如何创建一次发射项目然后从 BehaviorSubject onComplete 的 Observable？

Question

I need to create an Observable<T> from a BehaviorSubject<Observable<T>> . The created observable should only emit items once and then call onComplete() .

Is there a way to do this?

Thanks!

Edit

private BehaviorSubject<Observable<T>> subject = BehaviorSubject.create();

public Observable<T> getObservable(){
   //return the subject as Observable<T> which emits once and call onComplete()
}

Answer 1

So, you have a BehaviorSubject that emits Observable<T> s, and from this you want to create an Observable<T> that only emits the first item of the first Observable emitted by the Subject. You could do that like this:

public Observable<T> getObservableWithJustOneElement(){
    return subject.flatMap(new Func1<Observable<T>, Observable<T>>(){

        @Override
        public Observable<T> call(Observable<T> source) {
           return source;
        }
    })
    .take(1);
}

I know it looks a bit weird but it should be doing what you want. First, the flatMap flattens the Observable<Observable<T>> to just an Observable<T> (it just does not do any mapping in the sense of transforming the elements). Then, the take(1) makes sure that only one item will be emitted with onNext() and after that onCompleted() will be called.

If you had wanted only the items emitted by the first Observable emitted by the Subject, you could have used this instead:

public Observable<T> getObservableWithJustOneElement(){
    return subject.take(1)
    .flatMap(new Func1<Observable<T>, Observable<T>>(){

        @Override
        public Observable<T> call(Observable<T> source) {
           return source;
        }
    });
}

Answer 2

You can also create an Observable that emits once from for example a ReplaySubject or a BehaviourSubject with the first operator .

return behaviorSubject.pipe(first());

in case you want to return the first item that satisfies certain criteria you can use a predicate as an argument:

const predicate = (result: any): boolean => { 
  //your test to check the result should return true...
}
return behaviorSubject.pipe(first(predicate));