rxjs/src/internal/operators/delayWhen.ts

Reactive Extensions for modern JavaScript

import { Observable } from '../Observable';
import { MonoTypeOperatorFunction } from '../types';
import { concat } from '../observable/concat';
import { take } from './take';
import { ignoreElements } from './ignoreElements';
import { mapTo } from './mapTo';
import { mergeMap } from './mergeMap';

/** @deprecated The `subscriptionDelay` parameter will be removed in v8. */
export function delayWhen<T>(
  delayDurationSelector: (value: T, index: number) => Observable<any>,
  subscriptionDelay: Observable<any>
): MonoTypeOperatorFunction<T>;
export function delayWhen<T>(delayDurationSelector: (value: T, index: number) => Observable<any>): MonoTypeOperatorFunction<T>;

/**
 * Delays the emission of items from the source Observable by a given time span
 * determined by the emissions of another Observable.
 *
 * <span class="informal">It's like {@link delay}, but the time span of the
 * delay duration is determined by a second Observable.</span>
 *
 * ![](delayWhen.png)
 *
 * `delayWhen` time shifts each emitted value from the source Observable by a
 * time span determined by another Observable. When the source emits a value,
 * the `delayDurationSelector` function is called with the source value as
 * argument, and should return an Observable, called the "duration" Observable.
 * The source value is emitted on the output Observable only when the duration
 * Observable emits a value or completes.
 * The completion of the notifier triggering the emission of the source value
 * is deprecated behavior and will be removed in future versions.
 *
 * Optionally, `delayWhen` takes a second argument, `subscriptionDelay`, which
 * is an Observable. When `subscriptionDelay` emits its first value or
 * completes, the source Observable is subscribed to and starts behaving like
 * described in the previous paragraph. If `subscriptionDelay` is not provided,
 * `delayWhen` will subscribe to the source Observable as soon as the output
 * Observable is subscribed.
 *
 * ## Example
 * Delay each click by a random amount of time, between 0 and 5 seconds
 * ```ts
 * import { fromEvent, interval } from 'rxjs';
 * import { delayWhen } from 'rxjs/operators';
 *
 * const clicks = fromEvent(document, 'click');
 * const delayedClicks = clicks.pipe(
 *   delayWhen(event => interval(Math.random() * 5000)),
 * );
 * delayedClicks.subscribe(x => console.log(x));
 * ```
 *
 * @see {@link delay}
 * @see {@link throttle}
 * @see {@link throttleTime}
 * @see {@link debounce}
 * @see {@link debounceTime}
 * @see {@link sample}
 * @see {@link sampleTime}
 * @see {@link audit}
 * @see {@link auditTime}
 *
 * @param {function(value: T, index: number): Observable} delayDurationSelector A function that
 * returns an Observable for each value emitted by the source Observable, which
 * is then used to delay the emission of that item on the output Observable
 * until the Observable returned from this function emits a value.
 * @param {Observable} subscriptionDelay An Observable that triggers the
 * subscription to the source Observable once it emits any value.
 * @return A function that returns an Observable that delays the emissions of
 * the source Observable by an amount of time specified by the Observable
 * returned by `delayDurationSelector`.
 */
export function delayWhen<T>(
  delayDurationSelector: (value: T, index: number) => Observable<any>,
  subscriptionDelay?: Observable<any>
): MonoTypeOperatorFunction<T> {
  if (subscriptionDelay) {
    // DEPRECATED PATH
    return (source: Observable<T>) =>
      concat(subscriptionDelay.pipe(take(1), ignoreElements()), source.pipe(delayWhen(delayDurationSelector)));
  }

  return mergeMap((value, index) => delayDurationSelector(value, index).pipe(take(1), mapTo(value)));
}