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