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);
|
});
|
}
|
