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java – Making N sequential api calls using RxJava and Retrofit

Posted by: admin May 14, 2020 Leave a comment


I have a list of files that I’d like to upload to the backend from an Android device. Due to memory constraints, I’d like to make the second API call only after the first finished, the third after the second finished, and so on.

I wrote something like

private Observable<Integer> uploadFiles(List<File> files) {
        return Observable.create(subscriber -> {
            for (int i = 0, size = files.size(); i < size; i++) {
                UploadModel uploadModel = new UploadModel(files.get(0));
                int uploadResult = retrofitApi.uploadSynchronously(uploadModel);

But I feel like this might be going against the spirit of Rx, and the saying is if you’re using Observable.create, you’re probably doing it wrong…
Is this a reasonable approach? Is there a better way to achieve this with Retrofit’s RxJava integration?

How to&Answers:

Naively, I would do that (it does not work, though, see below):

return Observable.from(files).concatMap(file -> retrofitApi.upload(uploadModel));

Now the issue is that there is no way to tell retrofit to use only one thread for those calls.

reduce, however, passes the result of one function call to the next, along with the next emitted value from the original observable. That would work, but the function passed to reduce needs to be synchronous. Not good.

Another approach would be to modify the observable recursively:

void getNextFile(int i) {
    return retrofit.upload(i).
        onNext(result -> getNextFile(i + 1));

roughly. But I am not sure how to clean it to make it more readable.

The cleanest I would think would be something like:

Observable.from(files).map(file -> retrofitApi.uploadSynchronously(new UploadModel(file)));


The natives of RxJava would emit all items in Observable.from(...) as if in parallel. That’s the best way to think of it as parallel emission. However some cases require real consequent execution of the whole chain. I’ve come to the following solution, probably not the best one but working.

import rx.Observable;
import rx.Subscriber;

import java.util.Iterator;
import java.util.function.Function;

public class Rx {
    public static void ignore(Object arg) {

    public static <E, R> Observable<Void> sequential(Iterator<E> iterator, Function<E, Observable<R>> action) {
        return Observable.create(collectorSubscriber ->
                Observable.<Void>create(producerSubscriber ->
                        producerSubscriber.setProducer(ignoredCount -> {
                            if (!iterator.hasNext()) {

                            E model = iterator.next();
                                            () -> producerSubscriber.onNext(null));
                        .subscribe(new Subscriber<Void>() {
                            public void onStart() {

                            public void onCompleted() {

                            public void onError(Throwable e) {

                            public void onNext(Void aVoid) {

Example usage would be:

    Iterator<? extends Model> iterator = models.iterator();

    Rx.sequential(iterator, model -> someFunctionReturnsObservable(model))

This method guarantees chained executions of

Observable<Dummy> someFunctionReturnsObservable(Model model)


Currently the prefered way of creating observables is with fromAsync:

Observable.fromAsync(new Action1<AsyncEmitter<Object>>()
        public void call(final AsyncEmitter<Object> emitter)

            emitter.setCancellation(new AsyncEmitter.Cancellable()
                public void cancel() throws Exception
                    // on unSubscribe() callback
    }, AsyncEmitter.BackpressureMode.BUFFER);