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