1.注册监听键盘事件的通知

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
   
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardShow:)
                                                 name:UIKeyboardDidShowNotification
                                               object:nil];
   
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillHide:)
                                                 name:UIKeyboardWillHideNotification
                                               object:nil];
   
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardHide:)
                                                 name:UIKeyboardDidHideNotification
                                               object:nil];