tsx

@@ -1,6 +1,6 @@
+import { useValue } from '@tldraw/state'
 import classNames from 'classnames'
 import { useEffect, useLayoutEffect, useRef, useState } from 'react'
-import { useValue } from 'signia-react'
 import { Editor } from '../editor/Editor'
 import { EditorContext } from '../hooks/useEditor'
 import { hardResetEditor } from '../utils/hard-reset'

packages/editor/src/lib/components/LiveCollaborators.tsx

@@ -1,5 +1,5 @@
+import { track } from '@tldraw/state'
 import { useEffect, useRef, useState } from 'react'
-import { track } from 'signia-react'
 import { COLLABORATOR_CHECK_INTERVAL, COLLABORATOR_TIMEOUT } from '../constants'
 import { useEditor } from '../hooks/useEditor'
 import { useEditorComponents } from '../hooks/useEditorComponents'

packages/editor/src/lib/components/SelectionBg.tsx

@@ -1,6 +1,6 @@
 import { Matrix2d, toDomPrecision } from '@tldraw/primitives'
+import { track } from '@tldraw/state'
 import * as React from 'react'
-import { track } from 'signia-react'
 import { TLPointerEventInfo } from '../editor/types/event-types'
 import { useEditor } from '../hooks/useEditor'
 import { releasePointerCapture, setPointerCapture } from '../utils/dom'

packages/editor/src/lib/components/SelectionFg.tsx

@@ -1,7 +1,7 @@
 import { RotateCorner, toDomPrecision } from '@tldraw/primitives'
+import { track } from '@tldraw/state'
 import classNames from 'classnames'
 import { useRef } from 'react'
-import { track } from 'signia-react'
 import { EmbedShapeUtil } from '../editor/shapes/embed/EmbedShapeUtil'
 import { TextShapeUtil } from '../editor/shapes/text/TextShapeUtil'
 import { getCursor } from '../hooks/useCursor'

packages/editor/src/lib/components/Shape.tsx

@@ -1,15 +1,10 @@
 import { Matrix2d } from '@tldraw/primitives'
+import { track, useQuickReactor, useStateTracking } from '@tldraw/state'
 import { TLShape, TLShapeId } from '@tldraw/tlschema'
 import * as React from 'react'
-import {
-	track,
-	// @ts-expect-error 'private' export
-	useStateTracking,
-} from 'signia-react'
 import { useEditor } from '../..'
 import { ShapeUtil } from '../editor/shapes/ShapeUtil'
 import { useEditorComponents } from '../hooks/useEditorComponents'
-import { useQuickReactor } from '../hooks/useQuickReactor'
 import { useShapeEvents } from '../hooks/useShapeEvents'
 import { OptionalErrorBoundary } from './ErrorBoundary'
 

packages/editor/src/lib/components/ShapeIndicator.tsx

@@ -1,11 +1,7 @@
+import { useStateTracking, useValue } from '@tldraw/state'
 import { TLShape, TLShapeId } from '@tldraw/tlschema'
 import classNames from 'classnames'
 import * as React from 'react'
-import {
-	// @ts-expect-error 'private' export
-	useStateTracking,
-	useValue,
-} from 'signia-react'
 import { useEditor } from '../..'
 import type { Editor } from '../editor/Editor'
 import { ShapeUtil } from '../editor/shapes/ShapeUtil'

packages/editor/src/lib/config/TLSessionStateSnapshot.test.ts

@@ -1,4 +1,4 @@
-import { react } from 'signia'
+import { react } from '@tldraw/state'
 import { TestEditor } from '../test/TestEditor'
 import {
 	TLSessionStateSnapshot,

packages/editor/src/lib/config/TLSessionStateSnapshot.ts

@@ -1,3 +1,4 @@
+import { Signal, computed, transact } from '@tldraw/state'
 import {
 	RecordsDiff,
 	UnknownRecord,
@@ -18,7 +19,6 @@ import {
 } from '@tldraw/tlschema'
 import { objectMapFromEntries } from '@tldraw/utils'
 import { T } from '@tldraw/validate'
-import { Signal, computed, transact } from 'signia'
 import { uniqueId } from '../utils/data'
 
 const tabIdKey = 'TLDRAW_TAB_ID_v2' as const

packages/editor/src/lib/config/TLUserPreferences.test.ts

@@ -1,4 +1,4 @@
-import { atom } from 'signia'
+import { atom } from '@tldraw/state'
 import { TestEditor } from '../test/TestEditor'
 import { TLUserPreferences } from './TLUserPreferences'
 import { createTLUser } from './createTLUser'

packages/editor/src/lib/config/TLUserPreferences.ts

@@ -1,7 +1,7 @@
+import { atom } from '@tldraw/state'
 import { defineMigrations, migrate } from '@tldraw/store'
 import { getDefaultTranslationLocale } from '@tldraw/tlschema'
 import { T } from '@tldraw/validate'
-import { atom } from 'signia'
 import { uniqueId } from '../utils/data'
 
 const USER_DATA_KEY = 'TLDRAW_USER_DATA_v3'

packages/editor/src/lib/config/createTLUser.ts

@@ -1,5 +1,5 @@
+import { Signal, computed } from '@tldraw/state'
 import { TLInstancePresence, TLStore } from '@tldraw/tlschema'
-import { Signal, computed } from 'signia'
 import { TLUserPreferences, getUserPreferences, setUserPreferences } from './TLUserPreferences'
 
 /** @public */

packages/editor/src/lib/editor/Editor.ts

@@ -21,6 +21,7 @@ import {
 	intersectPolygonPolygon,
 	pointInPolygon,
 } from '@tldraw/primitives'
+import { EMPTY_ARRAY, atom, computed, transact } from '@tldraw/state'
 import { ComputedCache, RecordType } from '@tldraw/store'
 import {
 	Box2dModel,
@@ -79,7 +80,6 @@ import {
 } from '@tldraw/utils'
 import { EventEmitter } from 'eventemitter3'
 import { nanoid } from 'nanoid'
-import { EMPTY_ARRAY, atom, computed, transact } from 'signia'
 import { TLUser, createTLUser } from '../config/createTLUser'
 import { checkShapesAndAddCore } from '../config/defaultShapes'
 import { AnyTLShapeInfo } from '../config/defineShape'

packages/editor/src/lib/editor/derivations/arrowBindingsIndex.ts

@@ -1,5 +1,5 @@
+import { Computed, RESET_VALUE, computed, isUninitialized } from '@tldraw/state'
 import { TLArrowShape, TLShape, TLShapeId } from '@tldraw/tlschema'
-import { Computed, RESET_VALUE, computed, isUninitialized } from 'signia'
 import { Editor } from '../Editor'
 import { ArrowShapeUtil } from '../shapes/arrow/ArrowShapeUtil'
 

packages/editor/src/lib/editor/derivations/parentsToChildrenWithIndexes.ts

@@ -1,6 +1,6 @@
+import { computed, isUninitialized, RESET_VALUE } from '@tldraw/state'
 import { RecordsDiff } from '@tldraw/store'
 import { isShape, TLParentId, TLRecord, TLShapeId, TLStore } from '@tldraw/tlschema'
-import { computed, isUninitialized, RESET_VALUE } from 'signia'
 
 type Parents2Children = Record<TLParentId, [id: TLShapeId, index: string][]>
 

packages/editor/src/lib/editor/derivations/shapeIdsInCurrentPage.ts

@@ -1,3 +1,4 @@
+import { computed, isUninitialized, RESET_VALUE, withDiff } from '@tldraw/state'
 import { IncrementalSetConstructor } from '@tldraw/store'
 import {
 	isPageId,
@@ -8,7 +9,6 @@ import {
 	TLShapeId,
 	TLStore,
 } from '@tldraw/tlschema'
-import { computed, isUninitialized, RESET_VALUE, withDiff } from 'signia'
 
 /**
  * Get whether a shape is in the current page.

packages/editor/src/lib/editor/managers/HistoryManager.ts

@@ -1,5 +1,5 @@
+import { atom, transact } from '@tldraw/state'
 import { devFreeze } from '@tldraw/store'
-import { atom, transact } from 'signia'
 import { uniqueId } from '../../utils/data'
 import { TLCommandHandler, TLHistoryEntry } from '../types/history-types'
 import { Stack, stack } from './Stack'

packages/editor/src/lib/editor/managers/SnapManager.ts

@@ -11,9 +11,9 @@ import {
 	Vec2d,
 	VecLike,
 } from '@tldraw/primitives'
+import { atom, computed, EMPTY_ARRAY } from '@tldraw/state'
 import { TLParentId, TLShape, TLShapeId, Vec2dModel } from '@tldraw/tlschema'
 import { dedupe, deepCopy } from '@tldraw/utils'
-import { atom, computed, EMPTY_ARRAY } from 'signia'
 import { uniqueId } from '../../utils/data'
 import type { Editor } from '../Editor'
 import { GroupShapeUtil } from '../shapes/group/GroupShapeUtil'

packages/editor/src/lib/editor/managers/Stack.ts

@@ -1,4 +1,4 @@
-import { EMPTY_ARRAY } from 'signia'
+import { EMPTY_ARRAY } from '@tldraw/state'
 
 export type Stack<T> = StackItem<T> | EmptyStackItem<T>
 

packages/editor/src/lib/editor/shapes/arrow/ArrowShapeUtil.tsx

@@ -10,6 +10,7 @@ import {
 	Vec2d,
 	VecLike,
 } from '@tldraw/primitives'
+import { computed, EMPTY_ARRAY } from '@tldraw/state'
 import { ComputedCache } from '@tldraw/store'
 import {
 	TLArrowShape,
@@ -23,7 +24,6 @@ import {
 } from '@tldraw/tlschema'
 import { deepCopy, last, minBy } from '@tldraw/utils'
 import * as React from 'react'
-import { computed, EMPTY_ARRAY } from 'signia'
 import { SVGContainer } from '../../../components/SVGContainer'
 import {
 	ShapeUtil,

packages/editor/src/lib/editor/shapes/embed/EmbedShapeUtil.tsx

@@ -1,5 +1,6 @@
 /* eslint-disable react-hooks/rules-of-hooks */
 import { toDomPrecision } from '@tldraw/primitives'
+import { useValue } from '@tldraw/state'
 import {
 	TLEmbedShape,
 	TLEmbedShapePermissions,
@@ -7,7 +8,6 @@ import {
 } from '@tldraw/tlschema'
 import * as React from 'react'
 import { useMemo } from 'react'
-import { useValue } from 'signia-react'
 import { DefaultSpinner } from '../../../components/DefaultSpinner'
 import { HTMLContainer } from '../../../components/HTMLContainer'
 import { useIsEditing } from '../../../hooks/useIsEditing'

packages/editor/src/lib/editor/shapes/image/ImageShapeUtil.tsx

@@ -1,9 +1,9 @@
 /* eslint-disable react-hooks/rules-of-hooks */
 import { Vec2d, toDomPrecision } from '@tldraw/primitives'
+import { useValue } from '@tldraw/state'
 import { TLImageShape, TLShapePartial } from '@tldraw/tlschema'
 import { deepCopy } from '@tldraw/utils'
 import { useEffect, useState } from 'react'
-import { useValue } from 'signia-react'
 import { DefaultSpinner } from '../../../components/DefaultSpinner'
 import { HTMLContainer } from '../../../components/HTMLContainer'
 import { useIsCropping } from '../../../hooks/useIsCropping'

packages/editor/src/lib/editor/shapes/shared/ShapeFill.tsx

@@ -1,6 +1,6 @@
+import { useValue } from '@tldraw/state'
 import { TLDefaultColorStyle, TLDefaultFillStyle } from '@tldraw/tlschema'
 import * as React from 'react'
-import { useValue } from 'signia-react'
 import { HASH_PATERN_ZOOM_NAMES } from '../../../constants'
 import { useEditor } from '../../../hooks/useEditor'
 import { TLExportColors } from './TLExportColors'

packages/editor/src/lib/editor/shapes/shared/useEditableText.ts

@@ -1,7 +1,7 @@
 /* eslint-disable no-inner-declarations */
+import { useValue } from '@tldraw/state'
 import { TLShape, TLUnknownShape } from '@tldraw/tlschema'
 import React, { useCallback, useEffect, useRef } from 'react'
-import { useValue } from 'signia-react'
 import { useEditor } from '../../../hooks/useEditor'
 import { preventDefault, stopEventPropagation } from '../../../utils/dom'
 import { INDENT, TextHelpers } from '../text/TextHelpers'

packages/editor/src/lib/editor/shapes/shared/useForceSolid.ts

@@ -1,4 +1,4 @@
-import { useValue } from 'signia-react'
+import { useValue } from '@tldraw/state'
 import { useEditor } from '../../../hooks/useEditor'
 
 export function useForceSolid() {

packages/editor/src/lib/editor/shapes/video/VideoShapeUtil.tsx

@@ -1,7 +1,7 @@
 import { toDomPrecision } from '@tldraw/primitives'
+import { track } from '@tldraw/state'
 import { TLVideoShape } from '@tldraw/tlschema'
 import * as React from 'react'
-import { track } from 'signia-react'
 import { DefaultSpinner } from '../../../components/DefaultSpinner'
 import { HTMLContainer } from '../../../components/HTMLContainer'
 import { useIsEditing } from '../../../hooks/useIsEditing'

packages/editor/src/lib/editor/tools/StateNode.ts

@@ -1,5 +1,5 @@
+import { Atom, Computed, atom, computed } from '@tldraw/state'
 import { TLBaseShape } from '@tldraw/tlschema'
-import { Atom, Computed, atom, computed } from 'signia'
 import type { Editor } from '../Editor'
 import { TLShapeUtilConstructor } from '../shapes/ShapeUtil'
 import {

packages/editor/src/lib/hooks/useCursor.ts

@@ -1,8 +1,8 @@
 import { PI, radiansToDegrees } from '@tldraw/primitives'
+import { useQuickReactor } from '@tldraw/state'
 import { TLCursorType } from '@tldraw/tlschema'
 import { useContainer } from './useContainer'
 import { useEditor } from './useEditor'
-import { useQuickReactor } from './useQuickReactor'
 
 const DEFAULT_SVG = `<path d="m12 24.4219v-16.015l11.591 11.619h-6.781l-.411.124z" fill="white"/><path d="m21.0845 25.0962-3.605 1.535-4.682-11.089 3.686-1.553z" fill="white"/><path d="m19.751 24.4155-1.844.774-3.1-7.374 1.841-.775z" fill="black"/><path d="m13 10.814v11.188l2.969-2.866.428-.139h4.768z" fill="black"/>`
 const POINTER_SVG = `<path d="m13.3315 21.3799c-.284-.359-.629-1.093-1.243-1.984-.348-.504-1.211-1.453-1.468-1.935-.223-.426-.199-.617-.146-.97.094-.628.738-1.117 1.425-1.051.519.049.959.392 1.355.716.239.195.533.574.71.788.163.196.203.277.377.509.23.307.302.459.214.121-.071-.496-.187-1.343-.355-2.092-.128-.568-.159-.657-.281-1.093-.129-.464-.195-.789-.316-1.281-.084-.348-.235-1.059-.276-1.459-.057-.547-.087-1.439.264-1.849.275-.321.906-.418 1.297-.22.512.259.803 1.003.936 1.3.239.534.387 1.151.516 1.961.164 1.031.466 2.462.476 2.763.024-.369-.068-1.146-.004-1.5.058-.321.328-.694.666-.795.286-.085.621-.116.916-.055.313.064.643.288.766.499.362.624.369 1.899.384 1.831.086-.376.071-1.229.284-1.584.14-.234.497-.445.687-.479.294-.052.655-.068.964-.008.249.049.586.345.677.487.218.344.342 1.317.379 1.658.015.141.074-.392.293-.736.406-.639 1.843-.763 1.898.639.025.654.02.624.02 1.064 0 .517-.012.828-.04 1.202-.031.4-.117 1.304-.242 1.742-.086.301-.371.978-.652 1.384 0 0-1.074 1.25-1.191 1.813-.118.562-.079.566-.102.965-.023.398.121.922.121.922s-.802.104-1.234.035c-.391-.063-.875-.841-1-1.079-.172-.328-.539-.265-.682-.023-.225.383-.709 1.07-1.051 1.113-.668.084-2.054.031-3.139.02 0 0 .185-1.011-.227-1.358-.305-.259-.83-.784-1.144-1.06z" fill="white"/><g stroke="black" stroke-linecap="round" stroke-width=".75"><path d="m13.3315 21.3799c-.284-.359-.629-1.093-1.243-1.984-.348-.504-1.211-1.453-1.468-1.935-.223-.426-.199-.617-.146-.97.094-.628.738-1.117 1.425-1.051.519.049.959.392 1.355.716.239.195.533.574.71.788.163.196.203.277.377.509.23.307.302.459.214.121-.071-.496-.187-1.343-.355-2.092-.128-.568-.159-.657-.281-1.093-.129-.464-.195-.789-.316-1.281-.084-.348-.235-1.059-.276-1.459-.057-.547-.087-1.439.264-1.849.275-.321.906-.418 1.297-.22.512.259.803 1.003.936 1.3.239.534.387 1.151.516 1.961.164 1.031.466 2.462.476 2.763.024-.369-.068-1.146-.004-1.5.058-.321.328-.694.666-.795.286-.085.621-.116.916-.055.313.064.643.288.766.499.362.624.369 1.899.384 1.831.086-.376.071-1.229.284-1.584.14-.234.497-.445.687-.479.294-.052.655-.068.964-.008.249.049.586.345.677.487.218.344.342 1.317.379 1.658.015.141.074-.392.293-.736.406-.639 1.843-.763 1.898.639.025.654.02.624.02 1.064 0 .517-.012.828-.04 1.202-.031.4-.117 1.304-.242 1.742-.086.301-.371.978-.652 1.384 0 0-1.074 1.25-1.191 1.813-.118.562-.079.566-.102.965-.023.398.121.922.121.922s-.802.104-1.234.035c-.391-.063-.875-.841-1-1.079-.172-.328-.539-.265-.682-.023-.225.383-.709 1.07-1.051 1.113-.668.084-2.054.031-3.139.02 0 0 .185-1.011-.227-1.358-.305-.259-.83-.784-1.144-1.06z" stroke-linejoin="round"/><path d="m21.5664 21.7344v-3.459"/><path d="m19.5508 21.7461-.016-3.473"/><path d="m17.5547 18.3047.021 3.426"/></g>`

packages/editor/src/lib/hooks/useDarkMode.ts

@@ -1,5 +1,5 @@
+import { useValue } from '@tldraw/state'
 import React from 'react'
-import { useValue } from 'signia-react'
 import { debugFlags } from '../utils/debug-flags'
 import { useContainer } from './useContainer'
 import { useEditor } from './useEditor'

packages/editor/src/lib/hooks/useDocumentEvents.ts

@@ -1,5 +1,5 @@
+import { useValue } from '@tldraw/state'
 import { useEffect } from 'react'
-import { useValue } from 'signia-react'
 import { TLKeyboardEventInfo, TLPointerEventInfo } from '../editor/types/event-types'
 import { preventDefault } from '../utils/dom'
 import { useContainer } from './useContainer'

packages/editor/src/lib/hooks/useIsCropping.ts

@@ -1,5 +1,5 @@
+import { useValue } from '@tldraw/state'
 import { TLShapeId } from '@tldraw/tlschema'
-import { useValue } from 'signia-react'
 import { useEditor } from './useEditor'
 
 export function useIsCropping(shapeId: TLShapeId) {

packages/editor/src/lib/hooks/useIsEditing.ts

@@ -1,5 +1,5 @@
+import { useValue } from '@tldraw/state'
 import { TLShapeId } from '@tldraw/tlschema'
-import { useValue } from 'signia-react'
 import { useEditor } from './useEditor'
 
 export function useIsEditing(shapeId: TLShapeId) {

packages/editor/src/lib/hooks/usePeerIds.ts

@@ -1,6 +1,6 @@
+import { useComputed, useValue } from '@tldraw/state'
 import uniq from 'lodash.uniq'
 import { useMemo } from 'react'
-import { useComputed, useValue } from 'signia-react'
 import { useEditor } from './useEditor'
 
 // TODO: maybe move this to a computed property on the App class?

packages/editor/src/lib/hooks/usePresence.ts

@@ -1,6 +1,6 @@
+import { useValue } from '@tldraw/state'
 import { TLInstancePresence } from '@tldraw/tlschema'
 import { useMemo } from 'react'
-import { useValue } from 'signia-react'
 import { useEditor } from './useEditor'
 
 // TODO: maybe move this to a computed property on the App class?

packages/editor/src/lib/hooks/useZoomCss.ts

@@ -1,6 +1,6 @@
+import { EffectScheduler } from '@tldraw/state'
 import { debounce } from '@tldraw/utils'
 import * as React from 'react'
-import { EffectScheduler } from 'signia'
 import { useContainer } from './useContainer'
 import { useEditor } from './useEditor'
 

packages/editor/src/lib/utils/debug-flags.ts

@@ -1,4 +1,4 @@
-import { Atom, atom, react } from 'signia'
+import { Atom, atom, react } from '@tldraw/state'
 
