Determining the current foreground application from a background task or service

Question

I wish to have one application that runs in the background, which knows when any of the built-in applications (messaging, contacts, etc) is running.

So my questions

Answer 1

For cases when we need to check from our own service/background-thread whether our app is in foreground or not. This is how I implemented it, and it works fine for me:

public class TestApplication extends Application implements Application.ActivityLifecycleCallbacks {

    public static WeakReference foregroundActivityRef = null;

    @Override
    public void onActivityStarted(Activity activity) {
        foregroundActivityRef = new WeakReference<>(activity);
    }

    @Override
    public void onActivityStopped(Activity activity) {
        if (foregroundActivityRef != null && foregroundActivityRef.get() == activity) {
            foregroundActivityRef = null;
        }
    }

    // IMPLEMENT OTHER CALLBACK METHODS
}

Now to check from other classes, whether app is in foreground or not, simply call:

if(TestApplication.foregroundActivityRef!=null){
    // APP IS IN FOREGROUND!
    // We can also get the activity that is currently visible!
}

Update (as pointed out by SHS):

Do not forget to register for the callbacks in your Application class's onCreate method.

@Override
public void onCreate() {
    ...
    registerActivityLifecycleCallbacks(this);
}