RxJava: how to handle combineLatest() when one of the streams emits nothing

问题

I use combineLatest() to combine 3 streams of observables. All these are combined so that all data in the UI is shown at the same time. Now, there is a scenario in which one of the observables won't emit anything, since the data that gets fetched, can be null.

Is there a RxJava operator to let the subscriber know that there won't be any emits because of null data?

Edit

private fun retrieveData() {
    Observable.combineLatest(getCurrentUser.execute(), getLatestGoal.execute(), getLatestLog.execute(),
            Function3<User, Goal, Log, PersonalViewModel> { user, goal, log -> mapToViewModel(user, goal, log) })
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .doOnSubscribe { /*todo: animation*/ }
            .doOnNext { view.setViewModel(it) }
            .doOnComplete { view.stopLoading() }
            .doOnError { /*todo: error message*/ }
            .subscribe()
}

The third stream: getLatestLog.execute() emits nothing when the user has nog log. When this stream doesn't emit, the whole view will not be visible.

The data is fetched from FireBase Realtime database. The ChildEventListener has a method that looks like this:

override fun onChildAdded(dataSnapshot: DataSnapshot?, p1: String?) {
                val log = dataSnapshot?.getValue(Log::class.java)
                log?.let { subscriber.onNext(it) }
                subscriber.onComplete()
                firebaseDatabase.reference.removeEventListener(this)
            }

回答1:

If you have Java8 or some Optionals at your fingertips, you may use this construct:

  @Test
  void name() {
    TestScheduler scheduler = new TestScheduler();

    Observable<Optional<Integer>> o1$ =
        Observable.just(Optional.ofNullable(4)).mergeWith(Observable.never());
    Observable<Optional<Integer>> o2$ =
        Observable.just(Optional.ofNullable(2)).mergeWith(Observable.never());

    Observable<Optional<Integer>> o3$ =
        Observable.<Optional<Integer>>never()
            .timeout(1000, TimeUnit.MILLISECONDS, scheduler)
            .onErrorResumeNext(
                throwable -> {
                  return Observable.<Optional<Integer>>never()
                      .mergeWith(Observable.just(Optional.empty()));
                });

    Observable<Tuple3<Optional<Integer>, Optional<Integer>, Optional<Integer>>> result =
        Observable.combineLatest(
                o1$,
                o2$,
                o3$,
                (integer, integer2, integer3) -> Tuple.of(integer, integer2, integer3))
            .filter(t -> t._1.isPresent() && t._2.isPresent() && t._3.isPresent());

    TestObserver<Tuple3<Optional<Integer>, Optional<Integer>, Optional<Integer>>> test =
        result.test();

    scheduler.advanceTimeTo(10000, TimeUnit.SECONDS);

    test.assertNotComplete().assertNoErrors().assertNoValues();
  }

As you may no, null values are not allowed to be emitted through observables-pipelines. Therfore we need some other construct to represent null. In Java8 there is a construct called Optional (vavr calls it Option -> also Java8).

In this example o3$-Observable will not emit anything. It could also error, maybe this resembles your case a little bit more. We will catch the error (in this case: timeout-exception) and return a Observable with Optional.empty.

In the combination-callback we combine alle three values. In a later step we filter out all Tuples, which all have valid values (Optional with Value).

You will only get a value emitted, when all three values have been emitted with a value.

When you can not use a Optional-class, you can also define a INVALID-Object like in the following example:

class So51217041 {
  private static Integer INVALID_VALUE = 42;

  @Test
  void name() {
    Observable<Integer> o1$ = Observable.just(4).mergeWith(Observable.never());
    Observable<Integer> o2$ = Observable.just(2).mergeWith(Observable.never());

    Observable<Integer> o3$ =
        Observable.<Integer>never()
            .onErrorResumeNext(
                throwable -> {
                  return Observable.<Integer>never().mergeWith(Observable.just(INVALID_VALUE));
                });

    Observable<Tuple3<Integer, Integer, Integer>> result =
        Observable.combineLatest(
                o1$,
                o2$,
                o3$,
                (integer, integer2, integer3) -> Tuple.of(integer, integer2, integer3))
            .filter(t -> t._3 != INVALID_VALUE); // yeah I know, I want to compare reference, not the content

    TestObserver<Tuple3<Integer, Integer, Integer>> test = result.test();

    test.assertNotComplete().assertNoErrors().assertNoValues();
  }
}

Also, when you want a stream to start with INVALID or NULL, that the CombineLatest emits at least one value, you may use Observable#startWith(INVALID) oder Observable#startWith(Optional.empty()). This will guarantee, that the observable will at least emit one value.

回答2:

You can use public final Single first(T defaultItem) method. So the code may look like this

getLatestLog.execute()
.first(someDefaultNonNullLog)
.toObservable()

来源：https://stackoverflow.com/questions/51217041/rxjava-how-to-handle-combinelatest-when-one-of-the-streams-emits-nothing