142 lines
4.9 KiB
JavaScript
142 lines
4.9 KiB
JavaScript
import { Subscriber } from '../Subscriber';
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import { EmptyError } from '../util/EmptyError';
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/**
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* Emits only the first value (or the first value that meets some condition)
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* emitted by the source Observable.
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*
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* <span class="informal">Emits only the first value. Or emits only the first
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* value that passes some test.</span>
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*
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* <img src="./img/first.png" width="100%">
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*
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* If called with no arguments, `first` emits the first value of the source
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* Observable, then completes. If called with a `predicate` function, `first`
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* emits the first value of the source that matches the specified condition. It
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* may also take a `resultSelector` function to produce the output value from
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* the input value, and a `defaultValue` to emit in case the source completes
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* before it is able to emit a valid value. Throws an error if `defaultValue`
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* was not provided and a matching element is not found.
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*
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* @example <caption>Emit only the first click that happens on the DOM</caption>
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* var clicks = Rx.Observable.fromEvent(document, 'click');
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* var result = clicks.first();
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* result.subscribe(x => console.log(x));
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*
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* @example <caption>Emits the first click that happens on a DIV</caption>
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* var clicks = Rx.Observable.fromEvent(document, 'click');
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* var result = clicks.first(ev => ev.target.tagName === 'DIV');
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* result.subscribe(x => console.log(x));
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*
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* @see {@link filter}
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* @see {@link find}
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* @see {@link take}
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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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*
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* @param {function(value: T, index: number, source: Observable<T>): boolean} [predicate]
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* An optional function called with each item to test for condition matching.
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* @param {function(value: T, index: number): R} [resultSelector] A function to
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* produce the value on the output Observable based on the values
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* and the indices of the source Observable. The arguments passed to this
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* function are:
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* - `value`: the value that was emitted on the source.
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* - `index`: the "index" of the value from the source.
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* @param {R} [defaultValue] The default value emitted in case no valid value
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* was found on the source.
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* @return {Observable<T|R>} An Observable of the first item that matches the
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* condition.
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* @method first
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* @owner Observable
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*/
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export function first(predicate, resultSelector, defaultValue) {
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return (source) => source.lift(new FirstOperator(predicate, resultSelector, defaultValue, source));
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}
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class FirstOperator {
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constructor(predicate, resultSelector, defaultValue, source) {
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this.predicate = predicate;
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this.resultSelector = resultSelector;
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this.defaultValue = defaultValue;
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this.source = source;
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}
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call(observer, source) {
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return source.subscribe(new FirstSubscriber(observer, this.predicate, this.resultSelector, this.defaultValue, this.source));
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}
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}
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/**
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* We need this JSDoc comment for affecting ESDoc.
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* @ignore
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* @extends {Ignored}
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*/
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class FirstSubscriber extends Subscriber {
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constructor(destination, predicate, resultSelector, defaultValue, source) {
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super(destination);
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this.predicate = predicate;
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this.resultSelector = resultSelector;
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this.defaultValue = defaultValue;
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this.source = source;
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this.index = 0;
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this.hasCompleted = false;
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this._emitted = false;
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}
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_next(value) {
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const index = this.index++;
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if (this.predicate) {
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this._tryPredicate(value, index);
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}
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else {
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this._emit(value, index);
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}
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}
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_tryPredicate(value, index) {
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let result;
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try {
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result = this.predicate(value, index, this.source);
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}
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catch (err) {
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this.destination.error(err);
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return;
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}
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if (result) {
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this._emit(value, index);
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}
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}
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_emit(value, index) {
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if (this.resultSelector) {
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this._tryResultSelector(value, index);
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return;
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}
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this._emitFinal(value);
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}
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_tryResultSelector(value, index) {
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let result;
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try {
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result = this.resultSelector(value, index);
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}
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catch (err) {
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this.destination.error(err);
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return;
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}
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this._emitFinal(result);
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}
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_emitFinal(value) {
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const destination = this.destination;
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if (!this._emitted) {
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this._emitted = true;
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destination.next(value);
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destination.complete();
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this.hasCompleted = true;
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}
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}
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_complete() {
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const destination = this.destination;
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if (!this.hasCompleted && typeof this.defaultValue !== 'undefined') {
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destination.next(this.defaultValue);
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destination.complete();
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}
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else if (!this.hasCompleted) {
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destination.error(new EmptyError);
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}
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}
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}
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//# sourceMappingURL=first.js.map
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