Emit values within a loop (RxJava)

Question

I am new to Rx and I've been wondering how to properly emit values one by one (lazy load images in this example). What is the best way to do this without encountering concurrency issues?

This is my code:

@Override
public Observable<Bitmap> getPhotos(final String placeId) {
    return Observable.create(new ObservableOnSubscribe<Bitmap>() {
        @Override
        public void subscribe(ObservableEmitter<Bitmap> emitter) throws Exception {
            try {
                PlacePhotoMetadataResult res = Places.GeoDataApi
                        .getPlacePhotos(googleApiClient, placeId)
                        .await();
                PlacePhotoMetadataBuffer buffer = res.getPhotoMetadata();

                for (PlacePhotoMetadata photoMetadata : buffer) {
                    Bitmap photo = photoMetadata
                            .getPhoto(googleApiClient)
                            .await()
                            .getBitmap();

                    emitter.onNext(photo);
                }

                buffer.release();
            } catch (Exception ex) {
                emitter.onError(ex);
            } finally {
                emitter.onComplete();
            }
        }
    });
}

Thanks!

where is the loop here?

yosriz
– yosriz

2017-02-25 20:36:03 +00:00
Commented Feb 25, 2017 at 20:36 — yosriz
– yosriz, Commented Feb 25, 2017 at 20:36

Tassos Bassoukos · Accepted Answer · 2017-02-25 20:39:59Z

1

You really should invest some time in bridging PendingResult<T> to Observable<T> (or Single, or Flowable(my preference)). Then all your code can be expressed as:

private static <T> Observable<T> toObservable(PendingResult<T> result) {
    // 
}

public Observable<Bitmap> getPhotos(final String placeId) {
    return Observable
            .just(placeId)
            .flatMap(id -> toObservable(Places.GeoDataApi.getPlacePhotos(googleApiClient, placeId)))
            .flatMapIterable(PlacePhotoMetadataResult::getPhotoMetadata)
            .flatMap(metadata -> toObservable(metadata.getPhoto(googleApiClient)))
            .map(Photo::getBitmap);
}

A good place to start looking into how to do the bridge is this library, but it's RxJava 1 only.

answered Feb 25, 2017 at 20:39

Tassos Bassoukos

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2 Comments

Iliyan Kupenov Over a year ago

Is it fine to just return a Flowable instead of Observable and keep the rest of my code as it is? What backpressure strategy should I use?

Tassos Bassoukos Over a year ago

Sure you can use Flowable, I just used Observable because that's what you use! And in this case you don't need to specify a backpressure strategy, the flatMapIterable will take care of it. You'll need to adapt toObservable tho.

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