 // --- 1. DEFINE ---
 //

packages/editor/src/lib/utils/sync/TLLocalSyncClient.ts

@@ -1,3 +1,4 @@
+import { Signal, transact } from '@tldraw/state'
 import {
 	RecordsDiff,
 	SerializedSchema,
@@ -7,7 +8,6 @@ import {
 } from '@tldraw/store'
 import { TLStore } from '@tldraw/tlschema'
 import { assert } from '@tldraw/utils'
-import { Signal, transact } from 'signia'
 import {
 	TAB_ID,
 	TLSessionStateSnapshot,

packages/editor/tsconfig.json

@@ -16,6 +16,7 @@
 		{ "path": "../store" },
 		{ "path": "../validate" },
 		{ "path": "../utils" },
-		{ "path": "../indices" }
+		{ "path": "../indices" },
+		{ "path": "../state" }
 	]
 }

packages/state/CHANGELOG.md

@@ -0,0 +1,167 @@
+# v2.0.0-alpha.12 (Mon Apr 03 2023)
+
+#### 🐛 Bug Fix
+
+- Make sure all types and build stuff get run in CI [#1548](https://github.com/tldraw/tldraw-lite/pull/1548) ([@SomeHats](https://github.com/SomeHats))
+- add pre-commit api report generation [#1517](https://github.com/tldraw/tldraw-lite/pull/1517) ([@SomeHats](https://github.com/SomeHats))
+- [chore] restore api extractor [#1500](https://github.com/tldraw/tldraw-lite/pull/1500) ([@steveruizok](https://github.com/steveruizok))
+- Asset loading overhaul [#1457](https://github.com/tldraw/tldraw-lite/pull/1457) ([@SomeHats](https://github.com/SomeHats))
+- David/publish good [#1488](https://github.com/tldraw/tldraw-lite/pull/1488) ([@ds300](https://github.com/ds300))
+- [chore] alpha 10 [#1486](https://github.com/tldraw/tldraw-lite/pull/1486) ([@ds300](https://github.com/ds300))
+- [chore] package build improvements [#1484](https://github.com/tldraw/tldraw-lite/pull/1484) ([@ds300](https://github.com/ds300))
+- [chore] bump for alpha 8 [#1485](https://github.com/tldraw/tldraw-lite/pull/1485) ([@steveruizok](https://github.com/steveruizok))
+- [fix] page point offset [#1483](https://github.com/tldraw/tldraw-lite/pull/1483) ([@steveruizok](https://github.com/steveruizok))
+- stop using broken-af turbo for publishing [#1476](https://github.com/tldraw/tldraw-lite/pull/1476) ([@ds300](https://github.com/ds300))
+- [chore] add canary release script [#1423](https://github.com/tldraw/tldraw-lite/pull/1423) ([@ds300](https://github.com/ds300) [@steveruizok](https://github.com/steveruizok))
+- [chore] upgrade yarn [#1430](https://github.com/tldraw/tldraw-lite/pull/1430) ([@ds300](https://github.com/ds300))
+- flush store on attach [#1449](https://github.com/tldraw/tldraw-lite/pull/1449) ([@ds300](https://github.com/ds300))
+- [fix] dev version number for tldraw/tldraw [#1434](https://github.com/tldraw/tldraw-lite/pull/1434) ([@steveruizok](https://github.com/steveruizok))
+- repo cleanup [#1426](https://github.com/tldraw/tldraw-lite/pull/1426) ([@steveruizok](https://github.com/steveruizok))
+- [fix] use polyfill for `structuredClone` [#1408](https://github.com/tldraw/tldraw-lite/pull/1408) ([@TodePond](https://github.com/TodePond) [@steveruizok](https://github.com/steveruizok))
+- Run all the tests. Fix linting for tests. [#1389](https://github.com/tldraw/tldraw-lite/pull/1389) ([@MitjaBezensek](https://github.com/MitjaBezensek))
+
+#### Authors: 5
+
+- alex ([@SomeHats](https://github.com/SomeHats))
+- David Sheldrick ([@ds300](https://github.com/ds300))
+- Lu[ke] Wilson ([@TodePond](https://github.com/TodePond))
+- Mitja Bezenšek ([@MitjaBezensek](https://github.com/MitjaBezensek))
+- Steve Ruiz ([@steveruizok](https://github.com/steveruizok))
+
+---
+
+# @tldraw/tlstore
+
+## 2.0.0-alpha.11
+
+### Patch Changes
+
+- fix some package build scripting
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.10
+
+## 2.0.0-alpha.10
+
+### Patch Changes
+
+- 4b4399b6e: redeploy with yarn to prevent package version issues
+- Updated dependencies [4b4399b6e]
+  - @tldraw/utils@2.0.0-alpha.9
+
+## 2.0.0-alpha.9
+
+### Patch Changes
+
+- Release day!
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.8
+
+## 2.0.0-alpha.8
+
+### Patch Changes
+
+- 23dd81cfe: Make signia a peer dependency
+
+## 2.0.0-alpha.7
+
+### Patch Changes
+
+- Bug fixes.
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.7
+
+## 2.0.0-alpha.6
+
+### Patch Changes
+
+- Add licenses.
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.6
+
+## 2.0.0-alpha.5
+
+### Patch Changes
+
+- Add CSS files to tldraw/tldraw.
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.5
+
+## 2.0.0-alpha.4
+
+### Patch Changes
+
+- Add children to tldraw/tldraw
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.4
+
+## 2.0.0-alpha.3
+
+### Patch Changes
+
+- Change permissions.
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.3
+
+## 2.0.0-alpha.2
+
+### Patch Changes
+
+- Add tldraw, editor
+- Updated dependencies
+  - @tldraw/utils@2.0.0-alpha.2
+
+## 0.1.0-alpha.11
+
+### Patch Changes
+
+- Fix stale reactors.
+- Updated dependencies
+  - @tldraw/utils@0.1.0-alpha.11
+
+## 0.1.0-alpha.10
+
+### Patch Changes
+
+- Fix type export bug.
+- Updated dependencies
+  - @tldraw/utils@0.1.0-alpha.10
+
+## 0.1.0-alpha.9
+
+### Patch Changes
+
+- Fix import bugs.
+- Updated dependencies
+  - @tldraw/utils@0.1.0-alpha.9
+
+## 0.1.0-alpha.8
+
+### Patch Changes
+
+- Changes validation requirements, exports validation helpers.
+- Updated dependencies
+  - @tldraw/utils@0.1.0-alpha.8
+
+## 0.1.0-alpha.7
+
+### Patch Changes
+
+- - Pre-pre-release update
+- Updated dependencies
+  - @tldraw/utils@0.1.0-alpha.7
+
+## 0.0.2-alpha.1
+
+### Patch Changes
+
+- Fix error with HMR
+- Updated dependencies
+  - @tldraw/utils@0.0.2-alpha.1
+
+## 0.0.2-alpha.0
+
+### Patch Changes
+
+- Initial release
+- Updated dependencies
+  - @tldraw/utils@0.0.2-alpha.0

packages/state/README.md

@@ -0,0 +1,3 @@
+# @tldraw/state
+
+...

packages/state/api-extractor.json

@@ -0,0 +1,4 @@
+{
+	"$schema": "https://developer.microsoft.com/json-schemas/api-extractor/v7/api-extractor.schema.json",
+	"extends": "../../config/api-extractor.json"
+}

packages/state/api-report.md

@@ -0,0 +1,170 @@
+## API Report File for "@tldraw/state"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+
+import { FunctionComponent } from 'react';
+import { default as React_2 } from 'react';
+
+// @public
+export interface Atom<Value, Diff = unknown> extends Signal<Value, Diff> {
+    set(value: Value, diff?: Diff): Value;
+    update(updater: (value: Value) => Value): Value;
+}
+
+// @public
+export function atom<Value, Diff = unknown>(
+name: string,
+initialValue: Value,
+options?: AtomOptions<Value, Diff>): Atom<Value, Diff>;
+
+// @public
+export interface AtomOptions<Value, Diff> {
+    computeDiff?: ComputeDiff<Value, Diff>;
+    historyLength?: number;
+    isEqual?: (a: any, b: any) => boolean;
+}
+
+// @public
+export interface Computed<Value, Diff = unknown> extends Signal<Value, Diff> {
+    readonly isActivelyListening: boolean;
+    // @internal (undocumented)
+    readonly parentEpochs: number[];
+    // @internal (undocumented)
+    readonly parents: Signal<any, any>[];
+}
+
+// @public
+export function computed<Value, Diff = unknown>(name: string, compute: (previousValue: typeof UNINITIALIZED | Value, lastComputedEpoch: number) => Value | WithDiff<Value, Diff>, options?: ComputedOptions<Value, Diff>): Computed<Value, Diff>;
+
+// @public (undocumented)
+export function computed(target: any, key: string, descriptor: PropertyDescriptor): PropertyDescriptor;
+
+// @public (undocumented)
+export function computed<Value, Diff = unknown>(options?: ComputedOptions<Value, Diff>): (target: any, key: string, descriptor: PropertyDescriptor) => PropertyDescriptor;
+
+// @public
+export interface ComputedOptions<Value, Diff> {
+    computeDiff?: ComputeDiff<Value, Diff>;
+    historyLength?: number;
+    isEqual?: (a: any, b: any) => boolean;
+}
+
+// @public
+export class EffectScheduler<Result> {
+    constructor(name: string, runEffect: (lastReactedEpoch: number) => Result, options?: EffectSchedulerOptions);
+    attach(): void;
+    detach(): void;
+    execute(): Result;
+    get isActivelyListening(): boolean;
+    // @internal (undocumented)
+    lastTraversedEpoch: number;
+    // @internal (undocumented)
+    maybeScheduleEffect(): void;
+    // (undocumented)
+    readonly name: string;
+    // @internal (undocumented)
+    parentEpochs: number[];
+    // @internal (undocumented)
+    parents: Signal<any, any>[];
+    get scheduleCount(): number;
+    // @internal (undocumented)
+    scheduleEffect(): void;
+}
+
+// @public (undocumented)
+export const EMPTY_ARRAY: [];
+
+// @public
+export function getComputedInstance<Obj extends object, Prop extends keyof Obj>(obj: Obj, propertyName: Prop): Computed<Obj[Prop]>;
+
+// @public
+export function isAtom(value: unknown): value is Atom<unknown>;
+
+// @public
+export function isSignal(value: any): value is Signal<any>;
+
+// @public
+export const isUninitialized: (value: any) => value is typeof UNINITIALIZED;
+
+// @public
+export function react(name: string, fn: (lastReactedEpoch: number) => any, options?: EffectSchedulerOptions): () => void;
+
+// @public
+export interface Reactor<T = unknown> {
+    scheduler: EffectScheduler<T>;
+    start(options?: {
+        force?: boolean;
+    }): void;
+    stop(): void;
+}
+
+// @public
+export function reactor<Result>(name: string, fn: (lastReactedEpoch: number) => Result, options?: EffectSchedulerOptions): Reactor<Result>;
+
+// @public (undocumented)
+export const RESET_VALUE: unique symbol;
+
+// @public (undocumented)
+export type RESET_VALUE = typeof RESET_VALUE;
+
+// @public
+export interface Signal<Value, Diff = unknown> {
+    __unsafe__getWithoutCapture(): Value;
+    // @internal (undocumented)
+    children: ArraySet<Child>;
+    getDiffSince(epoch: number): Diff[] | RESET_VALUE;
+    lastChangedEpoch: number;
+    name: string;
+    readonly value: Value;
+}
+
+// @public
+export function track<T extends FunctionComponent<any>>(baseComponent: T): T extends React_2.MemoExoticComponent<any> ? T : React_2.MemoExoticComponent<T>;
+
+// @public
+export function transact<T>(fn: () => T): T;
+
+// @public
+export function transaction<T>(fn: (rollback: () => void) => T): T;
+
+// @public
+export function unsafe__withoutCapture<T>(fn: () => T): T;
+
+// @public
+export function useAtom<Value, Diff = unknown>(
+name: string,
+valueOrInitialiser: (() => Value) | Value,
+options?: AtomOptions<Value, Diff>): Atom<Value, Diff>;
+
+// @public
+export function useComputed<Value>(name: string, compute: () => Value, deps: any[]): Computed<Value>;
+
+// @public (undocumented)
+export function useComputed<Value, Diff = unknown>(name: string, compute: () => Value, opts: ComputedOptions<Value, Diff>, deps: any[]): Computed<Value>;
+
+// @public (undocumented)
+export function useQuickReactor(name: string, reactFn: () => void, deps?: any[]): void;
+
+// @public (undocumented)
+export function useReactor(name: string, reactFn: () => void, deps?: any[] | undefined): void;
+
+// @internal (undocumented)
+export function useStateTracking<T>(name: string, render: () => T): T;
+
+// @public
+export function useValue<Value>(value: Signal<Value>): Value;
+
+// @public (undocumented)
+export function useValue<Value>(name: string, fn: () => Value, deps: unknown[]): Value;
+
+// @public
+export function whyAmIRunning(): void;
+
+// @public
+export function withDiff<Value, Diff>(value: Value, diff: Diff): WithDiff<Value, Diff>;
+
+// (No @packageDocumentation comment for this package)
+
+```

packages/state/docs-ordering.json

@@ -0,0 +1,5 @@
+[
+	["functions", ["atom-1", "computed-1", "react", "reactor-1", "transact", "transaction"]],
+	["classes"],
+	["interfaces", ["Signal"]]
+]

packages/state/package.json

@@ -0,0 +1,71 @@
+{
+	"name": "@tldraw/state",
+	"description": "A tiny little drawing app (state).",
+	"version": "2.0.0-alpha.12",
+	"packageManager": "yarn@3.5.0",
+	"author": {
+		"name": "tldraw GB Ltd.",
+		"email": "hello@tldraw.com"
+	},
+	"homepage": "https://tldraw.dev",
+	"license": "Apache-2.0",
+	"repository": {
+		"type": "git",
+		"url": "https://github.com/tldraw/tldraw"
+	},
+	"bugs": {
+		"url": "https://github.com/tldraw/tldraw/issues"
+	},
+	"keywords": [
+		"tldraw",
+		"drawing",
+		"app",
+		"development",
+		"whiteboard",
+		"canvas",
+		"infinite"
+	],
+	"/* NOTE */": "These `main` and `types` fields are rewritten by the build script. They are not the actual values we publish",
+	"main": "./src/index.ts",
+	"types": "./.tsbuild/index.d.ts",
+	"/* GOTCHA */": "files will include ./dist and index.d.ts by default, add any others you want to include in here",
+	"files": [],
+	"scripts": {
+		"test": "lazy inherit",
+		"test-coverage": "lazy inherit",
+		"build": "yarn run -T tsx ../../scripts/build-package.ts",
+		"build-api": "yarn run -T tsx ../../scripts/build-api.ts",
+		"prepack": "yarn run -T tsx ../../scripts/prepack.ts",
+		"postpack": "../../scripts/postpack.sh",
+		"pack-tarball": "yarn pack",
+		"lint": "yarn run -T tsx ../../scripts/lint.ts"
+	},
+	"jest": {
+		"preset": "config/jest/node",
+		"setupFiles": [
+			"raf/polyfill"
+		],
+		"moduleNameMapper": {
+			"^~(.*)": "<rootDir>/src/$1"
+		},
+		"transformIgnorePatterns": [
+			"node_modules/(?!(nanoid|escape-string-regexp)/)"
+		]
+	},
+	"devDependencies": {
+		"@types/lodash": "^4.14.188",
+		"@types/react": "^18.0.24",
+		"@types/react-test-renderer": "^18.0.0",
+		"lodash": "^4.17.21",
+		"react-test-renderer": "^18.2.0"
+	},
+	"peerDependencies": {
+		"react": "^18"
+	},
+	"typedoc": {
+		"readmeFile": "none",
+		"entryPoint": "./src/index.ts",
+		"displayName": "@tldraw/state",
+		"tsconfig": "./tsconfig.json"
+	}
+}

packages/state/src/index.ts

@@ -0,0 +1,3 @@
+/* eslint-disable local/no-export-star */
+export * from './lib/core'
+export * from './lib/react'

packages/state/src/lib/core/ArraySet.ts

@@ -0,0 +1,146 @@
+// The maximum size for an array in an ArraySet
+export const ARRAY_SIZE_THRESHOLD = 8
+
+/**
+ * An ArraySet operates as an array until it reaches a certain size, after which a Set is used
+ * instead. In either case, the same methods are used to get, set, remove, and visit the items.
+ * @internal
+ */
+export class ArraySet<T> {
+	private arraySize = 0
+
+	private array: (T | undefined)[] | null = Array(ARRAY_SIZE_THRESHOLD)
+
+	private set: Set<T> | null = null
+
+	/**
+	 * Get whether this ArraySet has any elements.
+	 *
+	 * @returns True if this ArraySet has any elements, false otherwise.
+	 */
+	get isEmpty() {
+		if (this.array) {
+			return this.arraySize === 0
+		}
+
+		if (this.set) {
+			return this.set.size === 0
+		}
+
+		throw new Error('no set or array')
+	}
+
+	/**
+	 * Add an item to the ArraySet if it is not already present.
+	 *
+	 * @param elem - The element to add.
+	 */
+
+	add(elem: T) {
+		if (this.array) {
+			const idx = this.array.indexOf(elem)
+
+			// Return false if the element is already in the array.
+			if (idx !== -1) {
+				return false
+			}
+
+			if (this.arraySize < ARRAY_SIZE_THRESHOLD) {
+				// If the array is below the size threshold, push items into the array.
+
+				// Insert the element into the array's next available slot.
+				this.array[this.arraySize] = elem
+				this.arraySize++
+
+				return true
+			} else {
+				// If the array is full, convert it to a set and remove the array.
+				this.set = new Set(this.array as any)
+				this.array = null
+				this.set.add(elem)
+
+				return true
+			}
+		}
+
+		if (this.set) {
+			// Return false if the element is already in the set.
+			if (this.set.has(elem)) {
+				return false
+			}
+
+			this.set.add(elem)
+			return true
+		}
+
+		throw new Error('no set or array')
+	}
+
+	/**
+	 * Remove an item from the ArraySet if it is present.
+	 *
+	 * @param elem - The element to remove
+	 */
+	remove(elem: T) {
+		if (this.array) {
+			const idx = this.array.indexOf(elem)
+
+			// If the item is not in the array, return false.
+			if (idx === -1) {
+				return false
+			}
+
+			this.array[idx] = undefined
+			this.arraySize--
+
+			if (idx !== this.arraySize) {
+				// If the item is not the last item in the array, move the last item into the
+				// removed item's slot.
+				this.array[idx] = this.array[this.arraySize]
+				this.array[this.arraySize] = undefined
+			}
+
+			return true
+		}
+
+		if (this.set) {
+			// If the item is not in the set, return false.
+			if (!this.set.has(elem)) {
+				return false
+			}
+
+			this.set.delete(elem)
+
+			return true
+		}
+
+		throw new Error('no set or array')
+	}
+
+	/**
+	 * Run a callback for each element in the ArraySet.
+	 *
+	 * @param visitor - The callback to run for each element.
+	 */
+	visit(visitor: (item: T) => void) {
+		if (this.array) {
+			for (let i = 0; i < this.arraySize; i++) {
+				const elem = this.array[i]
+
+				if (typeof elem !== 'undefined') {
+					visitor(elem)
+				}
+			}
+
+			return
+		}
+
+		if (this.set) {
+			this.set.forEach(visitor)
+
+			return
+		}
+
+		throw new Error('no set or array')
+	}
+}

packages/state/src/lib/core/Atom.ts

@@ -0,0 +1,196 @@
+import { ArraySet } from './ArraySet'
+import { maybeCaptureParent } from './capture'
+import { EMPTY_ARRAY, equals } from './helpers'
+import { HistoryBuffer } from './HistoryBuffer'
+import { advanceGlobalEpoch, atomDidChange, globalEpoch } from './transactions'
+import { Child, ComputeDiff, RESET_VALUE, Signal } from './types'
+
+/**
+ * The options to configure an atom, passed into the [[atom]] function.
+ * @public
+ */
+export interface AtomOptions<Value, Diff> {
+	/**
+	 * The maximum number of diffs to keep in the history buffer.
+	 *
+	 * If you don't need to compute diffs, or if you will supply diffs manually via [[Atom.set]], you can leave this as `undefined` and no history buffer will be created.
+	 *
+	 * If you expect the value to be part of an active effect subscription all the time, and to not change multiple times inside of a single transaction, you can set this to a relatively low number (e.g. 10).
+	 *
+	 * Otherwise, set this to a higher number based on your usage pattern and memory constraints.
+	 *
+	 */
+	historyLength?: number
+	/**
+	 * A method used to compute a diff between the atom's old and new values. If provided, it will not be used unless you also specify [[AtomOptions.historyLength]].
+	 */
+	computeDiff?: ComputeDiff<Value, Diff>
+	/**
+	 * If provided, this will be used to compare the old and new values of the atom to determine if the value has changed.
+	 * By default, values are compared using first using strict equality (`===`), then `Object.is`, and finally any `.equals` method present in the object's prototype chain.
+	 * @param a - The old value
+	 * @param b - The new value
+	 * @returns
+	 */
+	isEqual?: (a: any, b: any) => boolean
+}
+
+/**
+ * An Atom is a signal that can be updated directly by calling [[Atom.set]] or [[Atom.update]].
+ *
+ * Atoms are created using the [[atom]] function.
+ *
+ * @example
+ * ```ts
+ * const name = atom('name', 'John')
+ *
+ * console.log(name.value) // 'John'
+ * ```
+ *
+ * @public
+ */
+export interface Atom<Value, Diff = unknown> extends Signal<Value, Diff> {
+	/**
+	 * Sets the value of this atom to the given value. If the value is the same as the current value, this is a no-op.
+	 *
+	 * @param value - The new value to set.
+	 * @param diff - The diff to use for the update. If not provided, the diff will be computed using [[AtomOptions.computeDiff]].
+	 */
+	set(value: Value, diff?: Diff): Value
+	/**
+	 * Updates the value of this atom using the given updater function. If the returned value is the same as the current value, this is a no-op.
+	 *
+	 * @param updater - A function that takes the current value and returns the new value.
+	 */
+	update(updater: (value: Value) => Value): Value
+}
+
+/**
+ * @internal
+ */
+export class _Atom<Value, Diff = unknown> implements Atom<Value, Diff> {
+	constructor(
+		public readonly name: string,
+		private current: Value,
+		options?: AtomOptions<Value, Diff>
+	) {
+		this.isEqual = options?.isEqual ?? null
+
+		if (!options) return
+
+		if (options.historyLength) {
+			this.historyBuffer = new HistoryBuffer(options.historyLength)
+		}
+
+		this.computeDiff = options.computeDiff
+	}
+
+	readonly isEqual: null | ((a: any, b: any) => boolean)
+
+	computeDiff?: ComputeDiff<Value, Diff>
+
+	lastChangedEpoch = globalEpoch
+
+	children = new ArraySet<Child>()
+
+	historyBuffer?: HistoryBuffer<Diff>
+
+	__unsafe__getWithoutCapture(): Value {
+		return this.current
+	}
+
+	get value() {
+		maybeCaptureParent(this)
+		return this.current
+	}
+
+	set(value: Value, diff?: Diff): Value {
+		// If the value has not changed, do nothing.
+		if (this.isEqual?.(this.current, value) ?? equals(this.current, value)) {
+			return this.current
+		}
+
+		// Tick forward the global epoch
+		advanceGlobalEpoch()
+
+		// Add the diff to the history buffer.
+		if (this.historyBuffer) {
+			this.historyBuffer.pushEntry(
+				this.lastChangedEpoch,
+				globalEpoch,
+				diff ??
+					this.computeDiff?.(this.current, value, this.lastChangedEpoch, globalEpoch) ??
+					RESET_VALUE
+			)
+		}
+
+		// Update the atom's record of the epoch when last changed.
+		this.lastChangedEpoch = globalEpoch
+
+		const oldValue = this.current
+		this.current = value
+
+		// Notify all children that this atom has changed.
+		atomDidChange(this, oldValue)
+
+		return value
+	}
+
+	update(updater: (value: Value) => Value): Value {
+		return this.set(updater(this.current))
+	}
+
+	getDiffSince(epoch: number): RESET_VALUE | Diff[] {
+		maybeCaptureParent(this)
+
+		// If no changes have occurred since the given epoch, return an empty array.
+		if (epoch >= this.lastChangedEpoch) {
+			return EMPTY_ARRAY
+		}
+
+		return this.historyBuffer?.getChangesSince(epoch) ?? RESET_VALUE
+	}
+}
+
+/**
+ * Creates a new [[Atom]].
+ *
+ * An Atom is a signal that can be updated directly by calling [[Atom.set]] or [[Atom.update]].
+ *
+ * @example
+ * ```ts
+ * const name = atom('name', 'John')
+ *
+ * name.value // 'John'
+ *
+ * name.set('Jane')
+ *
+ * name.value // 'Jane'
+ * ```
+ *
+ * @public
+ */
+export function atom<Value, Diff = unknown>(
+	/**
+	 * A name for the signal. This is used for debugging and profiling purposes, it does not need to be unique.
+	 */
+	name: string,
+	/**
+	 * The initial value of the signal.
+	 */
+	initialValue: Value,
+	/**
+	 * The options to configure the atom. See [[AtomOptions]].
+	 */
+	options?: AtomOptions<Value, Diff>
+): Atom<Value, Diff> {
+	return new _Atom(name, initialValue, options)
+}
+
+/**
+ * Returns true if the given value is an [[Atom]].
+ * @public
+ */
+export function isAtom(value: unknown): value is Atom<unknown> {
+	return value instanceof _Atom
+}

