Reacting to activity lifecycle in ViewModel

Question

I'm trying to create an app which will use MVVM architecture and there's one thing I quite don't understand.

Official Android docs say that's not a good idea to reference activity context in ViewModel's (as ViewModel may outlive activity) so I've started to wonder about usecase when I want to execute some action when my activity is resumed.

I know ViewModel's shouldn't do business logic themselves but even if I use some service class (let's say GPSService which has to start and pauseeach time activity is resumed on paused), and inside this service I react to activity onResume (using Lifecycle observer) I will still reference this activity from ViewModel as I'm referencing service which holds reference to activity being observed, this may cause activity leak (correct me if I'm wrong).

So my question is, how to react to activity or fragment lifecycle in MVVM architecture?

Answer 1

If you need to have a ViewModel be lifecycle aware, then you can have it implement LifeCycleObserver and override life cycle events as necessary. Example,

public class MyModel extends ViewModel implements
    LifecycleObserver {

  @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
  protected void onLifeCycleStop() {
      // do something
  }
}

In the activity or fragment then you can add the view model to the activity life cycle owner.

public class MyActivity extends AppCompatActivity {

  protected MyModel mMyModel;

  @Override
  public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);

      mMyModel = ViewModelProviders
          .of(this)
          .get(MyModel.class);

      getLifecycle().addObserver(mMyModel);
  }
}