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.

Answer 1

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.