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51 lines
2.2 KiB
51 lines
2.2 KiB
2 years ago
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import { Observable } from '../Observable';
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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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* <img src="./img/count.png" width="100%">
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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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* @example <caption>Counts how many seconds have passed before the first click happened</caption>
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* var seconds = Rx.Observable.interval(1000);
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* var clicks = Rx.Observable.fromEvent(document, 'click');
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* var secondsBeforeClick = seconds.takeUntil(clicks);
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* var result = secondsBeforeClick.count();
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* result.subscribe(x => console.log(x));
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*
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* @example <caption>Counts how many odd numbers are there between 1 and 7</caption>
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* var numbers = Rx.Observable.range(1, 7);
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* var result = numbers.count(i => i % 2 === 1);
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* result.subscribe(x => console.log(x));
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*
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* // Results in:
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* // 4
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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>(this: Observable<T>, predicate?: (value: T, index: number, source: Observable<T>) => boolean): Observable<number>;
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