62 lines
2.4 KiB
TypeScript
62 lines
2.4 KiB
TypeScript
import { Observable } from '../Observable';
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import { OperatorFunction } from '../types';
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/**
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* Counts the number of emissions on the source and emits that number when the
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* source completes.
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*
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* <span class="informal">Tells how many values were emitted, when the source
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* completes.</span>
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*
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* 
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*
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* `count` transforms an Observable that emits values into an Observable that
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* emits a single value that represents the number of values emitted by the
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* source Observable. If the source Observable terminates with an error, `count`
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* will pass this error notification along without emitting a value first. If
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* the source Observable does not terminate at all, `count` will neither emit
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* a value nor terminate. This operator takes an optional `predicate` function
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* as argument, in which case the output emission will represent the number of
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* source values that matched `true` with the `predicate`.
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*
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* ## Examples
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*
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* Counts how many seconds have passed before the first click happened
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* ```ts
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* import { fromEvent, interval } from 'rxjs';
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* import { count, takeUntil } from 'rxjs/operators';
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*
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* const seconds = interval(1000);
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* const clicks = fromEvent(document, 'click');
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* const secondsBeforeClick = seconds.pipe(takeUntil(clicks));
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* const result = secondsBeforeClick.pipe(count());
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* result.subscribe(x => console.log(x));
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* ```
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*
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* Counts how many odd numbers are there between 1 and 7
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* ```ts
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* import { range } from 'rxjs';
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* import { count } from 'rxjs/operators';
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*
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* const numbers = range(1, 7);
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* const result = numbers.pipe(count(i => i % 2 === 1));
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* result.subscribe(x => console.log(x));
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* // Results in:
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* // 4
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* ```
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*
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* @see {@link max}
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* @see {@link min}
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* @see {@link reduce}
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*
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* @param {function(value: T, i: number, source: Observable<T>): boolean} [predicate] A
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* boolean function to select what values are to be counted. It is provided with
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* arguments of:
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* - `value`: the value from the source Observable.
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* - `index`: the (zero-based) "index" of the value from the source Observable.
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* - `source`: the source Observable instance itself.
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* @return {Observable} An Observable of one number that represents the count as
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* described above.
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* @method count
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* @owner Observable
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*/
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export declare function count<T>(predicate?: (value: T, index: number, source: Observable<T>) => boolean): OperatorFunction<T, number>;
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