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2 API requests parallelly using Retrofit2 and RxJava

If you want to call 2 or 3 api parallelly (for parallel requests), then you can use RxJava with Retrofit2 and make this process simpler and easier. We can use zip() operator of Rxjava’s Observable class, this function accept 1, 2, or 3 Observable at same time, and then process them in serial. After processing all 3 Observables it combines 3 results and return them in callback.

here is example in Java:

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

Observable<ResponseType1> observable1 = retrofit.getApi_a();
Observable<ResponseType2> observable2 = retrofit.getApi_b();
Observable<ResponseType3> observable3 = retrofit.getApi_c();

2. Now Zip the observables to get a final observable.

Observable<List<String>> result = 
Observable.zip(observable1.subscribeOn(Schedulers.io()), observable2.subscribeOn(Schedulers
            .io()), observable3.subscribeOn(Schedulers.io()), new Function3<ResponseType1, ResponseType2, ResponseType3, List<String>>() {
    @Override
    public List<String> apply(ResponseType1 type1, ResponseType2 type2, ResponseType3 type3) {
        List<String> list = new ArrayList();
        list.add(type1.data);
        list.add(type2.data);
        list.add(type3.data);
        return list;
    }
});

3. now subscribe on the result observable.

result.observeOn(AndroidSchedulers.mainThread())
  .subscribeWith(new Observer<List<String>>() {
    @Override
    public void onSubscribe(Disposable d) {

    }

    @Override
    public void onNext(List<String> s) {
        Log.d(TAG, "s is the list with 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 results in ArrayList and we got List of results 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 observable1 = retrofit.getApi_a()
val observable2 = retrofit.getApi_b()
val observable3 = retrofit.getApi_c()

2. Now Zip the observables to get a final observable

val result = Observable.zip(
            observable1,
            observable2,
            observable3,
            BiFunction<ResponseType1, ResponseType2, ResponseType3, ArrayList<AddResponse>> { type1, type2, type3 ->
                arrayListOf(
                    type1.data,
                    type2.data,
                    type3.data
                )
            })

3. now subscribe on the result observable.

result.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread())
  .subscribe({
    Log.d(TAG, "**it** is the list with all the data")
  }, {
    Log.e(TAG, e.getMessage())
  })