rxjava switch observable if second observable start emits items

Question

I have some set of observable which I am executing in parallel, like localObservable and networkObservable. If the networkObservable starts emit items (from this time, I need only these items), then discard items emitted by localObservable (maybe localObservable has not yet started).

Observable<Integer> localObservable =
            Observable.defer(() -> Observable.range(1, 10)).subscribeOn(Schedulers.io());
Observable<Integer> networkObservable =
            Observable.defer(() -> Observable.range(11, 20)).subscribeOn(Schedulers.io());

score 7 · Accepted Answer · answered Apr 18 '17 at 06:14

you can do something like this:

 Observable<Long> networkObservable =
            Observable.interval(1000, 500, TimeUnit.MILLISECONDS)
                    .subscribeOn(Schedulers.io())
                    .share();
    Observable<Long> localObservable =
            Observable.interval(500, TimeUnit.MILLISECONDS)                       
                    .subscribeOn(Schedulers.io())
                    .takeUntil(networkObservable);

    Observable.merge(networkObservable, localObservable)
            .subscribe(System.out::println);

this will output:

0 // localObservable 
1 // localObservable 
0 // networkObservable from here on
1
2
...

takeUntil will make localObservable to stop and unsubscribe when the first emission from networkObservable happened, so the merged Observable will emit from localObservable as long networkObservable didn't started, and when it does, it will stop emitting from localObservable and switch to emit only from networkObservable.

If `networkObservable ` emits error, I wanna keep on the `localObservable `, how can I do that? — xymelon, Apr 21 '17 at 14:53
you can use mergeDelayError (you'll get onError at the end in this case, after localObservable emit something), or just catch all errors at the networkObservable with onErrorREsumeNext or similar — yosriz, Apr 22 '17 at 20:14

Sergej Isbrecht · Answer 2 · 2017-04-18T08:34:05.693

There is an easy solutions to this by an operator: AMB

Just look at the output of the System.out.

Documentation: http://reactivex.io/documentation/operators/amb.html

Basically you subscribe to both observable at the same time and whatever observable first emits gets through. The other observable will be unsubscribed.

@Test
public void ambTest() throws Exception {
    TestScheduler testScheduler = new TestScheduler();

    Observable<Integer> network = Observable.timer(1000, TimeUnit.MILLISECONDS, testScheduler)
                .concatMap(aLong -> Observable.just(1, 2, 3))
                .doOnSubscribe(disposable -> System.out.println("connect network"))
                .doOnDispose(() -> System.out.println("dispose network"));

    Observable<Integer> local = Observable.timer(500, TimeUnit.MILLISECONDS, testScheduler)
                .concatMap(aLong -> Observable.just(4, 5, 6))
                .doOnSubscribe(disposable -> System.out.println("connect local"))
                .doOnDispose(() -> System.out.println("dispose local"));

    Observable<Integer> integerObservable = Observable.ambArray(network, local);

    TestObserver<Integer> test = integerObservable.test();

    testScheduler.advanceTimeBy(600, TimeUnit.MILLISECONDS);

    test.assertValues(4, 5, 6);

    testScheduler.advanceTimeBy(1000, TimeUnit.MILLISECONDS);

    test.assertValues(4, 5, 6);
}

It's depends on the requirements, if you do want the emissions from local observable while network observable didn't emit yet, then amb will just chose local observable with its first emission and will never switch to network observable — yosriz, Apr 18 '17 at 12:48

rxjava switch observable if second observable start emits items

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