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import { Observable } from '../Observable';
import { SubscribableOrPromise, ObservedValueOf, ObservableInput } from '../types';
import { from } from './from'; // lol
import { empty } from './empty';
/**
* Creates an Observable that, on subscribe, calls an Observable factory to
* make an Observable for each new Observer.
*
* <span class="informal">Creates the Observable lazily, that is, only when it
* is subscribed.
* </span>
*
* 
*
* `defer` allows you to create the Observable only when the Observer
* subscribes, and create a fresh Observable for each Observer. It waits until
* an Observer subscribes to it, and then it generates an Observable,
* typically with an Observable factory function. It does this afresh for each
* subscriber, so although each subscriber may think it is subscribing to the
* same Observable, in fact each subscriber gets its own individual
* Observable.
*
* ## Example
* ### Subscribe to either an Observable of clicks or an Observable of interval, at random
* ```ts
* import { defer, fromEvent, interval } from 'rxjs';
*
* const clicksOrInterval = defer(function () {
* return Math.random() > 0.5
* ? fromEvent(document, 'click')
* : interval(1000);
* });
* clicksOrInterval.subscribe(x => console.log(x));
*
* // Results in the following behavior:
* // If the result of Math.random() is greater than 0.5 it will listen
* // for clicks anywhere on the "document"; when document is clicked it
* // will log a MouseEvent object to the console. If the result is less
* // than 0.5 it will emit ascending numbers, one every second(1000ms).
* ```
*
* @see {@link Observable}
*
* @param {function(): SubscribableOrPromise} observableFactory The Observable
* factory function to invoke for each Observer that subscribes to the output
* Observable. May also return a Promise, which will be converted on the fly
* to an Observable.
* @return {Observable} An Observable whose Observers' subscriptions trigger
* an invocation of the given Observable factory function.
* @static true
* @name defer
* @owner Observable
*/
export function defer<R extends ObservableInput<any> | void>(observableFactory: () => R): Observable<ObservedValueOf<R>> {
return new Observable<ObservedValueOf<R>>(subscriber => {
let input: R | void;
try {
input = observableFactory();
} catch (err) {
subscriber.error(err);
return undefined;
}
const source = input ? from(input as ObservableInput<ObservedValueOf<R>>) : empty();
return source.subscribe(subscriber);
});
}