factor out the useIsEncrypted hook

src/hooks/useIsEncrypted.ts

@@ -0,0 +1,36 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {useCallback, useState} from "react";
+import {MatrixClient} from "matrix-js-sdk/src/client";
+import {MatrixEvent} from "matrix-js-sdk/src/models/event";
+import {Room} from "matrix-js-sdk/src/models/room";
+
+import {useEventEmitter} from "./useEventEmitter";
+
+// Hook to simplify watching whether a Matrix room is encrypted, returns undefined if room is undefined
+export function useIsEncrypted(cli: MatrixClient, room?: Room): boolean | undefined {
+    const [isEncrypted, setIsEncrypted] = useState(room ? cli.isRoomEncrypted(room.roomId) : undefined);
+
+    const update = useCallback((event: MatrixEvent) => {
+        if (room && event.getType() === "m.room.encryption") {
+            setIsEncrypted(cli.isRoomEncrypted(room.roomId));
+        }
+    }, [cli, room]);
+    useEventEmitter(room ? room.currentState : undefined, "RoomState.events", update);
+
+    return isEncrypted;
+}