前几日发现一个iOS键盘通知方面的小bug，因此总结下iOS键盘通知的知识点。在iOS中，键盘通知目前的SDK里总共有如下6个：
- UIKeyboardWillShowNotification ： 显示键盘的时候立即发出该通知
- UIKeyboardDidShowNotification：显示键盘后才发出该通知
- UIKeyboardWillHideNotification：键盘即将消失的时候立即发出该通知
- UIKeyboardDidHideNotification：键盘消失后才发出该通知
- UIKeyboardWillChangeFrameNotification：键盘的frame值发生变化的时候立即发出该通知
- UIKeyboardDidChangeFrameNotification：键盘的frame值发生变化后才发出该通知
其中，以上通知的userInfo字典中包含了一些键盘frame以及动画相关的信息，字典的key如下：
NSString *const UIKeyboardFrameBeginUserInfoKey; //userInfo字典里该key对应一个NSValue，存储一个包含键盘初始frame值的CGRect结构（即键盘刚出现时的frame值）
NSString *const UIKeyboardFrameEndUserInfoKey; //userInfo字典里该key对应一个NSValue，存储一个包含键盘结束frame值的CGRect结构（即键盘动画结束后的frame值）
NSString *const UIKeyboardAnimationDurationUserInfoKey; //userInfo字典里该key对应一个NSNumber，存储一个包含键盘进入或离开屏幕的UIViewAnimationCurve结构
NSString *const UIKeyboardAnimationCurveUserInfoKey; //userInfo字典里该key对应一个NSNumber，存储一个包含键盘动画时间的double值，时间以秒为单位。
为了加深理解，下面分别打印出一个键盘显示、一个键盘消失以及两个屏幕旋转期间（这个在下面将会介绍）发出的键盘通知结构（模拟器打印的，真机诸位同学可以自己打印观察）：
{name = UIKeyboardDidShowNotification; userInfo = { //键盘显示时发出
UIKeyboardAnimationCurveUserInfoKey = 0;
UIKeyboardAnimationDurationUserInfoKey = "0.25";
UIKeyboardBoundsUserInfoKey = "NSRect: {{0, 0}, {480, 162}}";
UIKeyboardCenterBeginUserInfoKey = "NSPoint: {240, 401}";
UIKeyboardCenterEndUserInfoKey = "NSPoint: {240, 239}";
UIKeyboardFrameBeginUserInfoKey = "NSRect: {{-162, 0}, {162, 480}}";
UIKeyboardFrameChangedByUserInteraction = 0;
UIKeyboardFrameEndUserInfoKey = "NSRect: {{0, 0}, {162, 480}}";
}}
{name = UIKeyboardDidHideNotification; userInfo = {//键盘消失时发出
UIKeyboardAnimationCurveUserInfoKey = 0;
UIKeyboardAnimationDurationUserInfoKey = "0.25";
UIKeyboardBoundsUserInfoKey = "NSRect: {{0, 0}, {480, 162}}";
UIKeyboardCenterBeginUserInfoKey = "NSPoint: {240, 239}";
UIKeyboardCenterEndUserInfoKey = "NSPoint: {240, 401}";
UIKeyboardFrameBeginUserInfoKey = "NSRect: {{0, 0}, {162, 480}}";
UIKeyboardFrameChangedByUserInteraction = 0;
UIKeyboardFrameEndUserInfoKey = "NSRect: {{-162, 0}, {162, 480}}";
}}
{name = UIKeyboardWillChangeFrameNotification; userInfo = {//屏幕旋转期间并且键盘存在的时候发出
UIKeyboardAnimationCurveUserInfoKey = 0;
UIKeyboardAnimationDurationUserInfoKey = "0.25";
UIKeyboardBoundsUserInfoKey = "NSRect: {{0, 0}, {480, 162}}";
UIKeyboardCenterBeginUserInfoKey = "NSPoint: {240, 401}";
UIKeyboardCenterEndUserInfoKey = "NSPoint: {240, 239}";
UIKeyboardFrameBeginUserInfoKey = "NSRect: {{158, 0}, {162, 480}}";
UIKeyboardFrameChangedByUserInteraction = 0;
UIKeyboardFrameEndUserInfoKey = "NSRect: {{158, 0}, {162, 480}}";
}}
{name = UIKeyboardDidChangeFrameNotification; userInfo = {//屏幕旋转期间并且键盘存在的时候发出
UIKeyboardBoundsUserInfoKey = "NSRect: {{0, 0}, {480, 162}}";
UIKeyboardCenterBeginUserInfoKey = "NSPoint: {240, 401}";
UIKeyboardCenterEndUserInfoKey = "NSPoint: {240, 239}";
UIKeyboardFrameBeginUserInfoKey = "NSRect: {{320, 0}, {162, 480}}";
UIKeyboardFrameChangedByUserInteraction = 0;
UIKeyboardFrameEndUserInfoKey = "NSRect: {{158, 0}, {162, 480}}";
}}下面本文着重讲解关于键盘通知的发出顺序，当一个UITextView或UITextField变成第一焦点时，通知的发出顺序如下：UIKeyboardWillChangeFrameNotification、UIKeyboardWillShowNotification、UIKeyboardDidChangeFrameNotification、UIKeyboardDidShowNotification。
当一个UITextView或UITextField注销焦点状态时，发出通知顺序如下：UIKeyboardWillChangeFrameNotification、UIKeyboardWillHideNotification、UIKeyboardDidChangeFrameNotification、UIKeyboardDidHideNotification。
特别需要注意的是，当屏幕旋转的时候也会发出键盘通知，并且顺序如下：UIKeyboardWillChangeFrameNotification、UIKeyboardWillHideNotification、UIKeyboardDidChangeFrameNotification、UIKeyboardDidHideNotification、UIKeyboardWillChangeFrameNotification、UIKeyboardWillShowNotification、UIKeyboardDidChangeFrameNotification、UIKeyboardDidShowNotification。细心的读者肯定发现了，其顺序是一个UITextView或UITextField注销焦点状态时，发出键盘通知顺序和一个UITextView或UITextField变成第一焦点时发出键盘顺序组合起来一致，也就是说，在屏幕旋转期间，虽然我们看起来好像键盘没啥变化，但实际上通知已经经理了从“消失”到再次“显示”的路径了。所以，平时如果需要捕获键盘通知处理一些业务逻辑的话，需要特别注意通知的顺序，小心谨慎，避免意外的错误。笔者就是在这个地方犯了一个比较严重的错误，检查半天才找出来，希望各位童鞋引以为戒，比如：如果你希望在键盘弹出的时候捕获UIKeyboardDidShowNotification,以便添加一个透明的全屏的按钮（点击该按钮的时候让键盘消失）；然后捕获UIKeyboardDidHideNotification进行了一个动画，在该动画结束后移除上述全屏按钮；这样当屏幕旋转的时候，你就会发现，键盘明明没消失，但是上述全屏按钮已经不翼而飞（请聪明的读者朋友自己想想，这是为什么？）。
~~~~好，本篇结束。