packages/state/src/lib/core/Computed.ts

@@ -0,0 +1,378 @@
+/* eslint-disable prefer-rest-params */
+import { ArraySet } from './ArraySet'
+import { HistoryBuffer } from './HistoryBuffer'
+import { maybeCaptureParent, startCapturingParents, stopCapturingParents } from './capture'
+import { GLOBAL_START_EPOCH } from './constants'
+import { EMPTY_ARRAY, equals, haveParentsChanged } from './helpers'
+import { globalEpoch } from './transactions'
+import { Child, ComputeDiff, RESET_VALUE, Signal } from './types'
+
+const UNINITIALIZED = Symbol('UNINITIALIZED')
+/**
+ * The type of the first value passed to a computed signal function as the 'prevValue' parameter.
+ *
+ * @see [[isUninitialized]].
+ * @public
+ */
+type UNINITIALIZED = typeof UNINITIALIZED
+
+/**
+ * Call this inside a computed signal function to determine whether it is the first time the function is being called.
+ *
+ * Mainly useful for incremental signal computation.
+ *
+ * @example
+ * ```ts
+ * const count = atom('count', 0)
+ * const double = computed('double', (prevValue) => {
+ *   if (isUninitialized(prevValue)) {
+ *     console.log('First time!')
+ *   }
+ *   return count.value * 2
+ * })
+ * ```
+ *
+ * @param value - The value to check.
+ * @public
+ */
+export const isUninitialized = (value: any): value is UNINITIALIZED => {
+	return value === UNINITIALIZED
+}
+
+class WithDiff<Value, Diff> {
+	constructor(public value: Value, public diff: Diff) {}
+}
+
+/**
+ * When writing incrementally-computed signals it is convenient (and usually more performant) to incrementally compute the diff too.
+ *
+ * You can use this function to wrap the return value of a computed signal function to indicate that the diff should be used instead of calculating a new one with [[AtomOptions.computeDiff]].
+ *
+ * @example
+ * ```ts
+ * const count = atom('count', 0)
+ * const double = computed('double', (prevValue) => {
+ *   const nextValue = count.value * 2
+ *   if (isUninitialized(prevValue)) {
+ *     return nextValue
+ *   }
+ *   return withDiff(nextValue, nextValue - prevValue)
+ * }, { historyLength: 10 })
+ * ```
+ *
+ *
+ * @param value - The value.
+ * @param diff - The diff.
+ * @public
+ */
+export function withDiff<Value, Diff>(value: Value, diff: Diff): WithDiff<Value, Diff> {
+	return new WithDiff(value, diff)
+}
+
+/**
+ * Options for creating computed signals. Used when calling [[computed]].
+ * @public
+ */
+export interface ComputedOptions<Value, Diff> {
+	/**
+	 * The maximum number of diffs to keep in the history buffer.
+	 *
+	 * If you don't need to compute diffs, or if you will supply diffs manually via [[Atom.set]], you can leave this as `undefined` and no history buffer will be created.
+	 *
+	 * If you expect the value to be part of an active effect subscription all the time, and to not change multiple times inside of a single transaction, you can set this to a relatively low number (e.g. 10).
+	 *
+	 * Otherwise, set this to a higher number based on your usage pattern and memory constraints.
+	 *
+	 */
+	historyLength?: number
+	/**
+	 * A method used to compute a diff between the atom's old and new values. If provided, it will not be used unless you also specify [[AtomOptions.historyLength]].
+	 */
+	computeDiff?: ComputeDiff<Value, Diff>
+	/**
+	 * If provided, this will be used to compare the old and new values of the atom to determine if the value has changed.
+	 * By default, values are compared using first using strict equality (`===`), then `Object.is`, and finally any `.equals` method present in the object's prototype chain.
+	 * @param a - The old value
+	 * @param b - The new value
+	 * @returns
+	 */
+	isEqual?: (a: any, b: any) => boolean
+}
+
+/**
+ * A computed signal created via [[computed]].
+ *
+ * @public
+ */
+export interface Computed<Value, Diff = unknown> extends Signal<Value, Diff> {
+	/**
+	 * Whether this computed child is involved in an actively-running effect graph.
+	 * @public
+	 */
+	readonly isActivelyListening: boolean
+
+	/** @internal */
+	readonly parents: Signal<any, any>[]
+	/** @internal */
+	readonly parentEpochs: number[]
+}
+
+/**
+ * @internal
+ */
+export class _Computed<Value, Diff = unknown> implements Computed<Value, Diff> {
+	lastChangedEpoch = GLOBAL_START_EPOCH
+	lastTraversedEpoch = GLOBAL_START_EPOCH
+
+	/**
+	 * The epoch when the reactor was last checked.
+	 */
+	private lastCheckedEpoch = GLOBAL_START_EPOCH
+
+	parents: Signal<any, any>[] = []
+	parentEpochs: number[] = []
+
+	children = new ArraySet<Child>()
+
+	get isActivelyListening(): boolean {
+		return !this.children.isEmpty
+	}
+
+	historyBuffer?: HistoryBuffer<Diff>
+
+	// The last-computed value of this signal.
+	private state: Value = UNINITIALIZED as unknown as Value
+
+	private computeDiff?: ComputeDiff<Value, Diff>
+
+	private readonly isEqual: (a: any, b: any) => boolean
+
+	constructor(
+		/**
+		 * The name of the signal. This is used for debugging and performance profiling purposes. It does not need to be globally unique.
+		 */
+		public readonly name: string,
+		/**
+		 * The function that computes the value of the signal.
+		 */
+		private readonly derive: (
+			previousValue: Value | UNINITIALIZED,
+			lastComputedEpoch: number
+		) => Value | WithDiff<Value, Diff>,
+		options?: ComputedOptions<Value, Diff>
+	) {
+		if (options?.historyLength) {
+			this.historyBuffer = new HistoryBuffer(options.historyLength)
+		}
+		this.computeDiff = options?.computeDiff
+		this.isEqual = options?.isEqual ?? equals
+	}
+
+	__unsafe__getWithoutCapture(): Value {
+		const isNew = this.lastChangedEpoch === GLOBAL_START_EPOCH
+
+		if (!isNew && (this.lastCheckedEpoch === globalEpoch || !haveParentsChanged(this))) {
+			this.lastCheckedEpoch = globalEpoch
+			return this.state
+		}
+
+		try {
+			startCapturingParents(this)
+			const result = this.derive(this.state, this.lastCheckedEpoch)
+			const newState = result instanceof WithDiff ? result.value : result
+			if (this.state === UNINITIALIZED || !this.isEqual(newState, this.state)) {
+				if (this.historyBuffer && !isNew) {
+					const diff = result instanceof WithDiff ? result.diff : undefined
+					this.historyBuffer.pushEntry(
+						this.lastChangedEpoch,
+						globalEpoch,
+						diff ??
+							this.computeDiff?.(this.state, newState, this.lastCheckedEpoch, globalEpoch) ??
+							RESET_VALUE
+					)
+				}
+				this.lastChangedEpoch = globalEpoch
+				this.state = newState
+			}
+			this.lastCheckedEpoch = globalEpoch
+
+			return this.state
+		} finally {
+			stopCapturingParents()
+		}
+	}
+
+	get value(): Value {
+		const value = this.__unsafe__getWithoutCapture()
+		maybeCaptureParent(this)
+		return value
+	}
+
+	getDiffSince(epoch: number): RESET_VALUE | Diff[] {
+		// need to call .value to ensure both that this derivation is up to date
+		// and that tracking happens correctly
+		this.value
+
+		if (epoch >= this.lastChangedEpoch) {
+			return EMPTY_ARRAY
+		}
+
+		return this.historyBuffer?.getChangesSince(epoch) ?? RESET_VALUE
+	}
+}
+
+function computedAnnotation(
+	options: ComputedOptions<any, any> = {},
+	_target: any,
+	key: string,
+	descriptor: PropertyDescriptor
+) {
+	const originalMethod = descriptor.get
+	const derivationKey = Symbol.for('__@tldraw/state__computed__' + key)
+
+	descriptor.get = function (this: any) {
+		let d = this[derivationKey] as _Computed<any> | undefined
+
+		if (!d) {
+			d = new _Computed(key, originalMethod!.bind(this) as any, options)
+			Object.defineProperty(this, derivationKey, {
+				enumerable: false,
+				configurable: false,
+				writable: false,
+				value: d,
+			})
+		}
+		return d.value
+	}
+
+	return descriptor
+}
+
+/**
+ * Retrieves the underlying computed instance for a given property created with the [[computed]]
+ * decorator.
+ *
+ * @example
+ * ```ts
+ * class Counter {
+ *   max = 100
+ *   count = atom(0)
+ *
+ *   @computed get remaining() {
+ *     return this.max - this.count.value
+ *   }
+ * }
+ *
+ * const c = new Counter()
+ * const remaining = getComputedInstance(c, 'remaining')
+ * remaining.value === 100 // true
+ * c.count.set(13)
+ * remaining.value === 87 // true
+ * ```
+ *
+ * @param obj - The object
+ * @param propertyName - The property name
+ * @public
+ */
+export function getComputedInstance<Obj extends object, Prop extends keyof Obj>(
+	obj: Obj,
+	propertyName: Prop
+): Computed<Obj[Prop]> {
+	// deref to make sure it exists first
+	const key = Symbol.for('__@tldraw/state__computed__' + propertyName.toString())
+	let inst = obj[key as keyof typeof obj] as _Computed<Obj[Prop]> | undefined
+	if (!inst) {
+		// deref to make sure it exists first
+		obj[propertyName]
+		inst = obj[key as keyof typeof obj] as _Computed<Obj[Prop]> | undefined
+	}
+	return inst as any
+}
+
+/**
+ * Creates a computed signal.
+ *
+ * @example
+ * ```ts
+ * const name = atom('name', 'John')
+ * const greeting = computed('greeting', () => `Hello ${name.value}!`)
+ * console.log(greeting.value) // 'Hello John!'
+ * ```
+ *
+ * `computed` may also be used as a decorator for creating computed class properties.
+ *
+ * @example
+ * ```ts
+ * class Counter {
+ *   max = 100
+ *   count = atom<number>(0)
+ *
+ *   @computed get remaining() {
+ *     return this.max - this.count.value
+ *   }
+ * }
+ * ```
+ *
+ * You may optionally pass in a [[ComputedOptions]] when used as a decorator:
+ *
+ * @example
+ * ```ts
+ * class Counter {
+ *   max = 100
+ *   count = atom<number>(0)
+ *
+ *   @computed({isEqual: (a, b) => a === b})
+ *   get remaining() {
+ *     return this.max - this.count.value
+ *   }
+ * }
+ * ```
+ *
+ * @param name - The name of the signal.
+ * @param compute - The function that computes the value of the signal.
+ * @param options - Options for the signal.
+ *
+ * @public
+ */
+export function computed<Value, Diff = unknown>(
+	name: string,
+	compute: (
+		previousValue: Value | typeof UNINITIALIZED,
+		lastComputedEpoch: number
+	) => Value | WithDiff<Value, Diff>,
+	options?: ComputedOptions<Value, Diff>
+): Computed<Value, Diff>
+
+/** @public */
+export function computed(
+	target: any,
+	key: string,
+	descriptor: PropertyDescriptor
+): PropertyDescriptor
+/** @public */
+export function computed<Value, Diff = unknown>(
+	options?: ComputedOptions<Value, Diff>
+): (target: any, key: string, descriptor: PropertyDescriptor) => PropertyDescriptor
+/** @public */
+export function computed() {
+	if (arguments.length === 1) {
+		const options = arguments[0]
+		return (target: any, key: string, descriptor: PropertyDescriptor) =>
+			computedAnnotation(options, target, key, descriptor)
+	} else if (typeof arguments[0] === 'string') {
+		return new _Computed(arguments[0], arguments[1], arguments[2])
+	} else {
+		return computedAnnotation(undefined, arguments[0], arguments[1], arguments[2])
+	}
+}
+
+/**
+ * Returns true if the given value is a computed signal.
+ *
+ * @param value
+ * @returns {value is Computed<any>}
+ * @public
+ */
+export function isComputed(value: any): value is Computed<any> {
+	return value && value instanceof _Computed
+}

packages/state/src/lib/core/EffectScheduler.ts

@@ -0,0 +1,268 @@
+import { startCapturingParents, stopCapturingParents } from './capture'
+import { GLOBAL_START_EPOCH } from './constants'
+import { attach, detach, haveParentsChanged } from './helpers'
+import { globalEpoch } from './transactions'
+import { Signal } from './types'
+
+interface EffectSchedulerOptions {
+	/**
+	 * scheduleEffect is a function that will be called when the effect is scheduled.
+	 *
+	 * It can be used to defer running effects until a later time, for example to batch them together with requestAnimationFrame.
+	 *
+	 *
+	 * @example
+	 * ```ts
+	 * let isRafScheduled = false
+	 * const scheduledEffects: Array<() => void> = []
+	 * const scheduleEffect = (runEffect: () => void) => {
+	 * 	scheduledEffects.push(runEffect)
+	 * 	if (!isRafScheduled) {
+	 * 		isRafScheduled = true
+	 * 		requestAnimationFrame(() => {
+	 * 			isRafScheduled = false
+	 * 			scheduledEffects.forEach((runEffect) => runEffect())
+	 * 			scheduledEffects.length = 0
+	 * 		})
+	 * 	}
+	 * }
+	 * const stop = react('set page title', () => {
+	 * 	document.title = doc.title,
+	 * }, scheduleEffect)
+	 * ```
+	 *
+	 * @param execute - A function that will execute the effect.
+	 * @returns
+	 */
+	scheduleEffect?: (execute: () => void) => void
+}
+
+/**
+ * An EffectScheduler is responsible for executing side effects in response to changes in state.
+ *
+ * You probably don't need to use this directly unless you're integrating this library with a framework of some kind.
+ *
+ * Instead, use the [[react]] and [[reactor]] functions.
+ *
+ * @example
+ * ```ts
+ * const render = new EffectScheduler('render', drawToCanvas)
+ *
+ * render.attach()
+ * render.execute()
+ * ```
+ *
+ * @public
+ */
+export class EffectScheduler<Result> {
+	private _isActivelyListening = false
+	/**
+	 * Whether this scheduler is attached and actively listening to its parents.
+	 * @public
+	 */
+	get isActivelyListening() {
+		return this._isActivelyListening
+	}
+	/** @internal */
+	lastTraversedEpoch = GLOBAL_START_EPOCH
+
+	private lastReactedEpoch = GLOBAL_START_EPOCH
+	private _scheduleCount = 0
+
+	/**
+	 * The number of times this effect has been scheduled.
+	 * @public
+	 */
+	get scheduleCount() {
+		return this._scheduleCount
+	}
+
+	/** @internal */
+	parentEpochs: number[] = []
+	/** @internal */
+	parents: Signal<any, any>[] = []
+	private readonly _scheduleEffect?: (execute: () => void) => void
+	constructor(
+		public readonly name: string,
+		private readonly runEffect: (lastReactedEpoch: number) => Result,
+		options?: EffectSchedulerOptions
+	) {
+		this._scheduleEffect = options?.scheduleEffect
+	}
+
+	/** @internal */
+	maybeScheduleEffect() {
+		// bail out if we have been cancelled by another effect
+		if (!this._isActivelyListening) return
+		// bail out if no atoms have changed since the last time we ran this effect
+		if (this.lastReactedEpoch === globalEpoch) return
+
+		// bail out if we have parents and they have not changed since last time
+		if (this.parents.length && !haveParentsChanged(this)) {
+			this.lastReactedEpoch = globalEpoch
+			return
+		}
+		// if we don't have parents it's probably the first time this is running.
+		this.scheduleEffect()
+	}
+
+	/** @internal */
+	scheduleEffect() {
+		this._scheduleCount++
+		if (this._scheduleEffect) {
+			// if the effect should be deferred (e.g. until a react render), do so
+			this._scheduleEffect(this.maybeExecute)
+		} else {
+			// otherwise execute right now!
+			this.execute()
+		}
+	}
+
+	private maybeExecute = () => {
+		// bail out if we have been detached before this runs
+		if (!this._isActivelyListening) return
+		this.execute()
+	}
+
+	/**
+	 * Makes this scheduler become 'actively listening' to its parents.
+	 * If it has been executed before it will immediately become eligible to receive 'maybeScheduleEffect' calls.
+	 * If it has not executed before it will need to be manually executed once to become eligible for scheduling, i.e. by calling [[EffectScheduler.execute]].
+	 * @public
+	 */
+	attach() {
+		this._isActivelyListening = true
+		for (let i = 0, n = this.parents.length; i < n; i++) {
+			attach(this.parents[i], this)
+		}
+	}
+
+	/**
+	 * Makes this scheduler stop 'actively listening' to its parents.
+	 * It will no longer be eligible to receive 'maybeScheduleEffect' calls until [[EffectScheduler.attach]] is called again.
+	 */
+	detach() {
+		this._isActivelyListening = false
+		for (let i = 0, n = this.parents.length; i < n; i++) {
+			detach(this.parents[i], this)
+		}
+	}
+
+	/**
+	 * Executes the effect immediately and returns the result.
+	 * @returns The result of the effect.
+	 */
+	execute(): Result {
+		try {
+			startCapturingParents(this)
+			const result = this.runEffect(this.lastReactedEpoch)
+			this.lastReactedEpoch = globalEpoch
+			return result
+		} finally {
+			stopCapturingParents()
+		}
+	}
+}
+
+/**
+ * Starts a new effect scheduler, scheduling the effect immediately.
+ *
+ * Returns a function that can be called to stop the scheduler.
+ *
+ * @example
+ * ```ts
+ * const color = atom('color', 'red')
+ * const stop = react('set style', () => {
+ *   divElem.style.color = color.value
+ * })
+ * color.set('blue')
+ * // divElem.style.color === 'blue'
+ * stop()
+ * color.set('green')
+ * // divElem.style.color === 'blue'
+ * ```
+ *
+ *
+ * Also useful in React applications for running effects outside of the render cycle.
+ *
+ * @example
+ * ```ts
+ * useEffect(() => react('set style', () => {
+ *   divRef.current.style.color = color.value
+ * }), [])
+ * ```
+ *
+ * @public
+ */
+export function react(
+	name: string,
+	fn: (lastReactedEpoch: number) => any,
+	options?: EffectSchedulerOptions
+) {
+	const scheduler = new EffectScheduler(name, fn, options)
+	scheduler.attach()
+	scheduler.scheduleEffect()
+	return () => {
+		scheduler.detach()
+	}
+}
+
+/**
+ * The reactor is a user-friendly interface for starting and stopping an [[EffectScheduler]].
+ *
+ * Calling .start() will attach the scheduler and execute the effect immediately the first time it is called.
+ *
+ * If the reactor is stopped, calling `.start()` will re-attach the scheduler but will only execute the effect if any of its parents have changed since it was stopped.
+ *
+ * You can create a reactor with [[reactor]].
+ * @public
+ */
+export interface Reactor<T = unknown> {
+	/**
+	 * The underlying effect scheduler.
+	 * @public
+	 */
+	scheduler: EffectScheduler<T>
+	/**
+	 * Start the scheduler. The first time this is called the effect will be scheduled immediately.
+	 *
+	 * If the reactor is stopped, calling this will start the scheduler again but will only execute the effect if any of its parents have changed since it was stopped.
+	 *
+	 * If you need to force re-execution of the effect, pass `{ force: true }`.
+	 * @public
+	 */
+	start(options?: { force?: boolean }): void
+	/**
+	 * Stop the scheduler.
+	 * @public
+	 */
+	stop(): void
+}
+
+/**
+ * Creates a [[Reactor]], which is a thin wrapper around an [[EffectScheduler]].
+ *
+ * @public
+ */
+export function reactor<Result>(
+	name: string,
+	fn: (lastReactedEpoch: number) => Result,
+	options?: EffectSchedulerOptions
+): Reactor<Result> {
+	const scheduler = new EffectScheduler<Result>(name, fn, options)
+	return {
+		scheduler,
+		start: (options?: { force?: boolean }) => {
+			const force = options?.force ?? false
+			scheduler.attach()
+			if (force) {
+				scheduler.scheduleEffect()
+			} else {
+				scheduler.maybeScheduleEffect()
+			}
+		},
+		stop: () => {
+			scheduler.detach()
+		},
+	}
+}

packages/state/src/lib/core/HistoryBuffer.ts

@@ -0,0 +1,95 @@
+import { RESET_VALUE } from './types'
+
+type RangeTuple<Diff> = [fromEpoch: number, toEpoch: number, diff: Diff]
+
+/**
+ * A structure that stores diffs between values of an atom.
+ *
+ * @internal
+ */
+export class HistoryBuffer<Diff> {
+	private index = 0
+
+	// use a wrap around buffer to store the last N values
+	buffer: Array<RangeTuple<Diff> | undefined>
+
+	constructor(private readonly capacity: number) {
+		this.buffer = new Array(capacity)
+	}
+
+	/**
+	 * Add a diff to the history buffer.
+	 *
+	 * @param lastComputedEpoch - The epoch when the diff was computed.
+	 * @param currentEpoch - The current epoch.
+	 * @param diff - (optional) The diff to add, or else a reset value.
+	 */
+	pushEntry(lastComputedEpoch: number, currentEpoch: number, diff: Diff | RESET_VALUE) {
+		if (diff === undefined) {
+			return
+		}
+
+		if (diff === RESET_VALUE) {
+			this.clear()
+			return
+		}
+
+		// Add the diff to the buffer as a range tuple.
+		this.buffer[this.index] = [lastComputedEpoch, currentEpoch, diff]
+
+		// Bump the index, wrapping around if necessary.
+		this.index = (this.index + 1) % this.capacity
+	}
+
+	/**
+	 * Clear the history buffer.
+	 */
+	clear() {
+		this.index = 0
+		this.buffer.fill(undefined)
+	}
+
+	/**
+	 * Get the diffs since the given epoch.
+	 *
+	 * @param epoch - The epoch to get diffs since.
+	 * @returns An array of diffs or a flag to reset the history buffer.
+	 */
+	getChangesSince(sinceEpoch: number): RESET_VALUE | Diff[] {
+		const { index, capacity, buffer } = this
+
+		// For each item in the buffer...
+		for (let i = 0; i < capacity; i++) {
+			const offset = (index - 1 + capacity - i) % capacity
+
+			const elem = buffer[offset]
+
+			// If there's no element in the offset position, return the reset value
+			if (!elem) {
+				return RESET_VALUE
+			}
+
+			const [fromEpoch, toEpoch] = elem
+
+			// If the first element is already too early, bail
+			if (i === 0 && sinceEpoch >= toEpoch) {
+				return []
+			}
+
+			// If the element is since the given epoch, return an array with all diffs from this element and all following elements
+			if (fromEpoch <= sinceEpoch && sinceEpoch < toEpoch) {
+				const len = i + 1
+				const result = new Array(len)
+
+				for (let j = 0; j < len; j++) {
+					result[j] = buffer[(offset + j) % capacity]![2]
+				}
+
+				return result
+			}
+		}
+
+		// If we haven't returned yet, return the reset value
+		return RESET_VALUE
+	}
+}

