How to make multiple request and wait until data is come from all the requests in retrofit 2.0 - android

Question

current code:

Retrofit retrofit = new Retrofit.Builder()
                  .baseUrl(Constant.BASEURL)
                  .addConverterFactory(GsonConverterFactory.create())
                  .build();

APIService service = retrofit.create(APIService.class);

Call<ResponseWrap> call = service.getNewsData();

call.enqueue(new Callback<ResponseWrap>() {

  @Override
  public void onResponse(Call<ResponseWrap> call1, Response<ResponseWrap> response) {
    if (response.isSuccess()) {

        ResponseWrap finalRes = response.body();
        for(int i=0; i<finalRes.getResponse().getResults().size(); ++i){
            String title = finalRes.getResponse().getResults().get(i).getWebTitle();
            News n = new News(titleCategory, title, null);
            newsList.add(n);
        }

        AdapterRecommendation adapter = new AdapterRecommendation(getApplicationContext(), newsList);
        listView.setAdapter(adapter);

    }
    else{
        Toast.makeText(getApplicationContext(), "onResponse  - something wrong" + response.message(), Toast.LENGTH_LONG).show();
    }
  }

  @Override
  public void onFailure(Call<ResponseWrap> call1, Throwable t) {
      Toast.makeText(getApplicationContext(), "exception: " + t.getMessage(), Toast.LENGTH_LONG).show();
  }
});

works fine.

Now i want to make multiple calls (number of call will be decided at run time) and all calls gives data in same format. data from all calls needs to be add to newsList. Once data is available from all calls and added to newsList, call

AdapterRecommendation adapter = new AdapterRecommendation(getApplicationContext(), newsList);
listView.setAdapter(adapter);

Can anyone help me what is the best way to get data from multiple calls and wait until all request is not over in retrofit 2.0.

Answer 1

The clean and neat approach to wait until all your requests will be done is to use Retrofit2 in conjunction with RxJava2 and its zip function.

What zip does is basically constructs new observable that waits until all your retrofit Observable requests will be done and then it will emit its own result.

Here is an example Retrofit2 interface with Observables:

public interface MyBackendAPI {
  @GET("users/{user}")
  Observable<User> getUser(@Path("user") String user);

  @GET("users/{user}/photos")
  Observable<List<Photo>> listPhotos(@Path("user") String user);

  @GET("users/{user}/friends")
  Observable<List<User>> listFriends(@Path("user") String user);
}

In the code where you going to make multiple requests and only after all of them will complete do something else you can then write the following:

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://api.example.com/")
            .build();

    MyBackendAPI backendApi = retrofit.create(MyBackendAPI.class);

    List<Observable<?>> requests = new ArrayList<>();

    // Make a collection of all requests you need to call at once, there can be any number of requests, not only 3. You can have 2 or 5, or 100.
    requests.add(backendApi.getUser("someUserId"));
    requests.add(backendApi.listPhotos("someUserId"));
    requests.add(backendApi.listFriends("someUserId"));

    // Zip all requests with the Function, which will receive the results.
    Observable.zip(
            requests,
            new Function<Object[], Object>() {
                @Override
                public Object apply(Object[] objects) throws Exception {
                    // Objects[] is an array of combined results of completed requests

                    // do something with those results and emit new event
                    return new Object();
                }
            })
            // After all requests had been performed the next observer will receive the Object, returned from Function
            .subscribe(
                    // Will be triggered if all requests will end successfully (4xx and 5xx also are successful requests too)
                    new Consumer<Object>() {
                        @Override
                        public void accept(Object o) throws Exception {
                            //Do something on successful completion of all requests
                        }
                    },

                    // Will be triggered if any error during requests will happen
                    new Consumer<Throwable>() {
                        @Override
                        public void accept(Throwable e) throws Exception {
                            //Do something on error completion of requests
                        }
                    }
            );

That's all :)

---

Just in case wanna show how the same code looks like in Kotlin.

    val retrofit = Retrofit.Builder()
            .baseUrl("https://api.example.com/")
            .build()

    val backendApi = retrofit.create(MyBackendAPI::class.java)

    val requests = ArrayList<Observable<*>>()

    requests.add(backendApi.getUser())
    requests.add(backendApi.listPhotos())
    requests.add(backendApi.listFriends())

    Observable
            .zip(requests) {
                // do something with those results and emit new event
                Any() // <-- Here we emit just new empty Object(), but you can emit anything
            }
            // Will be triggered if all requests will end successfully (4xx and 5xx also are successful requests too)
            .subscribe({
                //Do something on successful completion of all requests
            }) {
                //Do something on error completion of requests
            }