RxJava groupBy和后续的阻塞操作（onComplete缺失？）

Question

I have boiled my problem down into the following snippet:

Observable<Integer> numbers = Observable.just(1, 2, 3);

Observable<GroupedObservable<Integer,Integer>> outer = numbers.groupBy(i->i%3);
System.out.println(outer.count().toBlocking().single());

which blocks interminably. I've been reading several posts and believe I understand the problem: GroupedObservables will not call onComplete until their inner Observables have also been completed. Unfortunately though I still can't get the above snippet to print!

For example, the following:

Observable<Integer> just = Observable.just(1, 2, 3);

Observable<GroupedObservable<Integer,Integer>> groupBy = just.groupBy(i->i%3);
groupBy.subscribe(inner -> inner.ignoreElements());
System.out.println(groupBy.count().toBlocking().single());

still does nothing. Have I misunderstood the problem? Is there another problem? In short, how can I get the above snippets to work?

Many thanks in advance, Dan.

Answer 1

Yes, you have to consume the groups in some fashion. Your second example doesn't work because you have two independent subscription to the grouping operation.

Usually, the solution is flatMap , but not not with ignoreElements because that will just complete and count won't get any elements. Instead, you can use takeLast(1) :

Observable.just(1, 2, 3)
    .groupBy(k -> k % 3)
    .flatMap(g -> g.takeLast(1))
    .count()
    .toBlocking()
    .forEach(System.out::println);