packages/state/src/lib/core/__tests__/EffectScheduler.test.ts

@@ -0,0 +1,82 @@
+import { atom } from '../Atom'
+import { EffectScheduler } from '../EffectScheduler'
+
+describe(EffectScheduler, () => {
+	test('when you detatch and reattach, it retains the parents without rerunning', () => {
+		const a = atom('a', 1)
+		let numReactions = 0
+		const scheduler = new EffectScheduler('test', () => {
+			a.value
+			numReactions++
+		})
+		scheduler.attach()
+		scheduler.execute()
+		expect(numReactions).toBe(1)
+		a.set(2)
+		expect(numReactions).toBe(2)
+		scheduler.detach()
+		expect(numReactions).toBe(2)
+		scheduler.attach()
+		expect(numReactions).toBe(2)
+		a.set(3)
+		expect(numReactions).toBe(3)
+	})
+
+	test('when you detatch and reattach, it retains the parents while rerunning if the parent has changed', () => {
+		const a = atom('a', 1)
+		let numReactions = 0
+		const scheduler = new EffectScheduler('test', () => {
+			a.value
+			numReactions++
+		})
+		scheduler.attach()
+		scheduler.execute()
+		expect(numReactions).toBe(1)
+		a.set(2)
+		expect(numReactions).toBe(2)
+		scheduler.detach()
+		a.set(3)
+		expect(numReactions).toBe(2)
+		scheduler.attach()
+		scheduler.execute()
+		expect(numReactions).toBe(3)
+		a.set(4)
+		expect(numReactions).toBe(4)
+	})
+
+	test('when an effect is scheduled it increments a schedule count, even if the effect never runs', () => {
+		const a = atom('a', 1)
+		let numReactions = 0
+		let numSchedules = 0
+		const scheduler = new EffectScheduler(
+			'test',
+			() => {
+				a.value
+				numReactions++
+			},
+			{
+				scheduleEffect: () => {
+					numSchedules++
+				},
+			}
+		)
+		scheduler.attach()
+		scheduler.execute()
+
+		expect(scheduler.scheduleCount).toBe(0)
+		expect(numSchedules).toBe(0)
+		expect(numReactions).toBe(1)
+
+		a.set(2)
+
+		expect(scheduler.scheduleCount).toBe(1)
+		expect(numSchedules).toBe(1)
+		expect(numReactions).toBe(1)
+
+		a.set(3)
+
+		expect(scheduler.scheduleCount).toBe(2)
+		expect(numSchedules).toBe(2)
+		expect(numReactions).toBe(1)
+	})
+})

packages/state/src/lib/core/__tests__/HistoryBuffer.test.ts

@@ -0,0 +1,56 @@
+import { HistoryBuffer } from '../HistoryBuffer'
+import { RESET_VALUE } from '../types'
+
+describe('HistoryBuffer', () => {
+	it('should wrap around', () => {
+		const buf = new HistoryBuffer<string>(3)
+		buf.pushEntry(0, 1, 'a')
+		expect(buf.getChangesSince(0)).toEqual(['a'])
+		expect(buf.getChangesSince(1)).toEqual([])
+
+		buf.pushEntry(1, 2, 'b')
+
+		expect(buf.getChangesSince(0)).toEqual(['a', 'b'])
+		expect(buf.getChangesSince(1)).toEqual(['b'])
+		expect(buf.getChangesSince(2)).toEqual([])
+
+		buf.pushEntry(2, 3, 'c')
+
+		expect(buf.getChangesSince(0)).toEqual(['a', 'b', 'c'])
+		expect(buf.getChangesSince(1)).toEqual(['b', 'c'])
+		expect(buf.getChangesSince(2)).toEqual(['c'])
+		expect(buf.getChangesSince(3)).toEqual([])
+
+		buf.pushEntry(3, 4, 'd')
+
+		expect(buf.getChangesSince(0)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(1)).toEqual(['b', 'c', 'd'])
+		expect(buf.getChangesSince(2)).toEqual(['c', 'd'])
+		expect(buf.getChangesSince(3)).toEqual(['d'])
+		expect(buf.getChangesSince(4)).toEqual([])
+
+		buf.pushEntry(4, 5, 'e')
+
+		expect(buf.getChangesSince(0)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(1)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(2)).toEqual(['c', 'd', 'e'])
+		expect(buf.getChangesSince(3)).toEqual(['d', 'e'])
+		expect(buf.getChangesSince(4)).toEqual(['e'])
+		expect(buf.getChangesSince(5)).toEqual([])
+	})
+
+	it('will clear if you push RESET_VALUE', () => {
+		const buf = new HistoryBuffer<string>(10)
+		buf.pushEntry(0, 1, 'a')
+		buf.pushEntry(1, 2, 'b')
+		buf.pushEntry(2, 3, 'c')
+		buf.pushEntry(3, 4, 'd')
+		buf.pushEntry(4, 5, RESET_VALUE)
+		expect(buf.getChangesSince(0)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(1)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(2)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(3)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(4)).toEqual(RESET_VALUE)
+		expect(buf.getChangesSince(5)).toEqual(RESET_VALUE)
+	})
+})

packages/state/src/lib/core/__tests__/arraySet.test.ts

@@ -0,0 +1,127 @@
+import { ARRAY_SIZE_THRESHOLD, ArraySet } from '../ArraySet'
+
+const get = <T>(set: ArraySet<T>) => {
+	const s = new Set<T>()
+
+	set.visit((i) => s.add(i))
+	return s
+}
+
+describe(ArraySet, () => {
+	it('works with small numbers of things', () => {
+		const as = new ArraySet<number>()
+
+		expect(as.isEmpty).toBe(true)
+
+		as.add(3)
+		as.add(5)
+		as.add(8)
+		as.add(9)
+
+		expect(get(as)).toEqual(new Set([3, 5, 8, 9]))
+
+		as.remove(8)
+
+		expect(get(as)).toEqual(new Set([3, 5, 9]))
+
+		as.add(10)
+
+		expect(get(as)).toEqual(new Set([3, 5, 9, 10]))
+
+		as.add(12)
+
+		expect(get(as)).toEqual(new Set([3, 5, 9, 10, 12]))
+
+		as.remove(5)
+		as.remove(9)
+
+		expect(get(as)).toEqual(new Set([3, 10, 12]))
+
+		as.remove(9)
+
+		expect(get(as)).toEqual(new Set([3, 10, 12]))
+
+		as.add(123)
+		as.add(234)
+
+		expect(get(as)).toEqual(new Set([3, 10, 12, 123, 234]))
+
+		expect(as.isEmpty).toBe(false)
+
+		as.remove(123)
+		as.remove(234)
+		as.remove(3)
+		as.remove(10)
+		expect(as.isEmpty).toBe(false)
+		as.remove(12)
+		expect(as.isEmpty).toBe(true)
+	})
+
+	it('works with large numbers of things', () => {
+		const as = new ArraySet<number>()
+
+		expect(as.isEmpty).toBe(true)
+		for (let i = 0; i < 100; i++) {
+			as.add(i)
+		}
+
+		expect(get(as)).toEqual(new Set(Array.from({ length: 100 }, (_, i) => i)))
+
+		expect(as.isEmpty).toBe(false)
+		for (let i = 0; i < 100; i++) {
+			as.remove(i)
+		}
+
+		expect(get(as)).toEqual(new Set())
+		expect(as.isEmpty).toBe(true)
+	})
+})
+
+function rng(seed: number) {
+	return () => {
+		const x = Math.sin(seed++) * 10000
+		return x - Math.floor(x)
+	}
+}
+
+function runTest(seed: number) {
+	const as = new ArraySet<number>()
+	const s = new Set<number>()
+	const r = rng(seed)
+
+	const nums = new Array(ARRAY_SIZE_THRESHOLD * 2).fill(0).map(() => Math.floor(r() * 100))
+
+	for (let i = 0; i < 1000; i++) {
+		const num = nums[Math.floor(r() * nums.length)]
+
+		if (r() > 0.5) {
+			as.add(num)
+			s.add(num)
+		} else {
+			as.remove(num)
+			s.delete(num)
+		}
+
+		try {
+			expect(get(as)).toEqual(s)
+		} catch (e) {
+			console.error('Failed on iteration', i, 'with seed', seed)
+			throw e
+		}
+	}
+}
+
+describe('fuzzing this thing (if this fails tell david)', () => {
+	new Array(10).fill(0).forEach(() => {
+		const seed = Math.floor(Math.random() * 1000000)
+		it(`fuzz with seed ${seed}`, () => {
+			runTest(seed)
+		})
+	})
+})
+
+describe('regression tests', () => {
+	it('passes with seed 354923', () => {
+		runTest(354923)
+	})
+})

packages/state/src/lib/core/__tests__/atom.test.ts

@@ -0,0 +1,199 @@
+import { atom } from '../Atom'
+import { reactor } from '../EffectScheduler'
+import { globalEpoch, transact, transaction } from '../transactions'
+import { RESET_VALUE } from '../types'
+
+describe('atoms', () => {
+	it('contain data', () => {
+		const a = atom('', 1)
+
+		expect(a.value).toBe(1)
+	})
+	it('can be updated', () => {
+		const a = atom('', 1)
+
+		a.set(2)
+
+		expect(a.value).toBe(2)
+	})
+	it('will not advance the global epoch on creation', () => {
+		const startEpoch = globalEpoch
+		atom('', 3)
+		expect(globalEpoch).toBe(startEpoch)
+	})
+	it('will advance the global epoch on .set', () => {
+		const startEpoch = globalEpoch
+		const a = atom('', 3)
+		a.set(4)
+		expect(globalEpoch).toBe(startEpoch + 1)
+	})
+	it('can store history', () => {
+		const a = atom('', 1, { historyLength: 3, computeDiff: (a, b) => b - a })
+
+		const startEpoch = globalEpoch
+
+		expect(a.getDiffSince(startEpoch)).toEqual([])
+
+		a.set(5)
+
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+
+		a.set(10)
+
+		expect(a.getDiffSince(startEpoch)).toEqual([+4, +5])
+
+		a.set(20)
+
+		expect(a.getDiffSince(startEpoch)).toEqual([+4, +5, +10])
+
+		a.set(30)
+
+		// will be RESET_VALUE because we don't have enough history
+		expect(a.getDiffSince(startEpoch)).toEqual(RESET_VALUE)
+	})
+	it('has history independent of other atoms', () => {
+		const a = atom('', 1, { historyLength: 3, computeDiff: (a, b) => b - a })
+		const b = atom('', 1, { historyLength: 3, computeDiff: (a, b) => b - a })
+
+		const startEpoch = globalEpoch
+
+		b.set(-5)
+		b.set(-10)
+		b.set(-20)
+		expect(b.getDiffSince(startEpoch)).toEqual([-6, -5, -10])
+		expect(b.getDiffSince(globalEpoch)).toEqual([])
+
+		expect(a.getDiffSince(startEpoch)).toEqual([])
+		a.set(5)
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+		expect(b.getDiffSince(startEpoch)).toEqual([-6, -5, -10])
+		expect(b.getDiffSince(globalEpoch)).toEqual([])
+	})
+	it('still updates history during transactions', () => {
+		const a = atom('', 1, { historyLength: 3, computeDiff: (a, b) => b - a })
+
+		const startEpoch = globalEpoch
+
+		transact(() => {
+			expect(a.getDiffSince(startEpoch)).toEqual([])
+
+			a.set(5)
+
+			expect(a.getDiffSince(startEpoch)).toEqual([+4])
+
+			a.set(10)
+
+			expect(a.getDiffSince(startEpoch)).toEqual([+4, +5])
+
+			a.set(20)
+
+			expect(a.getDiffSince(startEpoch)).toEqual([+4, +5, +10])
+		})
+
+		expect(a.getDiffSince(startEpoch)).toEqual([+4, +5, +10])
+	})
+	it('will clear the history if the transaction aborts', () => {
+		const a = atom('', 1, { historyLength: 3, computeDiff: (a, b) => b - a })
+
+		const startEpoch = globalEpoch
+
+		transaction((rollback) => {
+			expect(a.getDiffSince(startEpoch)).toEqual([])
+
+			a.set(5)
+
+			expect(a.getDiffSince(startEpoch)).toEqual([+4])
+
+			rollback()
+		})
+
+		expect(a.getDiffSince(startEpoch)).toEqual(RESET_VALUE)
+	})
+	it('supports an update operation', () => {
+		const startEpoch = globalEpoch
+		const a = atom('', 1)
+
+		a.update((value) => value + 1)
+
+		expect(a.value).toBe(2)
+		expect(globalEpoch).toBe(startEpoch + 1)
+	})
+	it('supports passing diffs in .set', () => {
+		const a = atom('', 1, { historyLength: 3 })
+
+		const startEpoch = globalEpoch
+
+		a.set(5, +4)
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+
+		a.set(6, +1)
+		expect(a.getDiffSince(startEpoch)).toEqual([+4, +1])
+	})
+	it('does not push history if nothing changed', () => {
+		const a = atom('', 1, { historyLength: 3 })
+
+		const startEpoch = globalEpoch
+
+		a.set(5, +4)
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+		a.set(5, +4)
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+	})
+	it('clears the history buffer if you fail to provide a diff', () => {
+		const a = atom('', 1, { historyLength: 3 })
+		const startEpoch = globalEpoch
+
+		a.set(5, +4)
+
+		expect(a.getDiffSince(startEpoch)).toEqual([+4])
+
+		a.set(6)
+
+		expect(a.getDiffSince(startEpoch)).toEqual(RESET_VALUE)
+	})
+})
+
+describe('reacting to atoms', () => {
+	it('should work', async () => {
+		const a = atom('', 234)
+
+		let val = 0
+		const r = reactor('', () => {
+			val = a.value
+		})
+
+		expect(val).toBe(0)
+
+		r.start()
+
+		expect(val).toBe(234)
+
+		a.set(939)
+
+		expect(val).toBe(939)
+
+		r.stop()
+
+		a.set(2342)
+
+		expect(val).toBe(939)
+		expect(a.value).toBe(2342)
+	})
+})
+
+test('isEqual can provide custom equality checks', () => {
+	const foo = { hello: true }
+	const bar = { hello: true }
+
+	const a = atom('a', foo)
+
+	a.set(bar)
+
+	expect(a.value).toBe(bar)
+
+	const b = atom('b', foo, { isEqual: (a, b) => a.hello === b.hello })
+
+	b.set(bar)
+
+	expect(b.value).toBe(foo)
+})

packages/state/src/lib/core/__tests__/capture.test.ts

@@ -0,0 +1,238 @@
+import { atom } from '../Atom'
+import { computed } from '../Computed'
+import { react } from '../EffectScheduler'
+import {
+	maybeCaptureParent,
+	startCapturingParents,
+	stopCapturingParents,
+	unsafe__withoutCapture,
+} from '../capture'
+import { advanceGlobalEpoch, globalEpoch } from '../transactions'
+import { Child } from '../types'
+
+const emptyChild = (props: Partial<Child> = {}) =>
+	({
+		parentEpochs: [],
+		parents: [],
+		isActivelyListening: false,
+		lastTraversedEpoch: 0,
+		...props,
+	} as Child)
+
+describe('capturing parents', () => {
+	it('can be started and stopped', () => {
+		const a = atom('', 1)
+		const startEpoch = globalEpoch
+
+		const child = emptyChild()
+		const originalParentEpochs = child.parentEpochs
+		const originalParents = child.parents
+
+		startCapturingParents(child)
+		maybeCaptureParent(a)
+		stopCapturingParents()
+
+		// the parents should be kept because no sharing is possible and we don't want to reallocate
+		// when parents change
+		expect(child.parentEpochs).toBe(originalParentEpochs)
+		expect(child.parents).toBe(originalParents)
+		expect(child.parentEpochs).toEqual([startEpoch])
+		expect(child.parents).toEqual([a])
+	})
+
+	it('can handle several parents', () => {
+		const atomA = atom('', 1)
+		const atomAEpoch = globalEpoch
+		advanceGlobalEpoch() // let's say time has passed
+		const atomB = atom('', 1)
+		const atomBEpoch = globalEpoch
+		advanceGlobalEpoch() // let's say time has passed
+		const atomC = atom('', 1)
+		const atomCEpoch = globalEpoch
+
+		expect(atomAEpoch < atomBEpoch).toBe(true)
+		expect(atomBEpoch < atomCEpoch).toBe(true)
+
+		const child = emptyChild()
+
+		const originalParentEpochs = child.parentEpochs
+		const originalParents = child.parents
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomA)
+		maybeCaptureParent(atomB)
+		maybeCaptureParent(atomC)
+		stopCapturingParents()
+
+		// the parents should be kept because no sharing is possible and we don't want to reallocate
+		// when parents change
+		expect(child.parentEpochs).toBe(originalParentEpochs)
+		expect(child.parents).toBe(originalParents)
+
+		expect(child.parentEpochs).toEqual([atomAEpoch, atomBEpoch, atomCEpoch])
+		expect(child.parents).toEqual([atomA, atomB, atomC])
+	})
+
+	it('will reorder if parents are captured in different orders each time', () => {
+		const atomA = atom('', 1)
+		advanceGlobalEpoch() // let's say time has passed
+		const atomB = atom('', 1)
+		advanceGlobalEpoch() // let's say time has passed
+		const atomC = atom('', 1)
+
+		const child = emptyChild()
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomA)
+		maybeCaptureParent(atomB)
+		maybeCaptureParent(atomC)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomA, atomB, atomC])
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomB)
+		maybeCaptureParent(atomA)
+		maybeCaptureParent(atomC)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomB, atomA, atomC])
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomA)
+		maybeCaptureParent(atomC)
+		maybeCaptureParent(atomB)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomA, atomC, atomB])
+	})
+
+	it('will shrink the parent arrays if the number of captured parents shrinks', () => {
+		const atomA = atom('', 1)
+		const atomAEpoch = globalEpoch
+		advanceGlobalEpoch() // let's say time has passed
+		const atomB = atom('', 1)
+		const atomBEpoch = globalEpoch
+		advanceGlobalEpoch() // let's say time has passed
+		const atomC = atom('', 1)
+		const atomCEpoch = globalEpoch
+
+		const child = emptyChild()
+
+		const originalParents = child.parents
+		const originalParentEpochs = child.parentEpochs
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomA)
+		maybeCaptureParent(atomB)
+		maybeCaptureParent(atomC)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomA, atomB, atomC])
+		expect(child.parents).toBe(originalParents)
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomB)
+		maybeCaptureParent(atomA)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomB, atomA])
+		expect(child.parentEpochs).toEqual([atomBEpoch, atomAEpoch])
+		expect(child.parents).toBe(originalParents)
+
+		startCapturingParents(child)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([])
+		expect(child.parentEpochs).toEqual([])
+		expect(child.parents).toBe(originalParents)
+		expect(child.parentEpochs).toBe(originalParentEpochs)
+
+		startCapturingParents(child)
+		maybeCaptureParent(atomC)
+		stopCapturingParents()
+
+		expect(child.parents).toEqual([atomC])
+		expect(child.parentEpochs).toEqual([atomCEpoch])
+		expect(child.parents).toBe(originalParents)
+		expect(child.parentEpochs).toBe(originalParentEpochs)
+	})
+
+	it('doesnt do anything if you dont start capturing', () => {
+		expect(() => {
+			maybeCaptureParent(atom('', 1))
+		}).not.toThrow()
+	})
+})
+
+describe(unsafe__withoutCapture, () => {
+	it('allows executing comptuer code in a context that short-circuits the current capture frame', () => {
+		const atomA = atom('a', 1)
+		const atomB = atom('b', 1)
+		const atomC = atom('c', 1)
+
+		const child = computed('', () => {
+			return atomA.value + atomB.value + unsafe__withoutCapture(() => atomC.value)
+		})
+
+		let lastValue: number | undefined
+		let numReactions = 0
+
+		react('', () => {
+			numReactions++
+			lastValue = child.value
+		})
+
+		expect(lastValue).toBe(3)
+		expect(numReactions).toBe(1)
+
+		atomA.set(2)
+
+		expect(lastValue).toBe(4)
+		expect(numReactions).toBe(2)
+
+		atomB.set(2)
+
+		expect(lastValue).toBe(5)
+		expect(numReactions).toBe(3)
+
+		atomC.set(2)
+
+		// The reaction should not have run because C was not captured
+		expect(lastValue).toBe(5)
+		expect(numReactions).toBe(3)
+	})
+
+	it('allows executing reactor code in a context that short-circuits the current capture frame', () => {
+		const atomA = atom('a', 1)
+		const atomB = atom('b', 1)
+		const atomC = atom('c', 1)
+
+		let lastValue: number | undefined
+		let numReactions = 0
+
+		react('', () => {
+			numReactions++
+			lastValue = atomA.value + atomB.value + unsafe__withoutCapture(() => atomC.value)
+		})
+
+		expect(lastValue).toBe(3)
+		expect(numReactions).toBe(1)
+
+		atomA.set(2)
+
+		expect(lastValue).toBe(4)
+		expect(numReactions).toBe(2)
+
+		atomB.set(2)
+
+		expect(lastValue).toBe(5)
+		expect(numReactions).toBe(3)
+
+		atomC.set(2)
+
+		// The reaction should not have run because C was not captured
+		expect(lastValue).toBe(5)
+		expect(numReactions).toBe(3)
+	})
+})

