1 import { Observable } from '../Observable';
2 import { SubscribableOrPromise, ObservedValueOf, ObservableInput } from '../types';
3 import { from } from './from'; // lol
4 import { empty } from './empty';
7 * Creates an Observable that, on subscribe, calls an Observable factory to
8 * make an Observable for each new Observer.
10 * <span class="informal">Creates the Observable lazily, that is, only when it
16 * `defer` allows you to create the Observable only when the Observer
17 * subscribes, and create a fresh Observable for each Observer. It waits until
18 * an Observer subscribes to it, and then it generates an Observable,
19 * typically with an Observable factory function. It does this afresh for each
20 * subscriber, so although each subscriber may think it is subscribing to the
21 * same Observable, in fact each subscriber gets its own individual
25 * ### Subscribe to either an Observable of clicks or an Observable of interval, at random
27 * import { defer, fromEvent, interval } from 'rxjs';
29 * const clicksOrInterval = defer(function () {
30 * return Math.random() > 0.5
31 * ? fromEvent(document, 'click')
34 * clicksOrInterval.subscribe(x => console.log(x));
36 * // Results in the following behavior:
37 * // If the result of Math.random() is greater than 0.5 it will listen
38 * // for clicks anywhere on the "document"; when document is clicked it
39 * // will log a MouseEvent object to the console. If the result is less
40 * // than 0.5 it will emit ascending numbers, one every second(1000ms).
43 * @see {@link Observable}
45 * @param {function(): SubscribableOrPromise} observableFactory The Observable
46 * factory function to invoke for each Observer that subscribes to the output
47 * Observable. May also return a Promise, which will be converted on the fly
49 * @return {Observable} An Observable whose Observers' subscriptions trigger
50 * an invocation of the given Observable factory function.
55 export function defer<R extends ObservableInput<any> | void>(observableFactory: () => R): Observable<ObservedValueOf<R>> {
56 return new Observable<ObservedValueOf<R>>(subscriber => {
59 input = observableFactory();
61 subscriber.error(err);
64 const source = input ? from(input as ObservableInput<ObservedValueOf<R>>) : empty();
65 return source.subscribe(subscriber);