监听键盘的弹出和收回通知

- (void)viewDidLoad
{
    [super viewDidLoad];

    //增加监听，当键盘出现或改变时触发方法
    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillShow:)
                                             name:UIKeyboardWillShowNotification
                                           object:nil];

    //增加监听，当键退出时调用代理方法
    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillHide:)
                                             name:UIKeyboardWillHideNotification
                                           object:nil];    
}

 //当键盘出现或改变时回调的代理方法
- (void)keyboardWillShow:(NSNotification *)aNotification
{
    //获取键盘的高度
    NSDictionary *userInfo = [aNotification userInfo];
    NSValue *aValue = [userInfo objectForKey:UIKeyboardFrameEndUserInfoKey];
    CGRect keyboardRect = [aValue CGRectValue];
   int height = keyboardRect.size.height;
}

//当键退出时回调
- (void)keyboardWillHide:(NSNotification *)aNotification{}