public class CurrentUser {
    private static final CurrentUser ourInstance = new CurrentUser();

    private CurrentUserInterface mInterface;

    private CurrentUser() {
    }

    public static CurrentUser getInstance() {
        return ourInstance;
    }

    public void setUserName(String userName) {
        if (mInterface != null) {
            mInterface.setUserName(userName);
        }
    }

    public void setInterface(CurrentUserInterface mInterface) {
        this.mInterface = mInterface;
    }

    public interface CurrentUserInterface {
        void setUserName(String username);
    }
}

public class UserViewModel extends AndroidViewModel implements CurrentUser.CurrentUserInterface {

    private MutableLiveData<String> userName;

    public UserViewModel(@NonNull Application application) {
        super(application);
        CurrentUser.getInstance().setInterface(this);     // <-- Is this a good practice ??
    }

    public LiveData<String> getUserName() {
        if (userName == null) {
            userName = new MutableLiveData<>();
            loadUser("username","password");
        }
        return userName;
    }

    @Override
    public void setUserName(String username) {
        if (this.userName == null) {
            return;
        }

        this.userName.postValue(username);
    }

    private void loadUser(String username, String password) {
        Intent service = new Intent(context, HttpService.class);

        service.putExtra(HTTP_SERVICE_LOGIN, true);

        service.putExtra(HTTP_SERVICE_LOGIN_USERNAME, username);
        service.putExtra(HTTP_SERVICE_LOGIN_PASSWORD, password);

        context.startService(service);
    }
}

public class HttpService extends IntentService {

    // Login constants
    public static final String HTTP_SERVICE_LOGIN = "HTTP_SERVICE_LOGIN";
    public static final String HTTP_SERVICE_LOGIN_USERNAME = "HTTP_SERVICE_LOGIN_USERNAME";
    public static final String HTTP_SERVICE_LOGIN_PASSWORD = "HTTP_SERVICE_LOGIN_PASSWORD";

    @Override
    protected void onHandleIntent(@Nullable Intent intent) {
        if (intent != null) {
            if (intent.getBooleanExtra(HTTP_SERVICE_LOGIN, false)) {

                String username = intent.getStringExtra(HTTP_SERVICE_LOGIN_USERNAME);
                String password = intent.getStringExtra(HTTP_SERVICE_LOGIN_PASSWORD);

                loginAction(username, password);
            }
        }
    }

    private void loginAction(String username, String password) {
        // Async call to http server - this part is working fine
        // Then
        CurrentUser.getInstance().setUserName("User connected");
    }
}

public class MainActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        /// initialization code

        final TextView tv = navigationView.getHeaderView(0).findViewById(R.id.textViewName);

        UserViewModel model = ViewModelProviders.of(this).get(UserViewModel.class);
        model.getUserName().observe(this, new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {
                tv.setText(s);
            }
        });
    }
}