CODEWITHSUNDEEP
androidrepositoryrx-java2android-room
Catalin Ghita
Catalin Ghita

Reputation: 826

RxJava2 concat operator doesn't return appropriate stream

In order to build my repository, I am retrieving 2 streams of data from Local Database and from remote API. I am trying to access the remote stream only if the mQuakesLocalDataSource has no items by using the concat operator as shown in many examples.

@NonNull
@Override
public Single<List<Quake>> getQuakes(){
    return Single.concat(mQuakesLocalDataSource.getQuakes(),
                            mQuakesRemoteDataSource.getQuakes())
                              .first(dummyList);
}

The issue I am facing is that mQuakesRemoteDataSource never returns a stream while mQuakesLocalDataSource is empty, therefore I am not having any data result. I have tested mQuakesRemoteDataSource in isolation and without the concat operator it seems to be retrieving its appropriate stream.

Why is this happening?

mQuakesLocalDataSource is based on Room and so therefore it should emit its stream and then complete, so it's not possible that the local source would emit a never-ending stream like SQLbrite does.

I have tried variations of this operator like concatArray and the result was the same, no data is being retrieved whatsoever.

An interesting note would be the fact that while debugging, I noticed that both mQuakesLocalDataSource and mQuakesRemoteDataSource get methods are triggered when passed into concat operator, before reaching the first() line. Shouldn't concat evaluate sources one by one (and filter the current one) in the presence of first operator?

I have also tried adding a filter with a Predicate in order to cache data into the Local Data Source, as below:

@NonNull
@Override
public Single<List<Quake>> getQuakes() {
    return Single.concat(mQuakesLocalDataSource.getQuakes(),
            mQuakesRemoteDataSource.getQuakes()).filter(new Predicate<List<Quake>>() {
        @Override
        public boolean test(List<Quake> quakes) throws Exception {
            boolean isValid = quakes != null && !quakes.isEmpty();

            // save items to local data source
            if (isValid) saveQuakes(quakes);

            return isValid;
        }
    }).first(new ArrayList<>());
}

The result was the same, no data is being retrieved.

Upvotes: 0

Views: 224

Answers (1)

akarnokd
akarnokd

Reputation: 69997

Returning an empty list in Single doesn't make the Single empty, thus first will correctly stop at the first item, the empty list, never calling the remote source. You have to decide on the item via flatMap whether or not to resume with the remote source instead of concat:

mQuakesLocalDataSource.getQuakes()
.flatMap(list -> {
    if (list.isEmpty()) {
        return mQuakesRemoteDataSource.getQuakes();
    }
    return Single.just(list);
})

Upvotes: 1

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