When does a LifecycleRegistry instance start listening to LifecycleOwner's lifecycle changes?

Question

I've started learning architecture components, but can't find one thing.

LifecycleFragment just creates a new LifecycleRegistry object, which does not start observing the fragment's lifecycle.

I guess the LifecycleRegistry object starts listening to the fragment's lifecycle when we, for example, put it into LiveData.observe() as first param, but I haven't found any proof of this in source code.

Question: When and how does a LifecycleRegistry object start to observe a fragment's lifecycle and refresh LifecycleRegistry.mState?

Answer 1

There is a ContentProvider called LifecycleRuntimeTrojanProvider that is merged into the app's AndroidManifest.xml. In its onCreate method it initializes a singleton called LifecycleDispatcher, which is responsible for updating all LifecycleRegistry instances.

LifecycleDispatcher uses the Application.registerActivityLifecycleCallbacks method that has been around since API 14 to get notified when a new activity is created. At this point it injects an instance of ReportFragment into the activity. The ReportFragment uses the Fragment lifecycle callbacks to update the activity's LifecycleRegistry if necessary, like this:

@Override
public void onStop() { // Showing onStop as example
    super.onStop();
    dispatch(Lifecycle.Event.ON_STOP);
}

private void dispatch(Lifecycle.Event event) {
    if (getActivity() instanceof LifecycleRegistryOwner) {
        ((LifecycleRegistryOwner) getActivity()).getLifecycle().handleLifecycleEvent(event);
    }
}

If the new activity is a FragmentActivity, the LifecycleDispatcher calls FragmentManager.registerFragmentLifecycleCallbacks to get notified of the activity's fragments lifecycle events. It relays the onFragmentCreated, onFragmentStarted and onFragmentResumed callbacks to the LifecycleRegistry in case the fragment is a LifecycleRegistryOwner, in the same way as before.

The onFragmentPaused, onFragmentStopped, and onFragmentDestroyed callbacks are called after the corresponding callbacks are called on the fragment, but the LifecycleObserver callbacks must be called before. So whenever a fragment is created, the LifecycleDispatcher injects an instance of LifecycleDispatcher.DestructionReportFragment into it. The DestructionReportFragment's lifecycle callbacks are used to update the registry for the pause, stop and destroy events.

I can't link to the code because it hasn't been released yet, but you can browse it in Android Studio after you add the library to your project.

Answer 2

As Mordag said, as of now, both the LifecycleActivity and LifecycleFragment are not yet implemented. In their documentation Google says:

Any custom fragment or activity can be turned into a LifecycleOwner by implementing the built-in LifecycleRegistryOwner interface (instead of extending LifecycleFragment or LifecycleActivity).

However, that is only half the story, because naturally you are using these Lifecycle Aware components to be able to react to your Activity/Fragment lifecycles and with their code snippet it just doesn't work, because initialising a LifecycleRegistry with the Activity/Fragment like this

LifecycleRegistry lifecycleRegistry = new LifecycleRegistry(this);

only gets you a Lifecycle in the INITIALIZED state.

So, long story short, in order for this to work right now (BEFORE their 1.0-release) it is you who have to implement the Lifecycle of the Activity/Fragment that implements the LifecycleRegistry. So, for each callback of the Activity/Fragment you need to do this:

public class ScoreMasterFragment extends Fragment
                                 implements LifecycleRegistryOwner {


    private LifecycleRegistry lifecycle;

    @Override
    public LifecycleRegistry getLifecycle() {
        return lifecycle;
    }

    public ScoreMasterFragment(){
         lifecycle = new LifecycleRegistry(this);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //more code here
        lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    }

    @Override
    public void onStart() {
        super.onStart();
        //more code here
        lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START);

    }

    @Override
    public void onResume() {
        super.onResume();
        //more code here
        lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    }

    @Override
    public void onPause() {
        super.onPause();
        //more code here
        lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
    }

    @Override
    public void onStop() {
        super.onStop();
        //more code here
        lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        //more code here
        _lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
    }

This will likely be in the code of the future LifecycleActivity and LifecycleFragment, but until then, if you put your Activities/Fragments observing some LifecycleAware object (like LiveData) you will have to do this.

In the case of LiveData, because it will not notify its observers unless they are at least in the STARTED state and in other cases because other LifecycleAware components cannot react to a Lifecycle if its only state is INITIALIZED.

Answer 3

The LifecycleFragment and LifecycleActivity are currently not fully implemented. Those classes will be implemented when the lib is reaching 1.0-release. Currently you can use those LifecycleRegistry to observe LiveData objects. Those objects are based on a future result which could e.g. be an object from your database.

The official documentation can be found here: https://developer.android.com/topic/libraries/architecture/index.html

Official statement regarding the two classes you mentioned:

Lifecycle Fragment and ActivityCompat in the Support Library do not yet implement LifecycleOwner interface. They will when Architecture Components reaches 1.0.0 version.

Answer 4

LifecycleActivity ,LifecycleFragment and LifecycleRegistryOwner interface are deprecated in API level 1.0.0. Use android.support.v7.app.AppCompatActivity and android.support.v4.app.Fragment instead of it. Official documentation here LifecycleActivity LifeCycleFragment