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

Reactive Extensions for modern JavaScript

import {Observable, ObservableInput} from '../Observable';
import {Operator} from '../Operator';
import {Subscriber} from '../Subscriber';
import {Subscription} from '../Subscription';
import {subscribeToResult} from '../util/subscribeToResult';
import {OuterSubscriber} from '../OuterSubscriber';
import {InnerSubscriber} from '../InnerSubscriber';

/**
 * Projects each source value to an Observable which is merged in the output
 * Observable.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables using {@link mergeAll}.</span>
 *
 * <img src="./img/mergeMap.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.
 *
 * @example <caption>Map and flatten each letter to an Observable ticking every 1 second</caption>
 * var letters = Rx.Observable.of('a', 'b', 'c');
 * var result = letters.mergeMap(x =>
 *   Rx.Observable.interval(1000).map(i => x+i)
 * );
 * result.subscribe(x => console.log(x));
 *
 * @see {@link concatMap}
 * @see {@link exhaustMap}
 * @see {@link merge}
 * @see {@link mergeAll}
 * @see {@link mergeMapTo}
 * @see {@link mergeScan}
 * @see {@link switchMap}
 *
 * @param {function(value: T, ?index: number): Observable} project A function
 * that, when applied to an item emitted by the source Observable, returns an
 * 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
 * @param {number} [concurrent=Number.POSITIVE_INFINITY] Maximum number of input
 * Observables being subscribed to concurrently.
 * @return {Observable} An Observable that emits the result of applying the
 * projection function (and the optional `resultSelector`) to each item emitted
 * by the source Observable and merging the results of the Observables obtained
 * from this transformation.
 * @method mergeMap
 * @owner Observable
 */
export function mergeMap<T, I, R>(project: (value: T, index: number) => ObservableInput<I>,
                                  resultSelector?: ((outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R) | number,
                                  concurrent: number = Number.POSITIVE_INFINITY): Observable<R> {
  if (typeof resultSelector === 'number') {
    concurrent = <number>resultSelector;
    resultSelector = null;
  }
  return this.lift(new MergeMapOperator(project, <any>resultSelector, concurrent));
}

export interface MergeMapSignature<T> {
  <R>(project: (value: T, index: number) => ObservableInput<R>, concurrent?: number): Observable<R>;
  <I, R>(project: (value: T, index: number) => ObservableInput<I>,
         resultSelector: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R,
         concurrent?: number): Observable<R>;
}

export class MergeMapOperator<T, I, R> implements Operator<T, I> {
  constructor(private project: (value: T, index: number) => ObservableInput<I>,
              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R,
              private concurrent: number = Number.POSITIVE_INFINITY) {
  }

  call(observer: Subscriber<I>, source: any): any {
    return source._subscribe(new MergeMapSubscriber(
      observer, this.project, this.resultSelector, this.concurrent
    ));
  }
}

/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
export class MergeMapSubscriber<T, I, R> extends OuterSubscriber<T, I> {
  private hasCompleted: boolean = false;
  private buffer: Observable<any>[] = [];
  private active: number = 0;
  protected index: number = 0;

  constructor(destination: Subscriber<I>,
              private project: (value: T, index: number) => ObservableInput<I>,
              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R,
              private concurrent: number = Number.POSITIVE_INFINITY) {
    super(destination);
  }

  protected _next(value: any): void {
    if (this.active < this.concurrent) {
      this._tryNext(value);
    } else {
      this.buffer.push(value);
    }
  }

  protected _tryNext(value: any) {
    let result: any;
    const index = this.index++;
    try {
      result = this.project(value, index);
    } catch (err) {
      this.destination.error(err);
      return;
    }
    this.active++;
    this._innerSub(result, value, index);
  }

  private _innerSub(ish: any, value: T, index: number): void {
    this.add(subscribeToResult<T, I>(this, ish, value, index));
  }

  protected _complete(): void {
    this.hasCompleted = true;
    if (this.active === 0 && this.buffer.length === 0) {
      this.destination.complete();
    }
  }

  notifyNext(outerValue: T, innerValue: I,
             outerIndex: number, innerIndex: number,
             innerSub: InnerSubscriber<T, I>): void {
    if (this.resultSelector) {
      this._notifyResultSelector(outerValue, innerValue, outerIndex, innerIndex);
    } else {
      this.destination.next(innerValue);
    }
  }

  _notifyResultSelector(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) {
    let result: R;
    try {
      result = this.resultSelector(outerValue, innerValue, outerIndex, innerIndex);
    } catch (err) {
      this.destination.error(err);
      return;
    }
    this.destination.next(result);
  }

  notifyComplete(innerSub: Subscription): void {
    const buffer = this.buffer;
    this.remove(innerSub);
    this.active--;
    if (buffer.length > 0) {
      this._next(buffer.shift());
    } else if (this.active === 0 && this.hasCompleted) {
      this.destination.complete();
    }
  }
}