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/RxJS

windowWhen

function stable

Branch out the source Observable values as a nested Observable using a factory function of closing Observables to determine when to start a new window.

`windowWhen<T>(closingSelector: () => Observable<any>): OperatorFunction<T, Observable<T>>`

Parameters

closingSelector

A function that takes no arguments and returns an Observable that signals (on either next or complete) when to close the previous window and start a new one.

Returns

OperatorFunction<T, Observable<T>>: An observable of windows, which in turn are Observables.

Description

It's like bufferWhen, but emits a nested Observable instead of an array.

windowWhen marble diagram

Returns an Observable that emits windows of items it collects from the source Observable. The output Observable emits connected, non-overlapping windows. It emits the current window and opens a new one whenever the Observable produced by the specified closingSelector function emits an item. The first window is opened immediately when subscribing to the output Observable.

Example

Emit only the first two clicks events in every window of [1-5] random seconds

import { fromEvent, interval } from 'rxjs';
import { windowWhen, map, mergeAll, take } from 'rxjs/operators';

const clicks = fromEvent(document, 'click');
const result = clicks.pipe(
  windowWhen(() => interval(1000 + Math.random() * 4000)),
  map(win => win.pipe(take(2))),     // each window has at most 2 emissions
  mergeAll()                         // flatten the Observable-of-Observables
);
result.subscribe(x => console.log(x));