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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.