键盘的frame发生改变时发出的通知（位置和尺寸）
UIKeyboardWillChangeFrameNotification
UIKeyboardDidChangeFrameNotification
键盘显示时发出的通知
UIKeyboardWillShowNotification
UIKeyboardDidShowNotification
键盘隐藏时发出的通知
UIKeyboardWillHideNotification
UIKeyboardDidHideNotification
原文：http://www.cnblogs.com/521it/p/5003255.html