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2 API requests serially using Retrofit2 and RxJava

If you want to call 2 or 3 api serially (one after another), then you can use RxJava with Retrofit2 and make this process simpler and easier. We can use flatMap() function of Rxjava’s Observable class, this function execute current Observable at same time, and then process them in serial. After processing all 3 Observables we can combines these 3 results and return them in callback.

here is example in Java:

This is simple chaining of requests.

List<String> result = new ArrayList<>();
Disposable disposable = retrofit.getApi_a()
  .subscribeOn(Schedulers.io())
  .flatMap((Function<ResponseType1, ObservableSource<ResponseType2>>) response1 -> {
    result.add(response1.data);
    return retrofit.getApi_b();
  })
  .flatMap((Function<ResponseType2, ObservableSource<ResponseType3>>) response2 -> {
      result.add(response2.data);
      return retrofit.getApi_c();
  })
  .map(response3 -> {
      result.add(response3.data);
      return response3;
  })
  .observeOn(AndroidSchedulers.mainThread())
  .subscribeWith(new DisposableObserver<Response3>() {

      @Override
      public void onNext(Response3 response3) {
          Log.d(TAG, "result variable will have all the data");
      }

      @Override
      public void onError(Throwable e) {
          Log.e(TAG, e.getMessage());
      }

      @Override
      public void onComplete() {

      }
  });

And you can see we added all result in ArrayList and we got List of result in callback function. So the onNext() function will call at the end of last API call.

here is example in Kotlin:

1. first of all get observable from retrofit interface class.

val result = new ArrayList<>()
val disposable = retrofit.getApi_a()
  .subscribeOn(Schedulers.io())
  .flatMap { 
    result.add(response1.data)
    retrofit.getApi_b() 
  }
  .flatMap { 
    result.add(response2.data)
    retrofit.getApi_c() 
  }
  .map { 
    result.add(response3.data)
    response3
  }
  .observeOn(AndroidSchedulers.mainThread())
  .subscribe({
    Log.d(TAG, "**result** is the list with all the data")
  }, {
    Log.e(TAG, e.getMessage())
  })