packages/state/src/lib/core/__tests__/computed.test.ts

@@ -0,0 +1,610 @@
+import { atom } from '../Atom'
+import { Computed, _Computed, computed, getComputedInstance, isUninitialized } from '../Computed'
+import { reactor } from '../EffectScheduler'
+import { assertNever } from '../helpers'
+import { advanceGlobalEpoch, globalEpoch, transact, transaction } from '../transactions'
+import { RESET_VALUE, Signal } from '../types'
+
+function getLastCheckedEpoch(derivation: Computed<any>): number {
+	return (derivation as any).lastCheckedEpoch
+}
+
+describe('derivations', () => {
+	it('will cache a value forever if it has no parents', () => {
+		const derive = jest.fn(() => 1)
+		const startEpoch = globalEpoch
+		const derivation = computed('', derive)
+
+		expect(derive).toHaveBeenCalledTimes(0)
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.value).toBe(1)
+		expect(derivation.value).toBe(1)
+
+		expect(derive).toHaveBeenCalledTimes(1)
+
+		advanceGlobalEpoch()
+		advanceGlobalEpoch()
+		advanceGlobalEpoch()
+		advanceGlobalEpoch()
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.value).toBe(1)
+		expect(derivation.value).toBe(1)
+		advanceGlobalEpoch()
+		advanceGlobalEpoch()
+		expect(derivation.value).toBe(1)
+		expect(derivation.value).toBe(1)
+
+		expect(derive).toHaveBeenCalledTimes(1)
+
+		expect(derivation.parents.length).toBe(0)
+
+		expect(derivation.lastChangedEpoch).toBe(startEpoch)
+	})
+
+	it('will update when parent atoms update', () => {
+		const a = atom('', 1)
+		const double = jest.fn(() => a.value * 2)
+		const derivation = computed('', double)
+		const startEpoch = globalEpoch
+		expect(double).toHaveBeenCalledTimes(0)
+
+		expect(derivation.value).toBe(2)
+		expect(double).toHaveBeenCalledTimes(1)
+
+		expect(derivation.lastChangedEpoch).toBe(startEpoch)
+
+		expect(derivation.value).toBe(2)
+		expect(derivation.value).toBe(2)
+		expect(double).toHaveBeenCalledTimes(1)
+		expect(derivation.lastChangedEpoch).toBe(startEpoch)
+
+		a.set(2)
+		const nextEpoch = globalEpoch
+		expect(nextEpoch > startEpoch).toBe(true)
+
+		expect(double).toHaveBeenCalledTimes(1)
+		expect(derivation.lastChangedEpoch).toBe(startEpoch)
+		expect(derivation.value).toBe(4)
+
+		expect(double).toHaveBeenCalledTimes(2)
+		expect(derivation.lastChangedEpoch).toBe(nextEpoch)
+
+		expect(derivation.value).toBe(4)
+		expect(double).toHaveBeenCalledTimes(2)
+		expect(derivation.lastChangedEpoch).toBe(nextEpoch)
+
+		// creating an unrelated atom and setting it will have no effect
+		const unrelatedAtom = atom('', 1)
+		unrelatedAtom.set(2)
+		unrelatedAtom.set(3)
+		unrelatedAtom.set(5)
+
+		expect(derivation.value).toBe(4)
+		expect(double).toHaveBeenCalledTimes(2)
+		expect(derivation.lastChangedEpoch).toBe(nextEpoch)
+	})
+
+	it('supports history', () => {
+		const startEpoch = globalEpoch
+		const a = atom('', 1)
+
+		const derivation = computed('', () => a.value * 2, {
+			historyLength: 3,
+			computeDiff: (a, b) => {
+				return b - a
+			},
+		})
+
+		derivation.value
+
+		expect(derivation.getDiffSince(startEpoch)).toHaveLength(0)
+
+		a.set(2)
+
+		expect(derivation.getDiffSince(startEpoch)).toEqual([+2])
+
+		a.set(3)
+
+		expect(derivation.getDiffSince(startEpoch)).toEqual([+2, +2])
+
+		a.set(5)
+
+		expect(derivation.getDiffSince(startEpoch)).toEqual([+2, +2, +4])
+
+		a.set(6)
+		// should fail now because we don't have enough hisstory
+		expect(derivation.getDiffSince(startEpoch)).toEqual(RESET_VALUE)
+	})
+
+	it('doesnt update history if it doesnt change', () => {
+		const startEpoch = globalEpoch
+		const a = atom('', 1)
+
+		const floor = jest.fn((n: number) => Math.floor(n))
+		const derivation = computed('', () => floor(a.value), {
+			historyLength: 3,
+			computeDiff: (a, b) => {
+				return b - a
+			},
+		})
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.getDiffSince(startEpoch)).toHaveLength(0)
+
+		a.set(1.2)
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.getDiffSince(startEpoch)).toHaveLength(0)
+		expect(floor).toHaveBeenCalledTimes(2)
+
+		a.set(1.5)
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.getDiffSince(startEpoch)).toHaveLength(0)
+		expect(floor).toHaveBeenCalledTimes(3)
+
+		a.set(1.9)
+
+		expect(derivation.value).toBe(1)
+		expect(derivation.getDiffSince(startEpoch)).toHaveLength(0)
+		expect(floor).toHaveBeenCalledTimes(4)
+
+		a.set(2.3)
+
+		expect(derivation.value).toBe(2)
+		expect(derivation.getDiffSince(startEpoch)).toEqual([+1])
+		expect(floor).toHaveBeenCalledTimes(5)
+	})
+
+	it('updates the lastCheckedEpoch whenever the globalEpoch advances', () => {
+		const startEpoch = globalEpoch
+		const a = atom('', 1)
+
+		const double = jest.fn(() => a.value * 2)
+		const derivation = computed('', double)
+
+		derivation.value
+
+		expect(getLastCheckedEpoch(derivation)).toEqual(startEpoch)
+
+		advanceGlobalEpoch()
+		derivation.value
+
+		expect(getLastCheckedEpoch(derivation)).toBeGreaterThan(startEpoch)
+
+		expect(double).toHaveBeenCalledTimes(1)
+	})
+
+	it('receives UNINTIALIZED as the previousValue the first time it computes', () => {
+		const a = atom('', 1)
+		const double = jest.fn((_prevValue) => a.value * 2)
+		const derivation = computed('', double)
+
+		expect(derivation.value).toBe(2)
+
+		expect(isUninitialized(double.mock.calls[0][0])).toBe(true)
+
+		a.set(2)
+
+		expect(derivation.value).toBe(4)
+		expect(isUninitialized(double.mock.calls[1][0])).toBe(false)
+		expect(double.mock.calls[1][0]).toBe(2)
+	})
+
+	it('receives the lastChangedEpoch as the second parameter each time it recomputes', () => {
+		const a = atom('', 1)
+		const double = jest.fn((_prevValue, lastChangedEpoch) => {
+			expect(lastChangedEpoch).toBe(derivation.lastChangedEpoch)
+			return a.value * 2
+		})
+		const derivation = computed('', double)
+
+		expect(derivation.value).toBe(2)
+
+		const startEpoch = globalEpoch
+
+		a.set(2)
+
+		expect(derivation.value).toBe(4)
+		expect(derivation.lastChangedEpoch).toBeGreaterThan(startEpoch)
+
+		expect(double).toHaveBeenCalledTimes(2)
+		expect.assertions(6)
+	})
+
+	it('can be reacted to', () => {
+		const firstName = atom('', 'John')
+		const lastName = atom('', 'Doe')
+
+		let numTimesComputed = 0
+		const fullName = computed('', () => {
+			numTimesComputed++
+			return `${firstName.value} ${lastName.value}`
+		})
+
+		let numTimesReacted = 0
+		let name = ''
+		const r = reactor('', () => {
+			name = fullName.value
+			numTimesReacted++
+		})
+
+		expect(numTimesReacted).toBe(0)
+		expect(name).toBe('')
+
+		r.start()
+
+		expect(numTimesReacted).toBe(1)
+		expect(numTimesComputed).toBe(1)
+		expect(name).toBe('John Doe')
+
+		firstName.set('Jane')
+
+		expect(numTimesComputed).toBe(2)
+		expect(numTimesReacted).toBe(2)
+		expect(name).toBe('Jane Doe')
+
+		firstName.set('Jane')
+		firstName.set('Jane')
+		firstName.set('Jane')
+
+		expect(numTimesComputed).toBe(2)
+		expect(numTimesReacted).toBe(2)
+		expect(name).toBe('Jane Doe')
+
+		transact(() => {
+			firstName.set('Wilbur')
+			expect(numTimesComputed).toBe(2)
+			expect(numTimesReacted).toBe(2)
+			expect(name).toBe('Jane Doe')
+			lastName.set('Jones')
+			expect(numTimesComputed).toBe(2)
+			expect(numTimesReacted).toBe(2)
+			expect(name).toBe('Jane Doe')
+			expect(fullName.value).toBe('Wilbur Jones')
+
+			expect(numTimesComputed).toBe(3)
+			expect(numTimesReacted).toBe(2)
+			expect(name).toBe('Jane Doe')
+		})
+
+		expect(numTimesComputed).toBe(3)
+		expect(numTimesReacted).toBe(3)
+		expect(name).toBe('Wilbur Jones')
+	})
+
+	it('will roll back to their initial value if a transaciton is aborted', () => {
+		const firstName = atom('', 'John')
+		const lastName = atom('', 'Doe')
+
+		const fullName = computed('', () => `${firstName.value} ${lastName.value}`)
+
+		transaction((rollback) => {
+			firstName.set('Jane')
+			lastName.set('Jones')
+			expect(fullName.value).toBe('Jane Jones')
+			rollback()
+		})
+
+		expect(fullName.value).toBe('John Doe')
+	})
+
+	it('will add history items if a transaction is aborted', () => {
+		const a = atom('', 1)
+		const b = atom('', 1)
+
+		const c = computed('', () => a.value + b.value, {
+			historyLength: 3,
+			computeDiff: (a, b) => b - a,
+		})
+
+		const startEpoch = globalEpoch
+
+		transaction((rollback) => {
+			expect(c.getDiffSince(startEpoch)).toEqual([])
+			a.set(2)
+			b.set(2)
+			expect(c.getDiffSince(startEpoch)).toEqual([+2])
+			rollback()
+		})
+
+		expect(c.getDiffSince(startEpoch)).toEqual([2, -2])
+	})
+
+	it('will return RESET_VALUE if .getDiffSince is called with an epoch before initialization', () => {
+		const a = atom('', 1)
+		const b = atom('', 1)
+
+		const c = computed('', () => a.value + b.value, {
+			historyLength: 3,
+			computeDiff: (a, b) => b - a,
+		})
+
+		expect(c.getDiffSince(globalEpoch - 1)).toEqual(RESET_VALUE)
+	})
+})
+
+type Difference =
+	| {
+			type: 'CHANGE'
+			path: string[]
+			value: any
+			oldValue: any
+	  }
+	| { type: 'CREATE'; path: string[]; value: any }
+	| { type: 'REMOVE'; path: string[]; oldValue: any }
+
+function getIncrementalRecordMapper<In, Out>(
+	obj: Signal<Record<string, In>, Difference[]>,
+	mapper: (t: In, k: string) => Out
+): Computed<Record<string, Out>> {
+	function computeFromScratch() {
+		const input = obj.value
+		return Object.fromEntries(Object.entries(input).map(([k, v]) => [k, mapper(v, k)]))
+	}
+	return computed('', (previousValue, lastComputedEpoch) => {
+		if (isUninitialized(previousValue)) {
+			return computeFromScratch()
+		}
+		const diff = obj.getDiffSince(lastComputedEpoch)
+		if (diff === RESET_VALUE) {
+			return computeFromScratch()
+		}
+		if (diff.length === 0) {
+			return previousValue
+		}
+
+		const newUpstream = obj.value
+
+		const result = { ...previousValue } as Record<string, Out>
+
+		const changedKeys = new Set<string>()
+		for (const change of diff.flat()) {
+			const key = change.path[0] as string
+			if (changedKeys.has(key)) {
+				continue
+			}
+			switch (change.type) {
+				case 'CHANGE':
+				case 'CREATE':
+					changedKeys.add(key)
+					if (key in newUpstream) {
+						result[key] = mapper(newUpstream[key], change.path[0] as string)
+					} else {
+						// key was removed later in this patch
+					}
+					break
+				case 'REMOVE':
+					if (key in result) {
+						delete result[key]
+					}
+					break
+				default:
+					assertNever(change)
+			}
+		}
+
+		return result
+	})
+}
+
+describe('incremental derivations', () => {
+	it('should be possible', () => {
+		type NumberMap = Record<string, number>
+
+		const nodes = atom<NumberMap, Difference[]>(
+			'',
+			{
+				a: 1,
+				b: 2,
+				c: 3,
+				d: 4,
+				e: 5,
+			},
+			{
+				historyLength: 10,
+				computeDiff: (valA, valB) => {
+					const result: Difference[] = []
+					for (const keyA in valA) {
+						if (!(keyA in valB)) {
+							result.push({
+								type: 'REMOVE',
+								oldValue: valA[keyA],
+								path: [keyA],
+							})
+						} else if (valA[keyA] != valB[keyA]) {
+							result.push({
+								type: 'CHANGE',
+								oldValue: valA[keyA],
+								path: [keyA],
+								value: valB[keyA],
+							})
+						}
+					}
+
+					for (const keyB in valB) {
+						if (!(keyB in valA)) {
+							result.push({
+								type: 'CREATE',
+								value: valB[keyB],
+								path: [keyB],
+							})
+						}
+					}
+					return result
+				},
+			}
+		)
+
+		const mapper = jest.fn((val) => val * 2)
+
+		const doubledNodes = getIncrementalRecordMapper(nodes, mapper)
+
+		expect(doubledNodes.value).toEqual({
+			a: 2,
+			b: 4,
+			c: 6,
+			d: 8,
+			e: 10,
+		})
+		expect(mapper).toHaveBeenCalledTimes(5)
+
+		nodes.update((ns) => ({ ...ns, a: 10 }))
+
+		expect(doubledNodes.value).toEqual({
+			a: 20,
+			b: 4,
+			c: 6,
+			d: 8,
+			e: 10,
+		})
+
+		expect(mapper).toHaveBeenCalledTimes(6)
+
+		// remove d
+		nodes.update(({ d: _d, ...others }) => others)
+
+		expect(doubledNodes.value).toEqual({
+			a: 20,
+			b: 4,
+			c: 6,
+			e: 10,
+		})
+		expect(mapper).toHaveBeenCalledTimes(6)
+
+		nodes.update((ns) => ({ ...ns, f: 50, g: 60 }))
+
+		expect(doubledNodes.value).toEqual({
+			a: 20,
+			b: 4,
+			c: 6,
+			e: 10,
+			f: 100,
+			g: 120,
+		})
+		expect(mapper).toHaveBeenCalledTimes(8)
+
+		nodes.set({ ...nodes.value })
+		// no changes so no new calls to mapper
+		expect(doubledNodes.value).toEqual({
+			a: 20,
+			b: 4,
+			c: 6,
+			e: 10,
+			f: 100,
+			g: 120,
+		})
+		expect(mapper).toHaveBeenCalledTimes(8)
+
+		// make several changes
+
+		nodes.update((ns) => ({ ...ns, a: 1 }))
+		nodes.update((ns) => ({ ...ns, b: 9 }))
+		nodes.update((ns) => ({ ...ns, c: 17 }))
+		nodes.update(({ f: _f, g: _g, ...others }) => ({ ...others }))
+		nodes.update((ns) => ({ ...ns, d: 4 }))
+		nodes.update((ns) => ({ ...ns, a: 4 }))
+
+		// nothing was called because we didn't deref yet
+		expect(mapper).toHaveBeenCalledTimes(8)
+
+		expect(doubledNodes.value).toEqual({
+			a: 8,
+			b: 18,
+			c: 34,
+			d: 8,
+			e: 10,
+		})
+
+		expect(mapper).toHaveBeenCalledTimes(12)
+	})
+})
+
+describe('computed as a decorator', () => {
+	it('can be used to decorate a class', () => {
+		class Foo {
+			a = atom('a', 1)
+			@computed
+			get b() {
+				return this.a.value * 2
+			}
+		}
+
+		const foo = new Foo()
+
+		expect(foo.b).toBe(2)
+
+		foo.a.set(2)
+
+		expect(foo.b).toBe(4)
+	})
+
+	it('can be used to decorate a class with custom properties', () => {
+		let numComputations = 0
+		class Foo {
+			a = atom('a', 1)
+
+			@computed({ isEqual: (a, b) => a.b === b.b })
+			get b() {
+				numComputations++
+				return { b: this.a.value * this.a.value }
+			}
+		}
+
+		const foo = new Foo()
+
+		const firstVal = foo.b
+		expect(firstVal).toEqual({ b: 1 })
+
+		foo.a.set(-1)
+
+		const secondVal = foo.b
+		expect(secondVal).toEqual({ b: 1 })
+
+		expect(firstVal).toBe(secondVal)
+		expect(numComputations).toBe(2)
+	})
+})
+
+describe(getComputedInstance, () => {
+	it('can retrieve the underlying computed instance', () => {
+		class Foo {
+			a = atom('a', 1)
+
+			@computed({ isEqual: (a, b) => a.b === b.b })
+			get b() {
+				return { b: this.a.value * this.a.value }
+			}
+		}
+
+		const foo = new Foo()
+
+		const bInst = getComputedInstance(foo, 'b')
+
+		expect(bInst).toBeDefined()
+		expect(bInst).toBeInstanceOf(_Computed)
+	})
+})
+
+describe('computed isEqual', () => {
+	it('does not get called for the initialization', () => {
+		const isEqual = jest.fn((a, b) => a === b)
+
+		const a = atom('a', 1)
+		const b = computed('b', () => a.value * 2, { isEqual })
+
+		expect(b.value).toBe(2)
+		expect(isEqual).not.toHaveBeenCalled()
+		expect(b.value).toBe(2)
+		expect(isEqual).not.toHaveBeenCalled()
+
+		a.set(2)
+
+		expect(b.value).toBe(4)
+		expect(isEqual).toHaveBeenCalledTimes(1)
+		expect(b.value).toBe(4)
+		expect(isEqual).toHaveBeenCalledTimes(1)
+	})
+})

