FlatMap and subjects - how to avoid duplicating subscribers?

Question

Let's say I have the following code:

BehaviorSubject<Integer> subject = BehaviorSubject.create();
BehaviorSubject<Integer> subject2 = BehaviorSubject.create();
subject.
        doOnNext(number -> subject2.onNext(number)).
        flatMap(number -> subject2).
        subscribe(number -> System.out.println("Number " + number));

for(int i = 0; i < 10; i++) {
    subject.onNext(i);
}

The output it produces is:

Number 0
Number 1
Number 1
Number 2
Number 2
Number 2
Number 3
Number 3
Number 3
Number 3
Number 4
Number 4
Number 4
Number 4
Number 4

And on and on.

I understand why it happens - it's because every time flatMap is called, new subscriber is added to subject2.

The question is - how to avoid this?

score 2 · Answer 1 · answered Aug 02 '16 at 12:13

2

try switchMap instead of flatMap.

switchMap is only ever subscribed to the last Observable it receives. It unsubscribes from all previously subscribed Observables.

http://reactivex.io/RxJava/javadoc/rx/Observable.html#switchMap

answered Aug 02 '16 at 12:13

Andrew G

2,596
2
19
26

score 1 · Answer 2 · answered Jul 28 '16 at 15:16

You can limit subject2 to one element inside flatMap like this:

flatMap(number -> subject2.first())

Code

    BehaviorSubject<Integer> subject = BehaviorSubject.create();
    BehaviorSubject<Integer> subject2 = BehaviorSubject.create();
    subject.
            doOnNext(number -> subject2.onNext(number)).
            flatMap(number -> subject2.first()).
            subscribe(number -> System.out.println("Number " + number));

    for(int i = 0; i < 10; i++) {
        subject.onNext(i);
    }

produces output which does not contain duplicates

FlatMap and subjects - how to avoid duplicating subscribers?

2 Answers2