Model-View-ViewModel (MVVM)

One Activity, one view

One Activity holds one view (Fragment) and ViewModel. I want to retain the ViewModel instance when the screen is rotated, but create the new one when backs from the other Activity.

• MainActivity

  public class MainActivity extends AppCompatActivity {
  
    private static final String MAIN_VIEWMODEL_TAG = "MAIN_VIEWMODEL_TAG";
  
    @Inject
    Lazy<MainViewModel> viewModelProvider;
  
    @Inject
    Lazy<MainFragment> fragmentProvider;
  
    @Override
    protected void onCreate(Bundle savedInstanceState) {
      AndroidInjection.inject(this);
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      findOrCreateViewThenDisplayIt(findOrCreateViewModel(MAIN_VIEWMODEL_TAG));
    }
  
    private void findOrCreateViewThenDisplayIt(@NonNull MainViewModel viewModel) {
      BaseView<MainViewModel> baseView = findView();
      if (baseView == null) {
        baseView = fragmentProvider.get();
        baseView.setViewModel(viewModel);
        ActivityUtils.addFragmentToActivity(
            getSupportFragmentManager(), baseView, R.id.contentFrame);
      } else {
        baseView.setViewModel(viewModel);
      }
    }
  
    @SuppressWarnings("unchecked")
    @Nullable
    private BaseView<MainViewModel> findView() {
      return (BaseView<MainViewModel>) getSupportFragmentManager()
          .findFragmentById(R.id.contentFrame);
    }
  
    @NonNull
    private MainViewModel findOrCreateViewModel(@NonNull String tag) {
      // In a configuration change we might have a ViewModel present. It's retained using the
      // Fragment Manager.
      @SuppressWarnings("unchecked")
      ViewModelHolder<MainViewModel> retainedViewModel =
          (ViewModelHolder<MainViewModel>) getSupportFragmentManager().findFragmentByTag(tag);
  
      if (retainedViewModel != null && retainedViewModel.getViewmodel() != null) {
        // If the model was retained, return it.
        return retainedViewModel.getViewmodel();
      } else {
        // There is no ViewModel yet, create it.
        MainViewModel viewModel = viewModelProvider.get();
        // and bind it to this Activity's lifecycle using the Fragment Manager.
        ActivityUtils.addFragmentToActivity(
            getSupportFragmentManager(),
            ViewModelHolder.createContainer(viewModel),
            tag);
        return viewModel;
      }
    }
  }
  

• MainViewModel

  @ActivityScoped
  public class MainViewModel extends BaseObservable implements BaseViewModel {
  
    @Inject
    MainViewModel() {
  
    }
  }
  

  Should set its scope same as Activity.

• MainFragment

  @ActivityScoped
  public class MainFragment extends BaseView<MainViewModel> {
  
    @Inject
    public MainFragment() {
      // Required empty public constructor
    }
  
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
      FragmentMainBinding binding = DataBindingUtil
          .inflate(inflater, R.layout.fragment_main, container, false);
      binding.setViewModel(viewModel);
      return binding.getRoot();
    }
  }
  

• LogCat

  I/main: onCreate()
  D/main: get ViewModel from Dagger!
  I/MainViewModel: MainViewModel()
  D/main: viewModel is com.hsy.d.main.MainViewModel@96bf940
  D/main: get View from Dagger!
  I/MainFragment: MainFragment()
  I/MainFragment: onCreate()
  I/MainFragment: onCreateView()
  I/main: onStart()
  D/MainFragment: onResume(): viewModel is com.hsy.d.main.MainViewModel@96bf940
  
  // rotation
  I/main: onStop()
  I/MainFragment: onDestroy()
  I/main: onDestroy()
  
  I/main: onCreate()
  I/MainFragment: MainFragment()
  I/MainFragment: onCreate()
  D/main: ViewModel was retained
  D/main: viewModel is com.hsy.d.main.MainViewModel@96bf940
  D/main: View was retained
  I/MainFragment: onCreateView()
  I/main: onStart()
  D/MainFragment: onResume(): viewModel is com.hsy.d.main.MainViewModel@96bf940
  
  // start other Activity and finish self
  I/main: onStop()
  I/MainFragment: onDestroy()
  I/main: onDestroy()
  
  // back to MainActivity
  I/main: onCreate()
  D/main: get ViewModel from Dagger!
  I/MainViewModel: MainViewModel()
  D/main: viewModel is com.hsy.d.main.MainViewModel@696feeb
  D/main: get View from Dagger!
  I/MainFragment: MainFragment()
  I/MainFragment: onCreate()
  I/MainFragment: onCreateView()
  I/main: onStart()
  D/MainFragment: onResume(): viewModel is com.hsy.d.main.MainViewModel@696feeb
  

android-architecture MVVM