packages/state/src/lib/core/__tests__/fuzz.tlstate.test.ts

@@ -0,0 +1,370 @@
+import { times } from 'lodash'
+import { Atom, atom, isAtom } from '../Atom'
+import { Computed, computed, isComputed } from '../Computed'
+import { Reactor, reactor } from '../EffectScheduler'
+import { transact } from '../transactions'
+import { Signal } from '../types'
+
+class RandomSource {
+	private seed: number
+
+	constructor(seed: number) {
+		this.seed = seed
+	}
+
+	nextFloat(): number {
+		this.seed = (this.seed * 9301 + 49297) % 233280
+		return this.seed / 233280
+	}
+
+	nextInt(max: number): number {
+		return Math.floor(this.nextFloat() * max)
+	}
+
+	nextIntInRange(min: number, max: number): number {
+		return this.nextInt(max - min) + min
+	}
+
+	nextId(): string {
+		return this.nextInt(Number.MAX_SAFE_INTEGER).toString(36)
+	}
+
+	selectOne<T>(arr: readonly T[]): T {
+		return arr[this.nextInt(arr.length)]
+	}
+
+	choice(probability: number): boolean {
+		return this.nextFloat() < probability
+	}
+
+	executeOne<Result>(
+		_choices: Record<string, (() => Result) | { weight?: number; do(): Result }>
+	): Result {
+		const choices = Object.values(_choices).map((choice) => {
+			if (typeof choice === 'function') {
+				return { weight: 1, do: choice }
+			}
+			return choice
+		})
+		const totalWeight = Object.values(choices).reduce(
+			(total, choice) => total + (choice.weight ?? 1),
+			0
+		)
+		const randomWeight = this.nextInt(totalWeight)
+		let weight = 0
+		for (const choice of Object.values(choices)) {
+			weight += choice.weight ?? 1
+			if (randomWeight < weight) {
+				return choice.do()
+			}
+		}
+		throw new Error('unreachable')
+	}
+}
+
+const LETTERS = ['a', 'b', 'c', 'd', 'e', 'f'] as const
+type Letter = (typeof LETTERS)[number]
+
+const unpack = (value: unknown): Letter => {
+	if (isComputed(value) || isAtom(value)) {
+		return unpack(value.value) as Letter
+	}
+	return value as Letter
+}
+
+type FuzzSystemState = {
+	atoms: Record<string, Atom<Letter>>
+	atomsInAtoms: Record<string, Atom<Atom<Letter>>>
+	derivations: Record<string, { derivation: Computed<Letter>; sneakyGet: () => Letter }>
+	derivationsInDerivations: Record<string, Computed<Computed<Letter>>>
+	atomsInDerivations: Record<string, Computed<Atom<Letter>>>
+	reactors: Record<string, { reactor: Reactor; result: string | null; dependencies: Signal<any>[] }>
+}
+
+type Op =
+	| { type: 'update_atom'; id: string; value: Letter }
+	| { type: 'update_atom_in_atom'; id: string; atomId: string }
+	| { type: 'deref_derivation'; id: string }
+	| { type: 'deref_derivation_in_derivation'; id: string }
+	| { type: 'deref_atom_in_derivation'; id: string }
+	| { type: 'run_several_ops_in_transaction'; ops: Op[] }
+	| { type: 'start_reactor'; id: string }
+	| { type: 'stop_reactor'; id: string }
+
+const MAX_ATOMS = 10
+const MAX_ATOMS_IN_ATOMS = 10
+const MAX_DERIVATIONS = 10
+const MAX_DERIVATIONS_IN_DERIVATIONS = 10
+const MAX_ATOMS_IN_DERIVATIONS = 10
+const MAX_REACTORS = 10
+const MAX_DEPENDENCIES_PER_ATOM = 3
+const MAX_OPS_IN_TRANSACTION = 10
+
+class Test {
+	source: RandomSource
+	systemState: FuzzSystemState = {
+		atoms: {},
+		atomsInAtoms: {},
+		derivations: {},
+		derivationsInDerivations: {},
+		atomsInDerivations: {},
+		reactors: {},
+	}
+
+	unpack_sneaky = (value: unknown): Letter => {
+		if (isComputed(value)) {
+			if (this.systemState.derivations[value.name]) {
+				return this.systemState.derivations[value.name].sneakyGet()
+			}
+			// @ts-expect-error
+			return this.unpack_sneaky(value.state) as Letter
+		} else if (isAtom(value)) {
+			// @ts-expect-error
+			return this.unpack_sneaky(value.current) as Letter
+		}
+		return value as Letter
+	}
+
+	getResultComparisons() {
+		const result: { expected: Record<string, string>; actual: Record<string, string | null> } = {
+			expected: {},
+			actual: {},
+		}
+		for (const [reactorId, { reactor, result: actualResult, dependencies }] of Object.entries(
+			this.systemState.reactors
+		)) {
+			if (!reactor.scheduler.isActivelyListening) continue
+			result.expected[reactorId] = dependencies.map(this.unpack_sneaky).join(':')
+			result.actual[reactorId] = actualResult
+		}
+
+		return result
+	}
+
+	constructor(seed: number) {
+		this.source = new RandomSource(seed)
+
+		times(this.source.nextIntInRange(1, MAX_ATOMS), () => {
+			const atomId = this.source.nextId()
+			this.systemState.atoms[atomId] = atom(atomId, this.source.selectOne(LETTERS))
+		})
+
+		times(this.source.nextIntInRange(1, MAX_ATOMS_IN_ATOMS), () => {
+			const atomId = this.source.nextId()
+			this.systemState.atomsInAtoms[atomId] = atom(
+				atomId,
+				this.source.selectOne(Object.values(this.systemState.atoms))
+			)
+		})
+
+		times(this.source.nextIntInRange(1, MAX_ATOMS_IN_DERIVATIONS), () => {
+			const derivationId = this.source.nextId()
+			const atom = this.source.selectOne(Object.values(this.systemState.atoms))
+			this.systemState.atomsInDerivations[derivationId] = computed(derivationId, () => atom)
+		})
+
+		times(this.source.nextIntInRange(1, MAX_DERIVATIONS), () => {
+			const derivationId = this.source.nextId()
+			const derivables = [
+				...Object.values(this.systemState.atoms),
+				...Object.values(this.systemState.atomsInAtoms),
+				...Object.values(this.systemState.atomsInDerivations),
+				...Object.values(this.systemState.derivations),
+			]
+			const inputA = this.source.selectOne(derivables)
+			const inputB = this.source.selectOne(derivables)
+			const inputC = this.source.selectOne(derivables)
+			const inputD = this.source.selectOne(derivables)
+			this.systemState.derivations[derivationId] = {
+				derivation: computed(derivationId, () => {
+					if (unpack(inputA) === unpack(inputB)) {
+						return unpack(inputC)
+					} else {
+						return unpack(inputD)
+					}
+				}),
+				sneakyGet: () => {
+					if (this.unpack_sneaky(inputA) === this.unpack_sneaky(inputB)) {
+						return this.unpack_sneaky(inputC)
+					} else {
+						return this.unpack_sneaky(inputD)
+					}
+				},
+			}
+		})
+
+		times(this.source.nextIntInRange(1, MAX_DERIVATIONS_IN_DERIVATIONS), () => {
+			const derivationId = this.source.nextId()
+			this.systemState.derivationsInDerivations[derivationId] = computed(derivationId, () =>
+				this.source.selectOne(Object.values(this.systemState.derivations).map((d) => d.derivation))
+			)
+		})
+
+		times(this.source.nextIntInRange(1, MAX_REACTORS), () => {
+			const reactorId = this.source.nextId()
+			const dependencies: Signal<any>[] = []
+
+			times(this.source.nextIntInRange(1, MAX_DEPENDENCIES_PER_ATOM), () => {
+				this.source.executeOne({
+					'add a random atom': () => {
+						dependencies.push(this.source.selectOne(Object.values(this.systemState.atoms)))
+					},
+					'add a random atom in atom': () => {
+						dependencies.push(this.source.selectOne(Object.values(this.systemState.atomsInAtoms)))
+					},
+					'add a random derivation': () => {
+						dependencies.push(
+							this.source.selectOne(
+								Object.values(this.systemState.derivations).map((d) => d.derivation)
+							)
+						)
+					},
+					'add a random derivation in derivation': () => {
+						dependencies.push(
+							this.source.selectOne(Object.values(this.systemState.derivationsInDerivations))
+						)
+					},
+					'add a random atom in derivation': () => {
+						dependencies.push(
+							this.source.selectOne(Object.values(this.systemState.atomsInDerivations))
+						)
+					},
+				})
+				dependencies.push(this.source.selectOne(Object.values(this.systemState.atoms)))
+			})
+
+			this.systemState.reactors[reactorId] = {
+				reactor: reactor(reactorId, () => {
+					this.systemState.reactors[reactorId].result = dependencies.map(unpack).join(':')
+				}),
+				result: '',
+				dependencies,
+			}
+		})
+	}
+
+	readonly ops: Op[] = []
+
+	getNextOp(): Op {
+		return this.source.executeOne<Op>({
+			'update atom': () => {
+				return {
+					type: 'update_atom',
+					id: this.source.selectOne(Object.keys(this.systemState.atoms)),
+					value: this.source.selectOne(LETTERS),
+				}
+			},
+			'update atom in atom': () => {
+				return {
+					type: 'update_atom_in_atom',
+					id: this.source.selectOne(Object.keys(this.systemState.atomsInAtoms)),
+					atomId: this.source.selectOne(Object.keys(this.systemState.atoms)),
+				}
+			},
+			'deref atom in derivation': () => {
+				return {
+					type: 'deref_atom_in_derivation',
+					id: this.source.selectOne(Object.keys(this.systemState.atomsInDerivations)),
+				}
+			},
+			'deref derivation in derivation': () => {
+				return {
+					type: 'deref_derivation_in_derivation',
+					id: this.source.selectOne(Object.keys(this.systemState.derivationsInDerivations)),
+				}
+			},
+			'deref derivation': () => {
+				return {
+					type: 'deref_derivation',
+					id: this.source.selectOne(Object.keys(this.systemState.derivations)),
+				}
+			},
+			'run several ops in a transaction': () => {
+				return {
+					type: 'run_several_ops_in_transaction',
+					ops: times(this.source.nextIntInRange(2, MAX_OPS_IN_TRANSACTION), () => this.getNextOp()),
+				}
+			},
+			start_reactor: () => {
+				return {
+					type: 'start_reactor',
+					id: this.source.selectOne(Object.keys(this.systemState.reactors)),
+				}
+			},
+			stop_reactor: () => {
+				return {
+					type: 'stop_reactor',
+					id: this.source.selectOne(Object.keys(this.systemState.reactors)),
+				}
+			},
+		})
+	}
+
+	applyOp(op: Op) {
+		switch (op.type) {
+			case 'update_atom': {
+				this.systemState.atoms[op.id].set(op.value)
+				break
+			}
+			case 'deref_atom_in_derivation': {
+				this.systemState.atomsInDerivations[op.id].value
+				break
+			}
+			case 'deref_derivation': {
+				this.systemState.derivations[op.id].derivation.value
+				break
+			}
+			case 'deref_derivation_in_derivation': {
+				this.systemState.derivationsInDerivations[op.id].value
+				break
+			}
+			case 'update_atom_in_atom': {
+				this.systemState.atomsInAtoms[op.id].set(this.systemState.atoms[op.atomId])
+				break
+			}
+			case 'run_several_ops_in_transaction': {
+				transact(() => {
+					op.ops.forEach((op) => this.applyOp(op))
+				})
+				break
+			}
+			case 'start_reactor': {
+				this.systemState.reactors[op.id].reactor.start()
+				break
+			}
+			case 'stop_reactor': {
+				this.systemState.reactors[op.id].reactor.stop()
+				break
+			}
+			default: {
+				throw new Error(`Unknown op type: ${op}`)
+			}
+		}
+	}
+
+	tick() {
+		const op = this.getNextOp()
+		this.ops.push(op)
+		this.applyOp(op)
+	}
+}
+
+const NUM_TESTS = 100
+const NUM_OPS_PER_TEST = 1000
+
+function runTest(seed: number) {
+	const test = new Test(seed)
+	// console.log(test.systemState)
+	for (let i = 0; i < NUM_OPS_PER_TEST; i++) {
+		test.tick()
+		const { expected, actual } = test.getResultComparisons()
+		expect(expected).toEqual(actual)
+	}
+}
+
+for (let i = 0; i < NUM_TESTS; i++) {
+	const seed = Math.floor(Math.random() * 1000000)
+	test('fuzzzzzz ' + seed, () => {
+		runTest(seed)
+	})
+}

packages/state/src/lib/core/__tests__/reactor.test.ts

@@ -0,0 +1,196 @@
+import { atom } from '../Atom'
+import { react, reactor } from '../EffectScheduler'
+import { advanceGlobalEpoch, transact } from '../transactions'
+
+describe('reactors', () => {
+	it('can be started and stopped ', () => {
+		const a = atom('', 1)
+		const r = reactor('', () => {
+			a.value
+		})
+		expect(r.scheduler.isActivelyListening).toBe(false)
+		r.start()
+		expect(r.scheduler.isActivelyListening).toBe(true)
+		r.stop()
+		expect(r.scheduler.isActivelyListening).toBe(false)
+		r.start()
+		expect(r.scheduler.isActivelyListening).toBe(true)
+	})
+
+	it('can not set atom values directly yet', () => {
+		const a = atom('', 1)
+		const r = reactor('', () => {
+			if (a.value < +Infinity) {
+				a.update((a) => a + 1)
+			}
+		})
+		expect(() => r.start()).toThrowErrorMatchingInlineSnapshot(
+			`"cannot change atoms during reaction cycle"`
+		)
+	})
+
+	it('will never be called twice after a single state update, even if that update affects multiple atoms to which the reactor is subscribed', () => {
+		const atomA = atom('', 1)
+		const atomB = atom('', 1)
+
+		const react = jest.fn(() => {
+			atomA.value
+			atomB.value
+		})
+		const r = reactor('', react)
+
+		r.start()
+		expect(react).toHaveBeenCalledTimes(1)
+
+		transact(() => {
+			atomA.set(2)
+			atomB.set(2)
+		})
+
+		expect(react).toHaveBeenCalledTimes(2)
+	})
+
+	it('will not react if stopped', () => {
+		const a = atom('', 1)
+		const react = jest.fn(() => {
+			a.value
+		})
+		const r = reactor('', react)
+
+		r.scheduler.maybeScheduleEffect()
+
+		expect(react).not.toHaveBeenCalled()
+	})
+
+	it('will not react if the parents have not changed', () => {
+		const a = atom('', 1)
+		const react = jest
+			.fn(() => {
+				a.value
+			})
+			.mockName('react')
+		const r = reactor('', react)
+
+		r.start()
+		expect(react).toHaveBeenCalledTimes(1)
+
+		advanceGlobalEpoch()
+		r.scheduler.maybeScheduleEffect()
+		expect(react).toHaveBeenCalledTimes(1)
+	})
+})
+
+describe('stopping', () => {
+	it('works', () => {
+		const a = atom('', 1)
+
+		const rfn = jest.fn(() => {
+			a.value
+		})
+		const r = reactor('', rfn)
+
+		expect(a.children.isEmpty).toBe(true)
+
+		r.start()
+
+		expect(a.children.isEmpty).toBe(false)
+
+		a.set(8)
+
+		expect(rfn).toHaveBeenCalledTimes(2)
+
+		r.stop()
+
+		expect(a.children.isEmpty).toBe(true)
+		expect(rfn).toHaveBeenCalledTimes(2)
+
+		a.set(2)
+
+		expect(rfn).toHaveBeenCalledTimes(2)
+
+		a.set(3)
+
+		expect(rfn).toHaveBeenCalledTimes(2)
+
+		expect(a.children.isEmpty).toBe(true)
+	})
+})
+
+test('.start() by default does not trigger a reaction if nothing has changed', () => {
+	const a = atom('', 1)
+
+	const rfn = jest.fn(() => {
+		a.value
+	})
+
+	const r = reactor('', rfn)
+	r.start()
+
+	expect(rfn).toHaveBeenCalledTimes(1)
+
+	r.stop()
+
+	r.start()
+
+	expect(rfn).toHaveBeenCalledTimes(1)
+})
+
+test('.start({force: true}) will trigger a reaction even if nothing has changed', () => {
+	const a = atom('', 1)
+
+	const rfn = jest.fn(() => {
+		a.value
+	})
+
+	const r = reactor('', rfn)
+	r.start()
+
+	expect(rfn).toHaveBeenCalledTimes(1)
+
+	r.stop()
+
+	r.start({ force: true })
+
+	expect(rfn).toHaveBeenCalledTimes(2)
+})
+
+test('.start with a custom scheduler only schedules an effect, it does not execute it immediately', () => {
+	let numSchedules = 0
+	let numExecutes = 0
+
+	const r = reactor(
+		'',
+		() => {
+			numExecutes++
+		},
+		{
+			scheduleEffect: () => {
+				numSchedules++
+			},
+		}
+	)
+	r.start()
+
+	expect(numSchedules).toBe(1)
+	expect(numExecutes).toBe(0)
+})
+
+test('react() with a custom scheduler only schedules an effect, it does not execute it immediately', () => {
+	let numSchedules = 0
+	let numExecutes = 0
+
+	react(
+		'',
+		() => {
+			numExecutes++
+		},
+		{
+			scheduleEffect: () => {
+				numSchedules++
+			},
+		}
+	)
+
+	expect(numSchedules).toBe(1)
+	expect(numExecutes).toBe(0)
+})

packages/state/src/lib/core/__tests__/transactions.test.ts

@@ -0,0 +1,202 @@
+import { atom } from '../Atom'
+import { computed } from '../Computed'
+import { react } from '../EffectScheduler'
+import { transact, transaction } from '../transactions'
+
+describe('transactions', () => {
+	it('should be abortable', () => {
+		const firstName = atom('', 'John')
+		const lastName = atom('', 'Doe')
+
+		let numTimesComputed = 0
+		const fullName = computed('', () => {
+			numTimesComputed++
+			return `${firstName.value} ${lastName.value}`
+		})
+
+		let numTimesReacted = 0
+		let name = ''
+
+		react('', () => {
+			name = fullName.value
+			numTimesReacted++
+		})
+
+		expect(numTimesReacted).toBe(1)
+		expect(numTimesComputed).toBe(1)
+		expect(name).toBe('John Doe')
+
+		transaction((rollback) => {
+			firstName.set('Wilbur')
+			expect(numTimesComputed).toBe(1)
+			expect(numTimesReacted).toBe(1)
+			expect(name).toBe('John Doe')
+			lastName.set('Jones')
+			expect(numTimesComputed).toBe(1)
+			expect(numTimesReacted).toBe(1)
+			expect(name).toBe('John Doe')
+			expect(fullName.value).toBe('Wilbur Jones')
+
+			expect(numTimesComputed).toBe(2)
+			expect(numTimesReacted).toBe(1)
+			expect(name).toBe('John Doe')
+
+			rollback()
+		})
+
+		// computes again
+		expect(numTimesComputed).toBe(3)
+		expect(numTimesReacted).toBe(2)
+
+		expect(fullName.value).toBe('John Doe')
+		expect(name).toBe('John Doe')
+	})
+
+	it('nested rollbacks work as expected', () => {
+		const atomA = atom('', 0)
+		const atomB = atom('', 0)
+
+		transaction((rollback) => {
+			atomA.set(1)
+			atomB.set(-1)
+			transaction((rollback) => {
+				atomA.set(2)
+				atomB.set(-2)
+				transaction((rollback) => {
+					atomA.set(3)
+					atomB.set(-3)
+					rollback()
+				})
+				rollback()
+			})
+			rollback()
+		})
+
+		expect(atomA.value).toBe(0)
+		expect(atomB.value).toBe(0)
+
+		transaction((rollback) => {
+			atomA.set(1)
+			atomB.set(-1)
+			transaction((rollback) => {
+				atomA.set(2)
+				atomB.set(-2)
+				transaction(() => {
+					atomA.set(3)
+					atomB.set(-3)
+				})
+				rollback()
+			})
+			rollback()
+		})
+
+		expect(atomA.value).toBe(0)
+		expect(atomB.value).toBe(0)
+
+		transaction((rollback) => {
+			atomA.set(1)
+			atomB.set(-1)
+			transaction(() => {
+				atomA.set(2)
+				atomB.set(-2)
+				transaction(() => {
+					atomA.set(3)
+					atomB.set(-3)
+				})
+			})
+			rollback()
+		})
+
+		expect(atomA.value).toBe(0)
+		expect(atomB.value).toBe(0)
+
+		transaction(() => {
+			atomA.set(1)
+			atomB.set(-1)
+			transaction((rollback) => {
+				atomA.set(2)
+				atomB.set(-2)
+				transaction((rollback) => {
+					atomA.set(3)
+					atomB.set(-3)
+					rollback()
+				})
+				rollback()
+			})
+		})
+
+		expect(atomA.value).toBe(1)
+		expect(atomB.value).toBe(-1)
+
+		transaction(() => {
+			atomA.set(1)
+			atomB.set(-1)
+			transaction(() => {
+				atomA.set(2)
+				atomB.set(-2)
+				transaction((rollback) => {
+					atomA.set(3)
+					atomB.set(-3)
+					rollback()
+				})
+			})
+		})
+
+		expect(atomA.value).toBe(2)
+		expect(atomB.value).toBe(-2)
+	})
+
+	it('should restore the original even if an inner commits', () => {
+		const a = atom('', 'a')
+
+		transaction((rollback) => {
+			transaction(() => {
+				a.set('b')
+			})
+			rollback()
+		})
+
+		expect(a.value).toBe('a')
+	})
+})
+
+describe('transact', () => {
+	it('executes things in a transaction', () => {
+		const a = atom('', 'a')
+
+		try {
+			transact(() => {
+				a.set('b')
+				throw new Error('blah')
+			})
+		} catch (e: any) {
+			expect(e.message).toBe('blah')
+		}
+
+		expect(a.value).toBe('a')
+
+		expect.assertions(2)
+	})
+
+	it('does not create nested transactions', () => {
+		const a = atom('', 'a')
+
+		transact(() => {
+			a.set('b')
+
+			try {
+				transact(() => {
+					a.set('c')
+					throw new Error('blah')
+				})
+			} catch (e: any) {
+				expect(e.message).toBe('blah')
+			}
+			expect(a.value).toBe('c')
+		})
+
+		expect(a.value).toBe('c')
+
+		expect.assertions(3)
+	})
+})

packages/state/src/lib/core/capture.ts

@@ -0,0 +1,179 @@
+import { attach, detach } from './helpers'
+import { Child, Signal } from './types'
+
+const globalKey = Symbol.for('__@tldraw/state__')
+const global = globalThis as { [globalKey]?: true }
+
+if (global[globalKey]) {
+	console.error(
+		'Multiple versions of @tldraw/state detected. This will cause unexpected behavior. Please add "resolutions" (yarn/pnpm) or "overrides" (npm) in your package.json to ensure only one version of @tldraw/state is loaded.'
+	)
+} else {
+	global[globalKey] = true
+}
+
+class CaptureStackFrame {
+	offset = 0
+	numNewParents = 0
+
+	maybeRemoved?: Signal<any>[]
+
+	constructor(public readonly below: CaptureStackFrame | null, public readonly child: Child) {}
+}
+
+let stack: CaptureStackFrame | null = null
+
+/**
+ * Executes the given function without capturing any parents in the current capture context.
+ *
+ * This is mainly useful if you want to run an effect only when certain signals change while also
+ * dereferencing other signals which should not cause the effect to rerun on their own.
+ *
+ * @example
+ * ```ts
+ * const name = atom('name', 'Sam')
+ * const time = atom('time', () => new Date().getTime())
+ *
+ * setInterval(() => {
+ *   time.set(new Date().getTime())
+ * })
+ *
+ * react('log name changes', () => {
+ * 	 console.log(name.value, 'was changed at', unsafe__withoutCapture(() => time.value))
+ * })
+ *
+ * ```
+ *
+ * @public
+ */
+export function unsafe__withoutCapture<T>(fn: () => T): T {
+	const oldStack = stack
+	stack = null
+	try {
+		return fn()
+	} finally {
+		stack = oldStack
+	}
+}
+
+export function startCapturingParents(child: Child) {
+	stack = new CaptureStackFrame(stack, child)
+}
+
+export function stopCapturingParents() {
+	const frame = stack!
+	stack = frame.below
+
+	const didParentsChange = frame.numNewParents > 0 || frame.offset !== frame.child.parents.length
+
+	if (!didParentsChange) {
+		return
+	}
+
+	for (let i = frame.offset; i < frame.child.parents.length; i++) {
+		const p = frame.child.parents[i]
+		const parentWasRemoved = frame.child.parents.indexOf(p) >= frame.offset
+		if (parentWasRemoved) {
+			detach(p, frame.child)
+		}
+	}
+
+	frame.child.parents.length = frame.offset
+	frame.child.parentEpochs.length = frame.offset
+
+	if (stack?.maybeRemoved) {
+		for (let i = 0; i < stack.maybeRemoved.length; i++) {
+			const maybeRemovedParent = stack.maybeRemoved[i]
+			if (frame.child.parents.indexOf(maybeRemovedParent) === -1) {
+				detach(maybeRemovedParent, frame.child)
+			}
+		}
+	}
+}
+
+// this must be called after the parent is up to date
+export function maybeCaptureParent(p: Signal<any, any>) {
+	if (stack) {
+		const idx = stack.child.parents.indexOf(p)
+		// if the child didn't deref this parent last time it executed, then idx will be -1
+		// if the child did deref this parent last time but in a different order relative to other parents, then idx will be greater than stack.offset
+		// if the child did deref this parent last time in the same order, then idx will be the same as stack.offset
+		// if the child did deref this parent already during this capture session then 0 <= idx < stack.offset
+
+		if (idx < 0) {
+			stack.numNewParents++
+			if (stack.child.isActivelyListening) {
+				attach(p, stack.child)
+			}
+		}
+
+		if (idx < 0 || idx >= stack.offset) {
+			if (idx !== stack.offset && idx > 0) {
+				const maybeRemovedParent = stack.child.parents[stack.offset]
+
+				if (!stack.maybeRemoved) {
+					stack.maybeRemoved = [maybeRemovedParent]
+				} else if (stack.maybeRemoved.indexOf(maybeRemovedParent) === -1) {
+					stack.maybeRemoved.push(maybeRemovedParent)
+				}
+			}
+
+			stack.child.parents[stack.offset] = p
+			stack.child.parentEpochs[stack.offset] = p.lastChangedEpoch
+			stack.offset++
+		}
+	}
+}
+
+/**
+ * A debugging tool that tells you why a computed signal or effect is running.
+ * Call in the body of a computed signal or effect function.
+ *
+ * @example
+ * ```ts
+ * const name = atom('name', 'Bob')
+ * react('greeting', () => {
+ * 	whyAmIRunning()
+ *	console.log('Hello', name.value)
+ * })
+ *
+ * name.set('Alice')
+ *
+ * // 'greeting' is running because:
+ * //     'name' changed => 'Alice'
+ * ```
+ *
+ * @public
+ */
+export function whyAmIRunning() {
+	const child = stack?.child
+	if (!child) {
+		throw new Error('whyAmIRunning() called outside of a reactive context')
+	}
+
+	const changedParents = []
+	for (let i = 0; i < child.parents.length; i++) {
+		const parent = child.parents[i]
+
+		if (parent.lastChangedEpoch > child.parentEpochs[i]) {
+			changedParents.push(parent)
+		}
+	}
+
+	if (changedParents.length === 0) {
+		// eslint-disable-next-line no-console
+		console.log((child as any).name, 'is running but none of the parents changed')
+	} else {
+		// eslint-disable-next-line no-console
+		console.log((child as any).name, 'is running because:')
+		for (const changedParent of changedParents) {
+			// eslint-disable-next-line no-console
+			console.log(
+				'\t',
+				(changedParent as any).name,
+				'changed =>',
+				changedParent.__unsafe__getWithoutCapture()
+			)
+		}
+	}
+}

