Android MVVM Commons

Helper classes to implement MVVM in Android with less common code

• Error handling

• Progress indication

• View lifecycle handling

• View components

• View layer dependency container

Error handling

The view component and the view model have to implement corresponding interfaces in ErrorContract. This class is mainly designed for simple error notification like toast message or an error dialog box. The view model can delegate the error event to the view by composing the ErrorVM class.

try {
    //tricky stuff
}catch (Exception e) {
    new ErrorVM(view).onErorr(view, e, "Error performing the action");
}

Progress indication

The view which has to show any dialog or progress indicator which is going to perform solely hide/show actions, should implement ProgressIndicatorView interface. An utility implementation which is showing a simple dialog box with a progress circle is ProgressDialogView

class MFragment extends Fragment implements ProgressIndicatorView{
    ProgressDialogView dialogView;
    public void onCreateView(...args..){
        dialogView = new ProgressDialogView(getContext());
    }
    @Override
    public void onShowProgress(){
        dialogView.onShowProgress()
    }
    @Override
    public void onHideProgress(){
        dialogView.onHideProgress();
    }
}

View lifecycle handling

If the view model is bound to the lifecycle of the container view component it has to implement LifecycleVM interface. To avoid any memory leak caused by backgorund thread and hard reference to the view component the view model needs to unsubscribe from any event emitter like event bus or RxJava observable.

If used with RxJava there is an implementation called RxLifecycleVM which wraps the CompositeDisposable class.

MyViewModel implements LifecycleVM {
    RxLifecycleVM lifecycleVM = new RxLifecycleVM();

    @Override public void onStart(){
        lifecycleVM.onStart();
        lifecycleVM.addSubscriber(getUsersListFromApi()
            .subscribeOn(...)
            .observeOn(...)
            .subscribe(...));
    }
    @Override public void onDestroy(){
        lifecycleVM.onDestroy();
    }
}

View components

All the fragments which use a single view model variable bound to the xml have to extend SingleVarBindingFragment. It takes care of calling the lifecycle methods on the view model if it implements LifecycleVM interface, and also is responsible for inflating the view and binding the view model to it.

public class MyFragment extends SingleVarBindingFragment<MyViewModel, MyFragmentBinding>  {
    @Override
    protected int getBindingVarId() {
        return BR.viewModel;
    }
    @Override
    protected MyViewModel onCreateVM() {
        return new MyViewModel();
    }

    @Override
    protected int getLayoutResource() {
        return R.layout.fragment_hours_tracker;
    }
    @Override
    protected void onInitView() {
        //Write any view initialization code here
    }
}

When using a fragment which is a standalone piece of UI and needs to be displayed in a separate activity, the class FragmentHolderActivity can be used. However if there is needed any customization and interaction between the activity the usage of this class is not encouraged.

FragmentHolderActivity.launch(getContext(), MyFragment.class, MyFragment.createArgs(consultant));

View layer dependency container

In order to avoid using complex DI injectors a little POJO can be written to contain all the data layer services to provide a way for creating mock testable app instances. However if the service classes are heavy and long to instantiate this method should be avoided.

In your application class

public class Application extends android.app.Application implements DiContextAware<DiContext>{
    private DiContext diContext;
    @Override
    public void onCreate() {
        super.onCreate();
        this.diContext = new DiContext();
    }
    @Override
    public DiContext getDiContext() {
        return diContext;
    }
}

DiContext.java

public class DiContext {
    private final UserService userService;
    public DiContext() {
        this.userService = new UserService();
    }
    public UserService getUserService() {
        return userService;
    }
}

UI component (fragment, activity)

    public void onCreate(){
        DiContext diContext = DiUtil.from(this);
        ViewModel vm = new ViewModel(diContext.getUserService())
    }