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import { Observable } from '../Observable';
import { ObservedValueOf, ObservableInput } from '../types';
import { innerFrom } from './innerFrom';
/**
* 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 an Observable only when the Observer
* subscribes. It waits until an Observer subscribes to it, calls the given
* factory function to get an Observable -- where a factory function typically
* generates a new Observable -- and subscribes the Observer to this Observable.
* In case the factory function returns a falsy value, then EMPTY is used as
* Observable instead. Last but not least, an exception during the factory
* function call is transferred to the Observer by calling `error`.
*
* ## 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(() => {
* 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(): ObservableInput} 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.
*/
export function defer<R extends ObservableInput<any>>(observableFactory: () => R): Observable<ObservedValueOf<R>> {
return new Observable<ObservedValueOf<R>>((subscriber) => {
innerFrom(observableFactory()).subscribe(subscriber);
});
}