packages/state/src/lib/core/constants.ts

@@ -0,0 +1,2 @@
+// Derivations start on GLOBAL_START_EPOCH so they are dirty before having been computed
+export const GLOBAL_START_EPOCH = -1

packages/state/src/lib/core/helpers.ts

@@ -0,0 +1,88 @@
+import { Child, Signal } from './types'
+
+/**
+ * Get whether the given value is a child.
+ *
+ * @param x The value to check.
+ * @returns True if the value is a child, false otherwise.
+ */
+function isChild(x: any): x is Child {
+	return x && typeof x === 'object' && 'parents' in x
+}
+
+/**
+ * Get whether a child's parents have changed.
+ *
+ * @param child The child to check.
+ * @returns True if the child's parents have changed, false otherwise.
+ */
+export function haveParentsChanged(child: Child) {
+	for (let i = 0, n = child.parents.length; i < n; i++) {
+		// Get the parent's value without capturing it.
+		child.parents[i].__unsafe__getWithoutCapture()
+
+		// If the parent's epoch does not match the child's view of the parent's epoch, then the parent has changed.
+		if (child.parents[i].lastChangedEpoch !== child.parentEpochs[i]) {
+			return true
+		}
+	}
+
+	return false
+}
+
+/**
+ * Detach a child from a parent.
+ *
+ * @param parent The parent to detach from.
+ * @param child The child to detach.
+ */
+export const detach = (parent: Signal<any>, child: Child) => {
+	// If the child is not attached to the parent, do nothing.
+	if (!parent.children.remove(child)) {
+		return
+	}
+
+	// If the parent has no more children, then detach the parent from its parents.
+	if (parent.children.isEmpty && isChild(parent)) {
+		for (let i = 0, n = parent.parents.length; i < n; i++) {
+			detach(parent.parents[i], parent)
+		}
+	}
+}
+
+/**
+ * Attach a child to a parent.
+ *
+ * @param parent The parent to attach to.
+ * @param child The child to attach.
+ */
+export const attach = (parent: Signal<any>, child: Child) => {
+	// If the child is already attached to the parent, do nothing.
+	if (!parent.children.add(child)) {
+		return
+	}
+
+	// If the parent itself is a child, add the parent to the parent's parents.
+	if (isChild(parent)) {
+		for (let i = 0, n = parent.parents.length; i < n; i++) {
+			attach(parent.parents[i], parent)
+		}
+	}
+}
+
+/**
+ * Get whether two values are equal (insofar as @tldraw/state is concerned).
+ *
+ * @param a The first value.
+ * @param b The second value.
+ */
+export function equals(a: any, b: any): boolean {
+	const shallowEquals =
+		a === b || Object.is(a, b) || Boolean(a && b && typeof a.equals === 'function' && a.equals(b))
+	return shallowEquals
+}
+
+export declare function assertNever(x: never): never
+
+/** @public */
+export const EMPTY_ARRAY: [] = Object.freeze([]) as any

packages/state/src/lib/core/index.ts

@@ -0,0 +1,12 @@
+export { atom, isAtom } from './Atom'
+export type { Atom, AtomOptions } from './Atom'
+export { computed, getComputedInstance, isUninitialized, withDiff } from './Computed'
+export type { Computed, ComputedOptions } from './Computed'
+export { EffectScheduler, react, reactor } from './EffectScheduler'
+export type { Reactor } from './EffectScheduler'
+export { unsafe__withoutCapture, whyAmIRunning } from './capture'
+export { EMPTY_ARRAY } from './helpers'
+export { isSignal } from './isSignal'
+export { transact, transaction } from './transactions'
+export { RESET_VALUE } from './types'
+export type { Signal } from './types'

packages/state/src/lib/core/isSignal.ts

@@ -0,0 +1,11 @@
+import { _Atom } from './Atom'
+import { _Computed } from './Computed'
+import { Signal } from './types'
+
+/**
+ * Returns true if the given value is a signal (either an Atom or a Computed).
+ * @public
+ */
+export function isSignal(value: any): value is Signal<any> {
+	return value instanceof _Atom || value instanceof _Computed
+}

packages/state/src/lib/core/transactions.ts

@@ -0,0 +1,251 @@
+import { _Atom } from './Atom'
+import { GLOBAL_START_EPOCH } from './constants'
+import { EffectScheduler } from './EffectScheduler'
+import { Child, Signal } from './types'
+
+// The current epoch (global to all atoms).
+export let globalEpoch = GLOBAL_START_EPOCH + 1
+
+// Whether any transaction is reacting.
+let globalIsReacting = false
+
+export function advanceGlobalEpoch() {
+	globalEpoch++
+}
+
+class Transaction {
+	constructor(public readonly parent: Transaction | null) {}
+	initialAtomValues = new Map<_Atom<any>, any>()
+
+	/**
+	 * Get whether this transaction is a root (no parents).
+	 *
+	 * @public
+	 */
+	get isRoot() {
+		return this.parent === null
+	}
+
+	/**
+	 * Commit the transaction's changes.
+	 *
+	 * @public
+	 */
+	commit() {
+		if (this.isRoot) {
+			// For root transactions, flush changes to each of the atom's initial values.
+			const atoms = this.initialAtomValues
+			this.initialAtomValues = new Map()
+			flushChanges(atoms.keys())
+		} else {
+			// For transaction's with parents, add the transaction's initial values to the parent's.
+			this.initialAtomValues.forEach((value, atom) => {
+				if (!this.parent!.initialAtomValues.has(atom)) {
+					this.parent!.initialAtomValues.set(atom, value)
+				}
+			})
+		}
+	}
+
+	/**
+	 * Abort the transaction.
+	 *
+	 * @public
+	 */
+	abort() {
+		globalEpoch++
+
+		// Reset each of the transaction's atoms to its initial value.
+		this.initialAtomValues.forEach((value, atom) => {
+			atom.set(value)
+			atom.historyBuffer?.clear()
+		})
+
+		// Commit the changes.
+		this.commit()
+	}
+}
+
+/**
+ * Collect all of the reactors that need to run for an atom and run them.
+ *
+ * @param atom The atom to flush changes for.
+ */
+function flushChanges(atoms: Iterable<_Atom<any>>) {
+	if (globalIsReacting) {
+		throw new Error('cannot change atoms during reaction cycle')
+	}
+
+	try {
+		globalIsReacting = true
+
+		// Collect all of the visited reactors.
+		const reactors = new Set<EffectScheduler<unknown>>()
+
+		// Visit each descendant of the atom, collecting reactors.
+		const traverse = (node: Child) => {
+			if (node.lastTraversedEpoch === globalEpoch) {
+				return
+			}
+
+			node.lastTraversedEpoch = globalEpoch
+
+			if ('maybeScheduleEffect' in node) {
+				reactors.add(node)
+			} else {
+				;(node as any as Signal<any>).children.visit(traverse)
+			}
+		}
+
+		for (const atom of atoms) {
+			atom.children.visit(traverse)
+		}
+
+		// Run each reactor.
+		for (const r of reactors) {
+			r.maybeScheduleEffect()
+		}
+	} finally {
+		globalIsReacting = false
+	}
+}
+
+/**
+ * Handle a change to an atom.
+ *
+ * @param atom The atom that changed.
+ * @param previousValue The atom's previous value.
+ *
+ * @internal
+ */
+export function atomDidChange(atom: _Atom<any>, previousValue: any) {
+	if (!currentTransaction) {
+		flushChanges([atom])
+	} else if (!currentTransaction.initialAtomValues.has(atom)) {
+		currentTransaction.initialAtomValues.set(atom, previousValue)
+	}
+}
+
+/**
+ * The current transaction, if there is one.
+ *
+ * @global
+ * @public
+ */
+export let currentTransaction = null as Transaction | null
+
+/**
+ * Batches state updates, deferring side effects until after the transaction completes.
+ *
+ * @example
+ * ```ts
+ * const firstName = atom('John')
+ * const lastName = atom('Doe')
+ *
+ * react('greet', () => {
+ *   console.log(`Hello, ${firstName.value} ${lastName.value}!`)
+ * })
+ *
+ * // Logs "Hello, John Doe!"
+ *
+ * transaction(() => {
+ *  firstName.set('Jane')
+ *  lastName.set('Smith')
+ * })
+ *
+ * // Logs "Hello, Jane Smith!"
+ * ```
+ *
+ * If the function throws, the transaction is aborted and any signals that were updated during the transaction revert to their state before the transaction began.
+ *
+ * @example
+ * ```ts
+ * const firstName = atom('John')
+ * const lastName = atom('Doe')
+ *
+ * react('greet', () => {
+ *   console.log(`Hello, ${firstName.value} ${lastName.value}!`)
+ * })
+ *
+ * // Logs "Hello, John Doe!"
+ *
+ * transaction(() => {
+ *  firstName.set('Jane')
+ *  throw new Error('oops')
+ * })
+ *
+ * // Does not log
+ * // firstName.value === 'John'
+ * ```
+ *
+ * A `rollback` callback is passed into the function.
+ * Calling this will prevent the transaction from committing and will revert any signals that were updated during the transaction to their state before the transaction began.
+ *
+ *  * @example
+ * ```ts
+ * const firstName = atom('John')
+ * const lastName = atom('Doe')
+ *
+ * react('greet', () => {
+ *   console.log(`Hello, ${firstName.value} ${lastName.value}!`)
+ * })
+ *
+ * // Logs "Hello, John Doe!"
+ *
+ * transaction((rollback) => {
+ *  firstName.set('Jane')
+ *  lastName.set('Smith')
+ *  rollback()
+ * })
+ *
+ * // Does not log
+ * // firstName.value === 'John'
+ * // lastName.value === 'Doe'
+ * ```
+ *
+ * @param fn - The function to run in a transaction, called with a function to roll back the change.
+ * @public
+ */
+export function transaction<T>(fn: (rollback: () => void) => T) {
+	const txn = new Transaction(currentTransaction)
+
+	// Set the current transaction to the transaction
+	currentTransaction = txn
+
+	try {
+		let rollback = false
+
+		// Run the function.
+		const result = fn(() => (rollback = true))
+
+		if (rollback) {
+			// If the rollback was triggered, abort the transaction.
+			txn.abort()
+		} else {
+			// Otherwise, commit the transaction.
+			txn.commit()
+		}
+
+		return result
+	} catch (e) {
+		// Abort the transaction if the function throws.
+		txn.abort()
+		throw e
+	} finally {
+		// Set the current transaction to the transaction's parent.
+		currentTransaction = currentTransaction.parent
+	}
+}
+
+/**
+ * Like [transaction](#transaction), but does not create a new transaction if there is already one in progress.
+ *
+ * @param fn - The function to run in a transaction.
+ * @public
+ */
+export function transact<T>(fn: () => T): T {
+	if (currentTransaction) {
+		return fn()
+	}
+	return transaction(fn)
+}

packages/state/src/lib/core/types.ts

@@ -0,0 +1,67 @@
+import { ArraySet } from './ArraySet'
+import { _Computed } from './Computed'
+import { EffectScheduler } from './EffectScheduler'
+
+/** @public */
+export const RESET_VALUE: unique symbol = Symbol('RESET_VALUE')
+
+/** @public */
+export type RESET_VALUE = typeof RESET_VALUE
+
+/**
+ * A Signal is a reactive value container. The value may change over time, and it may keep track of the diffs between sequential values.
+ *
+ * There are two types of signal:
+ *
+ * - Atomic signals, created using [[atom]]. These are mutable references to values that can be changed using [[Atom.set]].
+ * - Computed signals, created using [[computed]]. These are values that are computed from other signals. They are recomputed lazily if their dependencies change.
+ *
+ * @public
+ */
+export interface Signal<Value, Diff = unknown> {
+	/**
+	 * The name of the signal. This is used at runtime for debugging and perf profiling only. It does not need to be globally unique.
+	 */
+	name: string
+	/**
+	 * The current value of the signal. This is a reactive value, and will update when the signal changes.
+	 * Any computed signal that depends on this signal will be lazily recomputed if this signal changes.
+	 * Any effect that depends on this signal will be rescheduled if this signal changes.
+	 */
+	readonly value: Value
+	/**
+	 * The epoch when this signal's value last changed. Note tha this is not the same as when the value was last computed.
+	 * A signal may recopmute it's value without changing it.
+	 */
+	lastChangedEpoch: number
+	/**
+	 * Returns the sequence of diffs between the the value at the given epoch and the current value.
+	 * Returns the [[RESET_VALUE]] constant if there is not enough information to compute the diff sequence.
+	 * @param epoch - The epoch to get diffs since.
+	 */
+	getDiffSince(epoch: number): RESET_VALUE | Diff[]
+	/**
+	 * Returns the current value of the signal without capturing it as a dependency.
+	 * Use this if you need to retrieve the signal's value in a hot loop where the performance overhead of dependency tracking is too high.
+	 */
+	__unsafe__getWithoutCapture(): Value
+	/** @internal */
+	children: ArraySet<Child>
+}
+
+/** @internal */
+export type Child = EffectScheduler<any> | _Computed<any>
+
+/**
+ * Computes the diff between the previous and current value.
+ *
+ * If the diff cannot be computed for whatever reason, it should return [[RESET_VALUE]].
+ *
+ * @public
+ */
+export type ComputeDiff<Value, Diff> = (
+	previousValue: Value,
+	currentValue: Value,
+	lastComputedEpoch: number,
+	currentEpoch: number
+) => Diff | RESET_VALUE

packages/state/src/lib/react/index.ts

@@ -0,0 +1,7 @@
+export { track } from './track'
+export { useAtom } from './useAtom'
+export { useComputed } from './useComputed'
+export { useQuickReactor } from './useQuickReactor'
+export { useReactor } from './useReactor'
+export { useStateTracking } from './useStateTracking'
+export { useValue } from './useValue'

packages/state/src/lib/react/track.test.tsx

@@ -0,0 +1,227 @@
+import { createRef, forwardRef, memo, useEffect, useImperativeHandle } from 'react'
+import { ReactTestRenderer, act, create } from 'react-test-renderer'
+import { atom } from '../core/Atom'
+import { track } from './track'
+
+test("tracked components are memo'd", async () => {
+	let numRenders = 0
+	const Component = track(function Component({ a, b, c }: { a: string; b: string; c: string }) {
+		numRenders++
+		return (
+			<>
+				{a}
+				{b}
+				{c}
+			</>
+		)
+	})
+
+	let view: ReactTestRenderer
+	await act(() => {
+		view = create(<Component a="a" b="b" c="c" />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "a",
+		  "b",
+		  "c",
+		]
+	`)
+
+	expect(numRenders).toBe(1)
+
+	await act(() => {
+		view!.update(<Component a="a" b="b" c="c" />)
+	})
+
+	expect(numRenders).toBe(1)
+
+	await act(() => {
+		view!.update(<Component a="a" b="b" c="d" />)
+	})
+
+	expect(numRenders).toBe(2)
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "a",
+		  "b",
+		  "d",
+		]
+	`)
+})
+
+test("it's fine to call track on components that are already memo'd", async () => {
+	let numRenders = 0
+	const Component = track(
+		memo(function Component({ a, b, c }: { a: string; b: string; c: string }) {
+			numRenders++
+			return (
+				<>
+					{a}
+					{b}
+					{c}
+				</>
+			)
+		})
+	)
+
+	let view: ReactTestRenderer
+	await act(() => {
+		view = create(<Component a="a" b="b" c="c" />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "a",
+		  "b",
+		  "c",
+		]
+	`)
+
+	expect(numRenders).toBe(1)
+
+	await act(() => {
+		view!.update(<Component a="a" b="b" c="c" />)
+	})
+
+	expect(numRenders).toBe(1)
+
+	await act(() => {
+		view!.update(<Component a="a" b="b" c="d" />)
+	})
+
+	expect(numRenders).toBe(2)
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "a",
+		  "b",
+		  "d",
+		]
+	`)
+})
+
+test('tracked components can use refs', async () => {
+	const Component = track(
+		forwardRef<{ handle: string }, { prop: string }>(function Component({ prop }, ref) {
+			useImperativeHandle(ref, () => ({ handle: prop }), [prop])
+			return <>output</>
+		})
+	)
+
+	const ref = createRef<{ handle: string }>()
+
+	let view: ReactTestRenderer
+	await act(() => {
+		view = create(<Component prop="hello" ref={ref} />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot('"output"')
+
+	expect(ref.current?.handle).toBe('hello')
+
+	await act(() => {
+		view.update(<Component prop="world" ref={ref} />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot('"output"')
+
+	expect(ref.current?.handle).toBe('world')
+})
+
+test('tracked components update when the state they refernce updates', async () => {
+	const a = atom('a', 1)
+
+	const C = track(function Component() {
+		return <>{a.value}</>
+	})
+
+	let view: ReactTestRenderer
+
+	await act(() => {
+		view = create(<C />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"1"`)
+
+	await act(() => {
+		a.set(2)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+})
+
+test('things referenced in effects do not trigger updates', async () => {
+	const a = atom('a', 1)
+	let numRenders = 0
+
+	const Component = track(function Component() {
+		numRenders++
+		useEffect(() => {
+			a.value
+		}, [])
+		return <>hi</>
+	})
+
+	let view: ReactTestRenderer
+
+	await act(() => {
+		view = create(<Component />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"hi"`)
+	expect(numRenders).toBe(1)
+
+	await act(() => {
+		a.set(2)
+	})
+
+	expect(numRenders).toBe(1)
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"hi"`)
+})
+
+test("tracked zombie-children don't throw", async () => {
+	const theAtom = atom<Record<string, number>>('map', { a: 1, b: 2, c: 3 })
+	const Parent = track(function Parent() {
+		const ids = Object.keys(theAtom.value)
+		return (
+			<>
+				{ids.map((id) => (
+					<Child key={id} id={id} />
+				))}
+			</>
+		)
+	})
+	const Child = track(function Child({ id }: { id: string }) {
+		if (!(id in theAtom.value)) throw new Error('id not found!')
+		const value = theAtom.value[id]
+		return <>{value}</>
+	})
+
+	let view: ReactTestRenderer
+	await act(() => {
+		view = create(<Parent />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "1",
+		  "2",
+		  "3",
+		]
+	`)
+
+	// remove id 'b' creating a zombie-child
+	await act(() => {
+		theAtom?.update(({ b: _, ...rest }) => rest)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "1",
+		  "3",
+		]
+	`)
+})

packages/state/src/lib/react/track.ts

@@ -0,0 +1,60 @@
+import React, { forwardRef, FunctionComponent, memo } from 'react'
+import { useStateTracking } from './useStateTracking'
+
+export const ProxyHandlers = {
+	/**
+	 * This is a function call trap for functional components. When this is called, we know it means
+	 * React did run 'Component()', that means we can use any hooks here to setup our effect and
+	 * store.
+	 *
+	 * With the native Proxy, all other calls such as access/setting to/of properties will be
+	 * forwarded to the target Component, so we don't need to copy the Component's own or inherited
+	 * properties.
+	 *
+	 * @see https://github.com/facebook/react/blob/2d80a0cd690bb5650b6c8a6c079a87b5dc42bd15/packages/react-reconciler/src/ReactFiberHooks.old.js#L460
+	 */
+	apply(Component: FunctionComponent, thisArg: any, argumentsList: any) {
+		// eslint-disable-next-line react-hooks/rules-of-hooks
+		return useStateTracking(Component.displayName ?? Component.name ?? 'tracked(???)', () =>
+			Component.apply(thisArg, argumentsList)
+		)
+	},
+}
+
+export const ReactMemoSymbol = Symbol.for('react.memo')
+export const ReactForwardRefSymbol = Symbol.for('react.forward_ref')
+
+/**
+ * Returns a tracked version of the given component.
+ * Any signals whose values are read while the component renders will be tracked.
+ * If any of the tracked signals change later it will cause the component to re-render.
+ *
+ * This also wraps the component in a React.memo() call, so it will only re-render if the props change.
+ *
+ * @example
+ * ```ts
+ * const Counter = track(function Counter(props: CounterProps) {
+ *   const count = useAtom('count', 0)
+ *   const increment = useCallback(() => count.set(count.value + 1), [count])
+ *   return <button onClick={increment}>{count.value}</button>
+ * })
+ * ```
+ *
+ * @param baseComponent - The base component to track.
+ * @public
+ */
+export function track<T extends FunctionComponent<any>>(
+	baseComponent: T
+): T extends React.MemoExoticComponent<any> ? T : React.MemoExoticComponent<T> {
+	let compare = null
+	const $$typeof = baseComponent['$$typeof' as keyof typeof baseComponent]
+	if ($$typeof === ReactMemoSymbol) {
+		baseComponent = (baseComponent as any).type
+		compare = (baseComponent as any).compare
+	}
+	if ($$typeof === ReactForwardRefSymbol) {
+		return memo(forwardRef(new Proxy((baseComponent as any).render, ProxyHandlers) as any)) as any
+	}
+
+	return memo(new Proxy(baseComponent, ProxyHandlers) as any, compare) as any
+}

packages/state/src/lib/react/useAtom.test.tsx

@@ -0,0 +1,51 @@
+import ReactTestRenderer from 'react-test-renderer'
+import { Atom } from '../core/Atom'
+import { useAtom } from './useAtom'
+import { useValue } from './useValue'
+
+test('useAtom returns an atom', async () => {
+	let theAtom: null | Atom<any> = null as any
+	function Component() {
+		const a = useAtom('myAtom', 'a')
+		theAtom = a
+		return <>{useValue(a)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(theAtom).not.toBeNull()
+	expect(theAtom?.value).toBe('a')
+	expect(theAtom?.name).toBe('useAtom(myAtom)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"a"`)
+
+	// it doesn't create a new atom on re-render
+	const a = theAtom!
+	await ReactTestRenderer.act(() => {
+		theAtom?.set('b')
+	})
+	expect(a).toBe(theAtom)
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"b"`)
+})
+
+test('useAtom supports taking an initializer', async () => {
+	let theAtom: null | Atom<any> = null as any
+	function Component() {
+		const a = useAtom('myAtom', () => 'a')
+		theAtom = a
+		return <>{useValue(a)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(theAtom).not.toBeNull()
+	expect(theAtom?.value).toBe('a')
+
+	expect(theAtom?.name).toBe('useAtom(myAtom)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"a"`)
+})

