How can I update Activity/Fragment UI from Retrofit onResponse()?

Question

I want to use Retrofit 2.1.0 for my next Android project. I would like to use Design Pattern and therefore create separate classes for every REST call i.e

IBalanceService.java

public Interface IBalanceService{
  @GET("users/{username}")
  void getUser(@Path("username") String username, Callback<User> cb);
}  

BalanceRestClient.java

public class BalanceRestClient{  

  String username = "Makarov";  

  Retrofit retrofit = new Retrofit.Builder()
                               .baseUrl("http://hovermind.com")
                               .addConverterFactory(GsonConverterFactory.create())
                               .addCallAdapterFactory(rxAdapter)
                               .build();  

  IBalanceService bs = retrofit.create(IBalanceService.class);


  Call<User> call = apiService.getUser(username);
  call.enqueue(new Callback<User>() {  

      @Override
      public void onResponse(Call<User> call, Response<User> response) {

          User user = response.body();  

          // update Activity/Fragment from here  

      }

      @Override
      public void onFailure(Call<User> call, Throwable t) {
        // Log error here since request failed
      }  

   });
}

BalanceActivity.java

public BalanceActivity extends Activity{

   // BalanceRestClient will update some UI elements 
}  

How can I update UI in my Activity/fragment from onResponse()?

Additional Info:
min SDK : 19
target SDK : 23
Retrofit 2.1.0 with OkHttp and gson

Answer 1

 public class BalanceRestClient {

            private static BalanceRestClient instance;
            private IBalanceService request;

            private BalanceRestClient() {
                Retrofit retrofit = new Retrofit.Builder()
                        .baseUrl("http://hovermind.com")
                        .addConverterFactory(GsonConverterFactory.create())
                        .addCallAdapterFactory(rxAdapter)
                        .build();

                request = retrofit.create(IBalanceService.class);
            }

            public static synchronized BalanceRestClient getInstance() {
                if (instance == null)
                    instance = new BalanceRestClient();

                return instance;
            }

            public void getRequest(String userName, YourCustomeCallback callback) {

                Call<User> call = apiService.getUser(username);
                call.enqueue(new Callback<User>() {

                    @Override
                    public void onResponse(Call<User> call, Response<User> response) {

                        User user = response.body();

                        callback.onResponse(user);

                    }

                    @Override
                    public void onFailure(Call<User> call, Throwable t) {
                        // Log error here since request failed
                    }

                });
            }
        }

1. Create a singleton class. Its init a request only once.
2. Create a custom callback, which will give back your response to UI.
3. Call BalanceRestClient.getInstance.getRequest(params..) where you want to use it.

Answer 2

Make an interface like this on:

public interface BalanceListener {

    void onUserAvailable(User user);

}

In your BalanceRestClient have an instance of it. And in onResponse:

  @Override
  public void onResponse(Call<User> call, Response<User> response) {

      User user = response.body();  

      listener.onUserAvailable(user);

  }

Make your Activity implement BalanceListner:

 public class BalanceActivity extends Activity implements BalanceListener {
                ......

        @Override
        public void onUserAvailable(User user) {
            // update user views here
        }
    }