Handling applicationDidBecomeActive - "How can a view controller respond to the app becoming Active?"

Question

I have the UIApplicationDelegate protocol in my main AppDelegate.m class, with the applicationDidBecomeActive method defined.

I want to call a method when the application returns from the background, but the method is in another view controller. How can I check which view controller is currently showing in the applicationDidBecomeActive method and then make a call to a method within that controller?

Answer

Any class in your application can become an "observer" for different notifications in the application. When you create (or load) your view controller, you'll want to register it as an observer for the UIApplicationDidBecomeActiveNotification and specify which method that you want to call when that notification gets sent to your application.

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(someMethod:)
                                             name:UIApplicationDidBecomeActiveNotification object:nil];

Don't forget to clean up after yourself! Remember to remove yourself as the observer when your view is going away:

[[NSNotificationCenter defaultCenter] removeObserver:self 
                                                name:UIApplicationDidBecomeActiveNotification
                                              object:nil];

More information about the Notification Center.