键盘通知

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