packages/state/src/lib/react/useAtom.ts

@@ -0,0 +1,40 @@
+import { useState } from 'react'
+import { Atom, AtomOptions, atom } from '../core'
+
+/**
+ * Creates a new atom and returns it. The atom will be created only once.
+ *
+ * See [[atom]]
+ *
+ * @example
+ * ```ts
+ * const Counter = track(function Counter () {
+ *   const count = useAtom('count', 0)
+ *   const increment = useCallback(() => count.set(count.value + 1), [count])
+ *   return <button onClick={increment}>{count.value}</button>
+ * })
+ * ```
+ *
+ * @public
+ */
+export function useAtom<Value, Diff = unknown>(
+	/**
+	 * The name of the atom. This does not need to be globally unique. It is used for debugging and performance profiling.
+	 */
+	name: string,
+	/**
+	 * The initial value of the atom. If this is a function, it will be called to get the initial value.
+	 */
+	valueOrInitialiser: Value | (() => Value),
+	/**
+	 * Options for the atom.
+	 */
+	options?: AtomOptions<Value, Diff>
+): Atom<Value, Diff> {
+	return useState(() => {
+		const initialValue =
+			typeof valueOrInitialiser === 'function' ? (valueOrInitialiser as any)() : valueOrInitialiser
+
+		return atom(`useAtom(${name})`, initialValue, options)
+	})[0]
+}

packages/state/src/lib/react/useComputed.test.tsx

@@ -0,0 +1,117 @@
+import { useState } from 'react'
+import ReactTestRenderer from 'react-test-renderer'
+import { Atom } from '../core/Atom'
+import { Computed } from '../core/Computed'
+import { useAtom } from './useAtom'
+import { useComputed } from './useComputed'
+import { useValue } from './useValue'
+
+test('useComputed returns a computed value', async () => {
+	let theComputed = null as null | Computed<number>
+	let theAtom = null as null | Atom<number>
+	function Component() {
+		const a = useAtom('a', 1)
+		theAtom = a
+		const b = useComputed('a+1', () => a.value + 1, [])
+		theComputed = b
+		return <>{useValue(b)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(theComputed).not.toBeNull()
+	expect(theComputed?.value).toBe(2)
+	expect(theComputed?.name).toBe('useComputed(a+1)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"6"`)
+})
+
+test('useComputed has a dependencies array that allows creating a new computed', async () => {
+	let theComputed = null as null | Computed<number>
+	let theAtom = null as null | Atom<number>
+	let setCount = null as null | ((count: number) => void)
+	function Component() {
+		const [count, _setCount] = useState(0)
+		setCount = _setCount
+		const a = useAtom('a', 1)
+		theAtom = a
+		const b = useComputed('a+1', () => a.value + 1, [count])
+		theComputed = b
+		return <>{useValue(b)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	const initialComputed = theComputed
+
+	expect(theComputed).not.toBeNull()
+	expect(theComputed?.value).toBe(2)
+	expect(theComputed?.name).toBe('useComputed(a+1)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"6"`)
+
+	expect(initialComputed).toBe(theComputed)
+
+	await ReactTestRenderer.act(() => {
+		setCount?.(2)
+	})
+
+	expect(initialComputed).not.toBe(theComputed)
+})
+
+test('useComputed allows optionally passing options', async () => {
+	let theComputed = null as null | Computed<number>
+	let theAtom = null as null | Atom<number>
+	let setCount = null as null | ((count: number) => void)
+	const isEqual = jest.fn((a, b) => a === b)
+	function Component() {
+		const [count, _setCount] = useState(0)
+		setCount = _setCount
+		const a = useAtom('a', 1)
+		theAtom = a
+		const b = useComputed('a+1', () => a.value + 1, { isEqual }, [count])
+		theComputed = b
+		return <>{useValue(b)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	const initialComputed = theComputed
+
+	expect(theComputed).not.toBeNull()
+	expect(theComputed?.value).toBe(2)
+	expect(theComputed?.name).toBe('useComputed(a+1)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"6"`)
+
+	expect(initialComputed).toBe(theComputed)
+
+	await ReactTestRenderer.act(() => {
+		setCount?.(2)
+	})
+
+	expect(initialComputed).not.toBe(theComputed)
+
+	expect(isEqual).toHaveBeenCalled()
+})

packages/state/src/lib/react/useComputed.ts

@@ -0,0 +1,41 @@
+/* eslint-disable prefer-rest-params */
+import { useMemo } from 'react'
+import { Computed, ComputedOptions, computed } from '../core'
+
+/**
+ * Creates a new computed signal and returns it. The computed signal will be created only once.
+ *
+ * See [[computed]]
+ *
+ * @example
+ * ```ts
+ * type GreeterProps = {
+ *   firstName: Signal<string>
+ *   lastName: Signal<string>
+ * }
+ *
+ * const Greeter = track(function Greeter ({firstName, lastName}: GreeterProps) {
+ *   const fullName = useComputed('fullName', () => `${firstName.value} ${lastName.value}`)
+ *   return <div>Hello {fullName.value}!</div>
+ * })
+ * ```
+ *
+ * @public
+ */
+export function useComputed<Value>(name: string, compute: () => Value, deps: any[]): Computed<Value>
+/** @public */
+export function useComputed<Value, Diff = unknown>(
+	name: string,
+	compute: () => Value,
+	opts: ComputedOptions<Value, Diff>,
+	deps: any[]
+): Computed<Value>
+/** @public */
+export function useComputed() {
+	const name = arguments[0]
+	const compute = arguments[1]
+	const opts = arguments.length === 3 ? undefined : arguments[2]
+	const deps = arguments.length === 3 ? arguments[2] : arguments[3]
+	// eslint-disable-next-line react-hooks/exhaustive-deps
+	return useMemo(() => computed(`useComputed(${name})`, compute, opts), deps)
+}

packages/state/src/lib/react/useQuickReactor.ts

@@ -1,5 +1,5 @@
 import { useEffect } from 'react'
-import { EffectScheduler, EMPTY_ARRAY } from 'signia'
+import { EMPTY_ARRAY, EffectScheduler } from '../core'
 
 /** @public */
 export function useQuickReactor(name: string, reactFn: () => void, deps: any[] = EMPTY_ARRAY) {

packages/state/src/lib/react/useReactor.ts

@@ -1,5 +1,5 @@
 import { useEffect, useMemo } from 'react'
-import { EffectScheduler } from 'signia'
+import { EffectScheduler } from '../core'
 
 /** @public */
 export function useReactor(name: string, reactFn: () => void, deps: undefined | any[] = []) {

packages/state/src/lib/react/useStateTracking.test.tsx

@@ -0,0 +1,203 @@
+import * as React from 'react'
+import { act, create, ReactTestRenderer } from 'react-test-renderer'
+import { atom } from '../core/Atom'
+import { useStateTracking } from './useStateTracking'
+
+describe('useStateTracking', () => {
+	it('causes a rerender when a dependency changes', async () => {
+		const a = atom('', 0)
+
+		const Component = () => {
+			const val = useStateTracking('', () => {
+				return a.value
+			})
+			return <>You are {val} years old</>
+		}
+
+		let view: ReactTestRenderer
+		await act(() => {
+			view = create(<Component />)
+		})
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "0",
+		  " years old",
+		]
+	`)
+
+		act(() => {
+			a.set(1)
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "1",
+		  " years old",
+		]
+	`)
+	})
+
+	it('allows using hooks inside the callback', async () => {
+		const _age = atom('', 0)
+		let setHeight: (height: number) => void
+
+		const Component = () => {
+			let height
+			const age = useStateTracking('', () => {
+				// eslint-disable-next-line react-hooks/rules-of-hooks
+				;[height, setHeight] = React.useState(20)
+				return _age.value
+			})
+			return (
+				<>
+					You are {age} years old and {height} meters tall
+				</>
+			)
+		}
+
+		let view: ReactTestRenderer
+		await act(() => {
+			view = create(<Component />)
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "0",
+		  " years old and ",
+		  "20",
+		  " meters tall",
+		]
+	`)
+
+		act(() => {
+			_age.set(1)
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "1",
+		  " years old and ",
+		  "20",
+		  " meters tall",
+		]
+	`)
+
+		act(() => {
+			setHeight(21)
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "1",
+		  " years old and ",
+		  "21",
+		  " meters tall",
+		]
+	`)
+	})
+
+	it('allows throwing promises to trigger suspense boundaries', async () => {
+		const a = atom<null | number>('age', null)
+
+		let resolve = (_val: string) => {
+			// noop
+		}
+
+		const Component = () => {
+			const val = useStateTracking('', () => {
+				if (a.value === null) {
+					throw new Promise<string>((r) => {
+						resolve = r
+					})
+				}
+				return a.value
+			})
+			return <>You are {val} years old</>
+		}
+
+		let view: ReactTestRenderer = null as any
+		await act(() => {
+			view = create(
+				<React.Suspense fallback={<>fallback</>}>
+					<Component />
+				</React.Suspense>
+			)
+		})
+
+		expect(view.toJSON()).toMatchInlineSnapshot(`"fallback"`)
+
+		await act(() => {
+			a.set(1)
+		})
+		// merely setting the value won't trigger a rerender, the promise must resolve
+		expect(view.toJSON()).toMatchInlineSnapshot(`"fallback"`)
+
+		await act(() => {
+			resolve('resolved')
+		})
+
+		expect(view.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "1",
+		  " years old",
+		]
+	`)
+	})
+
+	it('stops reacting when the component unmounts', async () => {
+		const a = atom('', 0)
+		let numRenders = 0
+		const Component = () => {
+			const val = useStateTracking('', () => {
+				numRenders++
+				return a.value
+			})
+			return <>You are {val} years old</>
+		}
+
+		let view: ReactTestRenderer
+		await act(() => {
+			view = create(React.createElement(Component))
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "0",
+		  " years old",
+		]
+	`)
+
+		expect(numRenders).toBe(1)
+
+		await act(() => {
+			a.set(1)
+		})
+
+		expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "You are ",
+		  "1",
+		  " years old",
+		]
+	`)
+
+		expect(numRenders).toBe(2)
+
+		await act(() => {
+			view!.unmount()
+		})
+
+		await act(() => {
+			a.set(2)
+		})
+
+		expect(numRenders).toBe(2)
+	})
+})

packages/state/src/lib/react/useStateTracking.ts

@@ -0,0 +1,58 @@
+import React from 'react'
+import { EffectScheduler } from '../core'
+
+/** @internal */
+export function useStateTracking<T>(name: string, render: () => T): T {
+	// user render is only called at the bottom of this function, indirectly via scheduler.execute()
+	// we need it to always be up-to-date when calling scheduler.execute() but it'd be wasteful to
+	// instantiate a new EffectScheduler on every render, so we use an immediately-updated ref
+	// to wrap it
+	const renderRef = React.useRef(render)
+	renderRef.current = render
+
+	const [scheduler, subscribe, getSnapshot] = React.useMemo(() => {
+		let scheduleUpdate = null as null | (() => void)
+		// useSyncExternalStore requires a subscribe function that returns an unsubscribe function
+		const subscribe = (cb: () => void) => {
+			scheduleUpdate = cb
+			return () => {
+				scheduleUpdate = null
+			}
+		}
+
+		const scheduler = new EffectScheduler(
+			`useStateTracking(${name})`,
+			// this is what `scheduler.execute()` will call
+			() => renderRef.current?.(),
+			// this is what will be invoked when @tldraw/state detects a change in an upstream reactive value
+			{
+				scheduleEffect() {
+					scheduleUpdate?.()
+				},
+			}
+		)
+
+		// we use an incrementing number based on when this
+		const getSnapshot = () => scheduler.scheduleCount
+
+		return [scheduler, subscribe, getSnapshot]
+	}, [name])
+
+	React.useSyncExternalStore(subscribe, getSnapshot, getSnapshot)
+
+	// reactive dependencies are captured when `scheduler.execute()` is called
+	// and then to make it reactive we wait for a `useEffect` to 'attach'
+	// this allows us to avoid rendering outside of React's render phase
+	// and avoid 'zombie' components that try to render with bad/deleted data before
+	// react has a chance to umount them.
+	React.useEffect(() => {
+		scheduler.attach()
+		// do not execute, we only do that in render
+		scheduler.maybeScheduleEffect()
+		return () => {
+			scheduler.detach()
+		}
+	}, [scheduler])
+
+	return scheduler.execute()
+}

packages/state/src/lib/react/useValue.test.tsx

@@ -0,0 +1,135 @@
+import { useState } from 'react'
+import ReactTestRenderer from 'react-test-renderer'
+import { Atom, atom } from '../core/Atom'
+import { Computed } from '../core/Computed'
+import { useAtom } from './useAtom'
+import { useComputed } from './useComputed'
+import { useValue } from './useValue'
+
+test('useValue returns a value from a computed', async () => {
+	let theComputed = null as null | Computed<number>
+	let theAtom = null as null | Atom<number>
+	function Component() {
+		const a = useAtom('a', 1)
+		theAtom = a
+		const b = useComputed('a+1', () => a.value + 1, [])
+		theComputed = b
+		return <>{useValue(b)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(theComputed).not.toBeNull()
+	expect(theComputed?.value).toBe(2)
+	expect(theComputed?.name).toBe('useComputed(a+1)')
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"6"`)
+})
+
+test('useValue returns a value from an atom', async () => {
+	let theAtom = null as null | Atom<number>
+	function Component() {
+		const a = useAtom('a', 1)
+		theAtom = a
+		return <>{useValue(a)}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"1"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"5"`)
+})
+
+test('useValue returns a value from a compute function', async () => {
+	let theAtom = null as null | Atom<number>
+	let setB = null as null | ((b: number) => void)
+	function Component() {
+		const a = useAtom('a', 1)
+		const [b, _setB] = useState(1)
+		setB = _setB
+		theAtom = a
+		const c = useValue('a+b', () => a.value + b, [b])
+		return <>{c}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Component />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"2"`)
+
+	await ReactTestRenderer.act(() => {
+		theAtom?.set(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"6"`)
+
+	await ReactTestRenderer.act(() => {
+		setB!(5)
+	})
+	expect(view!.toJSON()).toMatchInlineSnapshot(`"10"`)
+})
+
+test("useValue doesn't throw when used in a zombie-child component", async () => {
+	const theAtom = atom<Record<string, number>>('map', { a: 1, b: 2, c: 3 })
+	function Parent() {
+		const ids = useValue('ids', () => Object.keys(theAtom.value), [])
+		return (
+			<>
+				{ids.map((id) => (
+					<Child key={id} id={id} />
+				))}
+			</>
+		)
+	}
+	function Child({ id }: { id: string }) {
+		const value = useValue(
+			'value',
+			() => {
+				if (!(id in theAtom.value)) throw new Error('id not found!')
+				return theAtom.value[id]
+			},
+			[id]
+		)
+		return <>{value}</>
+	}
+
+	let view: ReactTestRenderer.ReactTestRenderer
+	await ReactTestRenderer.act(() => {
+		view = ReactTestRenderer.create(<Parent />)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "1",
+		  "2",
+		  "3",
+		]
+	`)
+
+	// remove id 'b' creating a zombie-child
+	await ReactTestRenderer.act(() => {
+		theAtom?.update(({ b: _, ...rest }) => rest)
+	})
+
+	expect(view!.toJSON()).toMatchInlineSnapshot(`
+		Array [
+		  "1",
+		  "3",
+		]
+	`)
+})

packages/state/src/lib/react/useValue.ts

@@ -0,0 +1,98 @@
+/* eslint-disable prefer-rest-params */
+import { useMemo, useRef, useSyncExternalStore } from 'react'
+import { Signal, computed, react } from '../core'
+
+/**
+ * Extracts the value from a signal and subscribes to it.
+ *
+ * Note that you do not need to use this hook if you are wrapping the component with [[track]]
+ *
+ * @example
+ * ```ts
+ * const Counter: React.FC = () => {
+ *   const $count = useAtom('count', 0)
+ *   const increment = useCallback(() => $count.set($count.value + 1), [count])
+ *   const currentCount = useValue($count)
+ *   return <button onClick={increment}>{currentCount}</button>
+ * }
+ * ```
+ *
+ * You can also pass a function to compute the value and it will be memoized as in [[useComputed]]:
+ *
+ * @example
+ * ```ts
+ * type GreeterProps = {
+ *   firstName: Signal<string>
+ *   lastName: Signal<string>
+ * }
+ *
+ * const Greeter = track(function Greeter({ firstName, lastName }: GreeterProps) {
+ *   const fullName = useValue('fullName', () => `${firstName.value} ${lastName.value}`, [
+ *     firstName,
+ *     lastName,
+ *   ])
+ *   return <div>Hello {fullName}!</div>
+ * })
+ * ```
+ *
+ * @public
+ */
+export function useValue<Value>(value: Signal<Value>): Value
+/** @public */
+export function useValue<Value>(name: string, fn: () => Value, deps: unknown[]): Value
+/** @public */
+export function useValue() {
+	const args = arguments
+	// deps will be either the computed or the deps array
+	const deps = args.length === 3 ? args[2] : [args[0]]
+	const name = args.length === 3 ? args[0] : `useValue(${args[0].name})`
+
+	const isInRender = useRef(true)
+	isInRender.current = true
+
+	const $val = useMemo(() => {
+		if (args.length === 1) {
+			return args[0]
+		}
+		return computed(name, () => {
+			if (isInRender.current) {
+				return args[1]()
+			} else {
+				try {
+					return args[1]()
+				} catch {
+					// when getSnapshot is called outside of the render phase &
+					// subsequently throws an error, it might be because we're
+					// in a zombie-child state. in that case, we suppress the
+					// error and instead return a new dummy value to trigger a
+					// react re-render. if we were in a zombie child, react will
+					// unmount us instead of re-rendering so the error is
+					// irrelevant. if we're not in a zombie-child, react will
+					// call `getSnapshot` again in the render phase, and the
+					// error will be thrown as expected.å
+					return {}
+				}
+			}
+		})
+		// eslint-disable-next-line react-hooks/exhaustive-deps
+	}, deps)
+
+	try {
+		const { subscribe, getSnapshot } = useMemo(() => {
+			return {
+				subscribe: (listen: () => void) => {
+					return react(`useValue(${name})`, () => {
+						$val.value
+						listen()
+					})
+				},
+				getSnapshot: () => $val.value,
+			}
+			// eslint-disable-next-line react-hooks/exhaustive-deps
+		}, [$val])
+
+		return useSyncExternalStore(subscribe, getSnapshot, getSnapshot)
+	} finally {
+		isInRender.current = false
+	}
+}

packages/state/tsconfig.json

@@ -0,0 +1,10 @@
+{
+	"extends": "../../config/tsconfig.base.json",
+	"include": ["src"],
+	"exclude": ["node_modules", "dist", ".tsbuild*"],
+	"compilerOptions": {
+		"outDir": "./.tsbuild",
+		"rootDir": "src"
+	},
+	"references": [{ "path": "../utils" }]
+}

packages/store/api-report.md

@@ -4,8 +4,8 @@
 
 ```ts
 
-import { Atom } from 'signia';
-import { Computed } from 'signia';
+import { Atom } from '@tldraw/state';
+import { Computed } from '@tldraw/state';
 
 // @public
 export type AllRecords<T extends Store<any>> = ExtractR<ExtractRecordType<T>>;

packages/store/package.json

@@ -41,13 +41,11 @@
 		"lint": "yarn run -T tsx ../../scripts/lint.ts"
 	},
 	"dependencies": {
+		"@tldraw/state": "workspace:*",
 		"@tldraw/utils": "workspace:*",
 		"lodash.isequal": "^4.5.0",
 		"nanoid": "4.0.2"
 	},
-	"peerDependencies": {
-		"signia": "*"
-	},
 	"devDependencies": {
 		"@peculiar/webcrypto": "^1.4.0",
 		"@types/lodash.isequal": "^4.5.6",

packages/store/src/lib/Store.ts

@@ -1,3 +1,4 @@
+import { Atom, Computed, Reactor, atom, computed, reactor, transact } from '@tldraw/state'
 import {
 	filterEntries,
 	objectMapEntries,
@@ -7,7 +8,6 @@ import {
 	throttledRaf,
 } from '@tldraw/utils'
 import { nanoid } from 'nanoid'
-import { Atom, Computed, Reactor, atom, computed, reactor, transact } from 'signia'
 import { IdOf, RecordId, UnknownRecord } from './BaseRecord'
 import { Cache } from './Cache'
 import { RecordScope } from './RecordType'

packages/store/src/lib/StoreQueries.ts

@@ -1,5 +1,3 @@
-import { objectMapValues } from '@tldraw/utils'
-import isEqual from 'lodash.isequal'
 import {
 	Atom,
 	computed,
@@ -8,7 +6,9 @@ import {
 	isUninitialized,
 	RESET_VALUE,
 	withDiff,
-} from 'signia'
+} from '@tldraw/state'
+import { objectMapValues } from '@tldraw/utils'
+import isEqual from 'lodash.isequal'
 import { IdOf, UnknownRecord } from './BaseRecord'
 import { executeQuery, objectMatchesQuery, QueryExpression } from './executeQuery'
 import { IncrementalSetConstructor } from './IncrementalSetConstructor'

packages/store/src/lib/test/recordStore.test.ts

@@ -1,4 +1,4 @@
-import { Computed, react, RESET_VALUE, transact } from 'signia'
+import { Computed, react, RESET_VALUE, transact } from '@tldraw/state'
 import { BaseRecord, RecordId } from '../BaseRecord'
 import { createRecordType } from '../RecordType'
 import { CollectionDiff, RecordsDiff, Store } from '../Store'

packages/store/src/lib/test/recordStoreFuzzing.test.ts

@@ -1,4 +1,4 @@
-import { atom, EffectScheduler, RESET_VALUE } from 'signia'
+import { atom, EffectScheduler, RESET_VALUE } from '@tldraw/state'
 import { BaseRecord, IdOf, RecordId, UnknownRecord } from '../BaseRecord'
 import { executeQuery } from '../executeQuery'
 import { createRecordType } from '../RecordType'

packages/store/src/lib/test/recordStoreQueries.test.ts

@@ -1,4 +1,4 @@
-import { atom, RESET_VALUE } from 'signia'
+import { atom, RESET_VALUE } from '@tldraw/state'
 import { BaseRecord, RecordId } from '../BaseRecord'
 import { createRecordType } from '../RecordType'
 import { Store } from '../Store'

packages/store/tsconfig.json

@@ -6,5 +6,5 @@
 		"outDir": "./.tsbuild",
 		"rootDir": "src"
 	},
-	"references": [{ "path": "../utils" }]
+	"references": [{ "path": "../utils" }, { "path": "../state" }]
 }

packages/tlschema/api-report.md

@@ -9,7 +9,7 @@ import { Expand } from '@tldraw/utils';
 import { Migrations } from '@tldraw/store';
 import { RecordId } from '@tldraw/store';
 import { RecordType } from '@tldraw/store';
-import { Signal } from 'signia';
+import { Signal } from '@tldraw/state';
 import { Store } from '@tldraw/store';
 import { StoreSchema } from '@tldraw/store';
 import { StoreSnapshot } from '@tldraw/store';

packages/tlschema/package.json

@@ -56,12 +56,10 @@
 		]
 	},
 	"dependencies": {
+		"@tldraw/state": "workspace:*",
 		"@tldraw/store": "workspace:*",
 		"@tldraw/utils": "workspace:*",
 		"@tldraw/validate": "workspace:*",
 		"nanoid": "4.0.2"
-	},
-	"peerDependencies": {
-		"signia": "*"
 	}
 }