iOS get height of keyboard (notify changes when keyboard appears, changes and disappears)

1. - (void)viewDidLoad

   {

       [super viewDidLoad];

       

       // Increase monitoring, receive a message when the keyboard appears or changes

       [[NSNotificationCenter defaultCenter] addObserver:self

                                                selector:@selector(keyboardWillShow:)

                                                    name:UIKeyboardWillShowNotification

                                                  object:nil];

       

       // Increase monitoring, receive a message when the key exits

       [[NSNotificationCenter defaultCenter] addObserver:self

                                                selector:@selector(keyboardWillHide:)

                                                    name:UIKeyboardWillHideNotification

                                                  object:nil];

       

       

   }

   
   

   // Called when the keyboard appears or changes

   - (void)keyboardWillShow:(NSNotification *)aNotification

   {

       // Get the height of the keyboard

       NSDictionary *userInfo = [aNotification userInfo];

       NSValue  *aValue = [userInfo  objectForKey : UIKeyboardFrameEndUserIn foKey ];

       CGRect keyboardRect = [aValue CGRectValue];

       float height = keyboardRect.size.height;

   }

   
   

   // Called when the key exits

   - (void)keyboardWillHide:(NSNotification *)aNotification

   {

       

   }