es6/operator/expand.js

import { expand as higherOrder } from '../operators/expand';
/* tslint:enable:max-line-length */
/**
 * Recursively projects each source value to an Observable which is merged in
 * the output Observable.
 *
 * <span class="informal">It's similar to {@link mergeMap}, but applies the
 * projection function to every source value as well as every output value.
 * It's recursive.</span>
 *
 * <img src="./img/expand.png" width="100%">
 *
 * Returns an Observable that emits items based on applying a function that you
 * supply to each item emitted by the source Observable, where that function
 * returns an Observable, and then merging those resulting Observables and
 * emitting the results of this merger. *Expand* will re-emit on the output
 * Observable every source value. Then, each output value is given to the
 * `project` function which returns an inner Observable to be merged on the
 * output Observable. Those output values resulting from the projection are also
 * given to the `project` function to produce new output values. This is how
 * *expand* behaves recursively.
 *
 * @example <caption>Start emitting the powers of two on every click, at most 10 of them</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var powersOfTwo = clicks
 *   .mapTo(1)
 *   .expand(x => Rx.Observable.of(2 * x).delay(1000))
 *   .take(10);
 * powersOfTwo.subscribe(x => console.log(x));
 *
 * @see {@link mergeMap}
 * @see {@link mergeScan}
 *
 * @param {function(value: T, index: number) => Observable} project A function
 * that, when applied to an item emitted by the source or the output Observable,
 * returns an Observable.
 * @param {number} [concurrent=Number.POSITIVE_INFINITY] Maximum number of input
 * Observables being subscribed to concurrently.
 * @param {Scheduler} [scheduler=null] The IScheduler to use for subscribing to
 * each projected inner Observable.
 * @return {Observable} An Observable that emits the source values and also
 * result of applying the projection function to each value emitted on the
 * output Observable and and merging the results of the Observables obtained
 * from this transformation.
 * @method expand
 * @owner Observable
 */
export function expand(project, concurrent = Number.POSITIVE_INFINITY, scheduler = undefined) {
    concurrent = (concurrent || 0) < 1 ? Number.POSITIVE_INFINITY : concurrent;
    return higherOrder(project, concurrent, scheduler)(this);
}
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