rxjs/src/internal/operators/switchMap.ts

Reactive Extensions for modern JavaScript

import { Subscriber } from '../Subscriber';
import { ObservableInput, OperatorFunction, ObservedValueOf } from '../types';
import { innerFrom } from '../observable/innerFrom';
import { operate } from '../util/lift';
import { OperatorSubscriber } from './OperatorSubscriber';

/* tslint:disable:max-line-length */
export function switchMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O
): OperatorFunction<T, ObservedValueOf<O>>;
/** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
export function switchMap<T, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: undefined
): OperatorFunction<T, ObservedValueOf<O>>;
/** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
export function switchMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, R>;
/* tslint:enable:max-line-length */

/**
 * Projects each source value to an Observable which is merged in the output
 * Observable, emitting values only from the most recently projected Observable.
 *
 * <span class="informal">Maps each value to an Observable, then flattens all of
 * these inner Observables.</span>
 *
 * ![](switchMap.png)
 *
 * 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 (so-called "inner") Observable. Each time it observes one of these
 * inner Observables, the output Observable begins emitting the items emitted by
 * that inner Observable. When a new inner Observable is emitted, `switchMap`
 * stops emitting items from the earlier-emitted inner Observable and begins
 * emitting items from the new one. It continues to behave like this for
 * subsequent inner Observables.
 *
 * ## Example
 * Generate new Observable according to source Observable values
 * ```typescript
 * import { of } from 'rxjs';
 * import { switchMap } from 'rxjs/operators';
 *
 * const switched = of(1, 2, 3).pipe(switchMap((x: number) => of(x, x ** 2, x ** 3)));
 * switched.subscribe(x => console.log(x));
 * // outputs
 * // 1
 * // 1
 * // 1
 * // 2
 * // 4
 * // 8
 * // ... and so on
 * ```
 *
 * Restart an interval Observable on every click event
 * ```ts
 * import { fromEvent, interval } from 'rxjs';
 * import { switchMap } from 'rxjs/operators';
 *
 * const clicks = fromEvent(document, 'click');
 * const result = clicks.pipe(switchMap((ev) => interval(1000)));
 * result.subscribe(x => console.log(x));
 * ```
 *
 * @see {@link concatMap}
 * @see {@link exhaustMap}
 * @see {@link mergeMap}
 * @see {@link switchAll}
 * @see {@link switchMapTo}
 *
 * @param {function(value: T, ?index: number): ObservableInput} project A function
 * that, when applied to an item emitted by the source Observable, returns an
 * Observable.
 * @return A function that returns an Observable that emits the result of
 * applying the projection function (and the optional deprecated
 * `resultSelector`) to each item emitted by the source Observable and taking
 * only the values from the most recently projected inner Observable.
 */
export function switchMap<T, R, O extends ObservableInput<any>>(
  project: (value: T, index: number) => O,
  resultSelector?: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
): OperatorFunction<T, ObservedValueOf<O> | R> {
  return operate((source, subscriber) => {
    let innerSubscriber: Subscriber<ObservedValueOf<O>> | null = null;
    let index = 0;
    // Whether or not the source subscription has completed
    let isComplete = false;

    // We only complete the result if the source is complete AND we don't have an active inner subscription.
    // This is called both when the source completes and when the inners complete.
    const checkComplete = () => isComplete && !innerSubscriber && subscriber.complete();

    source.subscribe(
      new OperatorSubscriber(
        subscriber,
        (value) => {
          // Cancel the previous inner subscription if there was one
          innerSubscriber?.unsubscribe();
          let innerIndex = 0;
          const outerIndex = index++;
          // Start the next inner subscription
          innerFrom(project(value, outerIndex)).subscribe(
            (innerSubscriber = new OperatorSubscriber(
              subscriber,
              // When we get a new inner value, next it through. Note that this is
              // handling the deprecate result selector here. This is because with this architecture
              // it ends up being smaller than using the map operator.
              (innerValue) => subscriber.next(resultSelector ? resultSelector(value, innerValue, outerIndex, innerIndex++) : innerValue),
              () => {
                // The inner has completed. Null out the inner subcriber to
                // free up memory and to signal that we have no inner subscription
                // currently.
                innerSubscriber = null!;
                checkComplete();
              }
            ))
          );
        },
        () => {
          isComplete = true;
          checkComplete();
        }
      )
    );
  });
}