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import { Observable } from '../Observable';
import { Unsubscribable, ObservableInput, ObservedValueOf } from '../types';
import { innerFrom } from './innerFrom';
import { EMPTY } from './empty';
/**
* Creates an Observable that uses a resource which will be disposed at the same time as the Observable.
*
* <span class="informal">Use it when you catch yourself cleaning up after an Observable.</span>
*
* `using` is a factory operator, which accepts two functions. First function returns a disposable resource.
* It can be an arbitrary object that implements `unsubscribe` method. Second function will be injected with
* that object and should return an Observable. That Observable can use resource object during its execution.
* Both functions passed to `using` will be called every time someone subscribes - neither an Observable nor
* resource object will be shared in any way between subscriptions.
*
* When Observable returned by `using` is subscribed, Observable returned from the second function will be subscribed
* as well. All its notifications (nexted values, completion and error events) will be emitted unchanged by the output
* Observable. If however someone unsubscribes from the Observable or source Observable completes or errors by itself,
* the `unsubscribe` method on resource object will be called. This can be used to do any necessary clean up, which
* otherwise would have to be handled by hand. Note that complete or error notifications are not emitted when someone
* cancels subscription to an Observable via `unsubscribe`, so `using` can be used as a hook, allowing you to make
* sure that all resources which need to exist during an Observable execution will be disposed at appropriate time.
*
* @see {@link defer}
*
* @param {function(): ISubscription} resourceFactory A function which creates any resource object
* that implements `unsubscribe` method.
* @param {function(resource: ISubscription): Observable<T>} observableFactory A function which
* creates an Observable, that can use injected resource object.
* @return {Observable<T>} An Observable that behaves the same as Observable returned by `observableFactory`, but
* which - when completed, errored or unsubscribed - will also call `unsubscribe` on created resource object.
*/
export function using<T extends ObservableInput<any>>(
resourceFactory: () => Unsubscribable | void,
observableFactory: (resource: Unsubscribable | void) => T | void
): Observable<ObservedValueOf<T>> {
return new Observable<ObservedValueOf<T>>((subscriber) => {
const resource = resourceFactory();
const result = observableFactory(resource);
const source = result ? innerFrom(result) : EMPTY;
source.subscribe(subscriber);
return () => {
// NOTE: Optional chaining did not work here.
// Related TS Issue: https://github.com/microsoft/TypeScript/issues/40818
if (resource) {
resource.unsubscribe();
}
};
});
}