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];