Question

Is there a way to register for an activity’s events? I’m specifically interested in the onStart / onStop events, and I don’t want to add special operations in the activity for that.

Answer 1

One way to get events from the lifecycle of other activities is to register your class as an Application.ActivityLifecycleCallbacks with the main Application instance and filter events for the Activity you’re interested in.

This is a short example (you may want to register the callbacks from another method/class other than MainActivity.onCreate or you’ll miss that message 😉 and you may have a dependency there that you don’t want)

On the activity you want to spy:

public class MainActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Register a spy for this activity
        getApplication().registerActivityLifecycleCallbacks(new ActivitySpy(this));
    }
}

Then the Spy code looks something like:

public class ActivitySpy implements ActivityLifecycleCallbacks {

    private final Activity mActivity;

    public ActivitySpy(Activity activity) {
        mActivity = activity;
    }

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        if (mActivity == activity)
            Log.i("SPY", "Activity Created");
    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        if (mActivity == activity)
            Log.i("SPY", "Activity Destroyed");
    }

    // (...) Other overrides

}

You can also register the spy from another place if you have a reference to the Activity you want to follow.

I hope this helps 🙂

EDIT: I forgot to mention, this will only work on API Level 14 and above…