键盘通知

1.在登录页面，或者编辑页面，常常会出现弹出的键盘将内容遮住的状况。出现此种状况。能够经过键盘的通知来处理。

UIKeyboardWillShowNotification
UIKeyboardDidShowNotification
UIKeyboardWillHideNotification
UIKeyboardDidHideNotification

通知消息 NSNotification中的 userInfo字典中包含键盘的位置和大小信息，对应的key为

UIKeyboardFrameBeginUserInfoKey
UIKeyboardFrameEndUserInfoKey
UIKeyboardAnimationDurationUserInfoKey
UIKeyboardAnimationCurveUserInfoKey