RxJava - upload files sequentially - emit next item, when onNext called

Question

I have a method that uploads concurrently multiple files to the cloud storage. It looks something like this:

List<String> files = Arrays.asList("file0", "file1", "file2");

Observable.from(files)
        .flatMap(file -> uploadFile(file)
                .flatMap(done -> notifyFinished(file)))
        .subscribe(this::onNext, this::onError, this::onCompleted);


private Observable<Boolean> uploadFile(String file) {
    Timber.d("Uploading: " + file);
    return Observable.just(true).delay(6, TimeUnit.SECONDS);
}

private Observable<Boolean> notifyFinished(String file) {
    Timber.d("Notify finished: " + file);
    return Observable.just(true).delay(3, TimeUnit.SECONDS);
}

The output of this is:

06-09 02:10:04.779 D: Uploading: file0
06-09 02:10:04.780 D: Uploading: file1
06-09 02:10:04.781 D: Uploading: file2
06-09 02:10:10.782 D: Notify finished: file1
06-09 02:10:10.782 D: Notify finished: file0
06-09 02:10:10.783 D: Notify finished: file2
06-09 02:10:13.784 D: onNext
06-09 02:10:13.786 D: onNext
06-09 02:10:13.786 D: onNext
06-09 02:10:13.787 D: onCompleted

I want to make it work sequentially, eg:

1) Uploading: file0
2) Notify finished: file0
3) onNext
4) Uploading: file1
5) Notify finished: file1
6) onNext
   ...

Is it possible to do something like this with Rx?

EDIT

Replacing first flatMap with concatMap did the job. I thought that I knew the difference between those operators, but this example just shown that I know nothing... Now the output is:

06-09 02:15:00.581 D: Uploading: file0
06-09 02:15:06.584 D: Notify finished: file0
06-09 02:15:09.586 D: onNext
06-09 02:15:09.587 D: Uploading: file1
06-09 02:15:15.590 D: Notify finished: file1
06-09 02:15:18.593 D: onNext
06-09 02:15:18.595 D: Uploading: file2
06-09 02:15:24.598 D: Notify finished: file2
06-09 02:15:27.599 D: onNext
06-09 02:15:27.601 D: onCompleted

Answer 1

if you want 'ordered' consecutive sequential, just use concatMap() instead of flatMap()

Answer 2

Create an observable per file and concat the three observables

@Test
public void testContact() {

    Observable.concat(Observable.just(uploadFile(file1)),
                      Observable.just(uploadFile(file2)),
                      Observable.just(uploadFile(file3)))
              .flatMap(file -> notifyFinished(file)))
              .subscribe(this::onNext, this::onError, this::onCompleted);
}

You will have to make the method notifyFinished return the observable file instead boolean.

You can also use merge or zip, you have more examples of combining observables here https://github.com/politrons/reactive/tree/master/src/test/java/rx/observables/combining