Start emitting after another observable emitted its first item

Question

I have this simple usecase that I can't seem to figure out.

Given 2 observables, A and B

A starts emitting items after B emits it's first item, and B can continue to emit items, which should no longer affect A.

I need this because A needs the first value emitted by B to start emitting.

I've tried both skipUntil and startWith to no avail.

Myk Willis Myk Willis · Accepted Answer · 2017-12-12T22:57:53

I think flatMap can help you do what you want:

Observable<Integer> createA(Observable<Integer> B) {
    return B.take(1).flatMap(b -> {
        /* replace with your Observable A implementation here */
        BehaviorSubject<Integer> subjectA = BehaviorSubject.create();
        return subjectA;
    });
}

This method returns an Observable A, which begins emitting only after it has been initialized with the first value emitted by Observable B.

Start emitting after another observable emitted its first item

1 Answers