How to detect when keyboard is shown and hidden

In the ViewDidLoad method of your class set up to listen for messages about the keyboard:

// Listen for keyboard appearances and disappearances
[[NSNotificationCenter defaultCenter] addObserver:self 
                                         selector:@selector(keyboardDidShow:)
                                             name:UIKeyboardDidShowNotification
                                           object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardDidHide:)
                                             name:UIKeyboardDidHideNotification
                                           object:nil];

Then in the methods you specify (in this case keyboardDidShow and keyboardDidHide) you can do something about it:

- (void)keyboardDidShow: (NSNotification *) notif{
    // Do something here
}

- (void)keyboardDidHide: (NSNotification *) notif{
    // Do something here
}