What's the point of AsyncSubject in RXJS?

问题:

The documentation for RxJS defines AsyncSubject as follows:

The AsyncSubject is a variant where only the last value of the Observable execution is sent to its observers, and only when the execution completes.

I don't see where / why I would ever need to use this variant of subject. Can someone provide an explanation or a real-world example to illustrate why it exists and its advantages?

回答1:

It looks like it could be useful for fetching and caching (one-shot) resources, since generally http.get will emit one response then complete.

From rxjs/spec/subjects/AsyncSubject-spec.ts

it('should emit the last value when complete', () => {
it('should emit the last value when subscribing after complete', () => {
it('should keep emitting the last value to subsequent subscriptions', () => {

Components that subscribe after the fetch will then pick up value, which is not the case for Subject

const subject = new Rx.Subject();
const asyncSubject = new Rx.AsyncSubject();

// Subscribe before
subject.subscribe(x => console.log('before complete - subject', x))
asyncSubject.subscribe(x => console.log('before complete - asyncSubject', x))

subject.next('value 1');
subject.complete();
subject.next('value 2');

asyncSubject.next('value 1');
asyncSubject.complete();
asyncSubject.next('value 2');

// Subscribe after
subject.subscribe(x => console.log('after complete - subject', x))
asyncSubject.subscribe(x => console.log('after complete - asyncSubject', x))

.as-console-wrapper { max-height: 100% ! important; top: 0 }

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.5.6/Rx.js"></script>