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50 lines
1.9 KiB
50 lines
1.9 KiB
4 years ago
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import { Observable } from '../Observable';
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import { MonoTypeOperatorFunction } from '../types';
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/**
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* Returns an Observable that emits the single item emitted by the source Observable that matches a specified
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* predicate, if that Observable emits one such item. If the source Observable emits more than one such item or no
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* items, notify of an IllegalArgumentException or NoSuchElementException respectively. If the source Observable
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* emits items but none match the specified predicate then `undefined` is emitted.
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*
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* <span class="informal">Like {@link first}, but emit with error notification if there is more than one value.</span>
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* 
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*
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* ## Example
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* emits 'error'
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* ```ts
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* import { range } from 'rxjs';
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* import { single } from 'rxjs/operators';
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*
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* const numbers = range(1,5).pipe(single());
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* numbers.subscribe(x => console.log('never get called'), e => console.log('error'));
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* // result
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* // 'error'
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* ```
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*
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* emits 'undefined'
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* ```ts
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* import { range } from 'rxjs';
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* import { single } from 'rxjs/operators';
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*
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* const numbers = range(1,5).pipe(single(x => x === 10));
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* numbers.subscribe(x => console.log(x));
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* // result
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* // 'undefined'
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* ```
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*
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* @see {@link first}
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* @see {@link find}
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* @see {@link findIndex}
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* @see {@link elementAt}
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*
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* @throws {EmptyError} Delivers an EmptyError to the Observer's `error`
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* callback if the Observable completes before any `next` notification was sent.
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* @param {Function} predicate - A predicate function to evaluate items emitted by the source Observable.
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* @return {Observable<T>} An Observable that emits the single item emitted by the source Observable that matches
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* the predicate or `undefined` when no items match.
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*
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* @method single
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* @owner Observable
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*/
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export declare function single<T>(predicate?: (value: T, index: number, source: Observable<T>) => boolean): MonoTypeOperatorFunction<T>;
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