以UITextField的键盘弹出顺序为例：
先是[textField becomeFirstResponsder]----------->调用textField的代理事件（textFieldShouldBeginEditing)----------->假如textFieldShouldBeginEditing返回YES，,调用代理事件textFieldDidBeginEditing，那么[textField isFirstResponsder]返回YES------------>键盘开始弹出，发送键盘出现通知
UIKeyboardWillShowNotification,当键盘完全出现在屏幕上时，发送UIKeyboardDidShowNotification
键盘消失顺序(与弹出顺序类似):先是[textField resignFirstResponsder]----------->调用（textFieldShouldEndEditing)--------------->假如textFieldShouldEndEditing返回YES，调用textFieldDidEndEditing,[textField isFirstResponsder]返回NO，------------->键盘消失，发送键盘消失通知