获取键盘高度

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.titleTextField.delegate = self;
    self.contextTextView.delegate = self;
    
    //增加监听，当键盘出现或改变时收出消息
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillShow:)
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
    
    //增加监听，当键退出时收出消息
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWillHide:)
                                                 name:UIKeyboardWillHideNotification
                                               object:nil];
}
//当键盘出现或改变时调用
- (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;
}

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