iOS 监听键盘的弹出，收起，并且拿到键盘的高度

//监听当键盘将要出现时
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
    
    //监听当键将要退出时
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillHide:)
                                                 name:UIKeyboardWillHideNotification
                                               object:nil];


//当键盘出现
- (void)keyboardWillShow:(NSNotification *)notification
{
    //获取键盘的高度
    NSDictionary *userInfo = [notification userInfo];
    NSValue *value = [userInfo objectForKey:UIKeyboardFrameEndUserInfoKey];
    CGRect keyboardRect = [value CGRectValue];
    int height = keyboardRect.size.height;
}

//当键退出
- (void)keyboardWillHide:(NSNotification *)notification
{
    //获取键盘的高度
    NSDictionary *userInfo = [notification userInfo];
    NSValue *value = [userInfo objectForKey:UIKeyboardFrameEndUserInfoKey];
    CGRect keyboardRect = [value CGRectValue];
    int height = keyboardRect.size.height;
  
}