@reactivex/rxjs/src/operator/concatMapTo.ts

Reactive Extensions for modern JavaScript

import {Observable, ObservableInput} from '../Observable';
import {MergeMapToOperator} from './mergeMapTo';

/**
 * Projects each source value to the same Observable which is merged multiple
 * times in a serialized fashion on the output Observable.
 *
 * <span class="informal">It's like {@link concatMap}, but maps each value
 * always to the same inner Observable.</span>
 *
 * <img src="./img/concatMapTo.png" width="100%">
 *
 * Maps each source value to the given Observable `innerObservable` regardless
 * of the source value, and then flattens those resulting Observables into one
 * single Observable, which is the output Observable. Each new `innerObservable`
 * instance emitted on the output Observable is concatenated with the previous
 * `innerObservable` instance.
 *
 * __Warning:__ if source values arrive endlessly and faster than their
 * corresponding inner Observables can complete, it will result in memory issues
 * as inner Observables amass in an unbounded buffer waiting for their turn to
 * be subscribed to.
 *
 * Note: `concatMapTo` is equivalent to `mergeMapTo` with concurrency parameter
 * set to `1`.
 *
 * @example <caption>For each click event, tick every second from 0 to 3, with no concurrency</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var result = clicks.concatMapTo(Rx.Observable.interval(1000).take(4));
 * result.subscribe(x => console.log(x));
 *
 * @see {@link concat}
 * @see {@link concatAll}
 * @see {@link concatMap}
 * @see {@link mergeMapTo}
 * @see {@link switchMapTo}
 *
 * @param {Observable} innerObservable An Observable to replace each value from
 * the source Observable.
 * @param {function(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number): any} [resultSelector]
 * A function to produce the value on the output Observable based on the values
 * and the indices of the source (outer) emission and the inner Observable
 * emission. The arguments passed to this function are:
 * - `outerValue`: the value that came from the source
 * - `innerValue`: the value that came from the projected Observable
 * - `outerIndex`: the "index" of the value that came from the source
 * - `innerIndex`: the "index" of the value from the projected Observable
 * @return {Observable} An observable of values merged together by joining the
 * passed observable with itself, one after the other, for each value emitted
 * from the source.
 * @method concatMapTo
 * @owner Observable
 */
export function concatMapTo<T, I, R>(innerObservable: Observable<I>,
                                     resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R): Observable<R> {
  return this.lift(new MergeMapToOperator(innerObservable, resultSelector, 1));
}

export interface ConcatMapToSignature<T> {
  <R>(observable: ObservableInput<R>): Observable<R>;
  <I, R>(observable: ObservableInput<I>,
         resultSelector: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R): Observable<R>;
}