如何得到软键盘收起的事件

点击软键盘右下角的收起键时，如何才可以得到这个事件？

用通知：

 

NSNotificationCenter *defaultCenter = [NSNotificationCenter defaultCenter];
        [defaultCenter addObserver:self
                          selector:@selector(keyboardWillShow:)
                              name:UIKeyboardWillShowNotification
                            object:nil];
        [defaultCenter addObserver:self
                          selector:@selector(keyboardWillHide:)
                              name:UIKeyboardWillHideNotification
                            object:nil];
        [defaultCenter addObserver:self
                          selector:@selector(keyboardDidHide:)
                              name:UIKeyboardDidHideNotification
                            object:nil];
        [defaultCenter addObserver:self 
                          selector:@selector(keyboardDidShow:) 
                              name:UIKeyboardDidShowNotification 
                            object:nil];