iOS全局处理键盘事件

1. 注册监听键盘事件的通知
   
   [[NSNotificationCenter defaultCenter] addObserver:self
   selector:@selector(keyboardWillShow:)
   name:UIKeyboardWillShowNotification
   object:nil];
   
   [[NSNotificationCenter defaultCenter] addObserver:self
   selector:@selector(keyboardShow:)
   name:UIKeyboardDidShowNotification
   object:nil];
   
   [[NSNotificationCenter defaultCenter] addObserver:self
   selector:@selector(keyboardWillHide:)
   name:UIKeyboardWillHideNotification
   object:nil];
   
   [[NSNotificationCenter defaultCenter] addObserver:self
   selector:@selector(keyboardHide:)
   name:UIKeyboardDidHideNotification
   object:nil];
2. 在键盘将要出现和隐藏的回调中，加入动画。
   
   - (void)keyboardWillShow:(NSNotification *)notif {
   if (self.hidden == YES) {
   return;
   }
   
   CGRect rect = [[notif.userInfo objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
   CGFloat y = rect.origin.y;
   [UIView beginAnimations:nil context:nil];
   [UIView setAnimationDuration:0.25];
   NSArray *subviews = [self subviews];
   for (UIView *sub in subviews) {
   
   CGFloat maxY = CGRectGetMaxY(sub.frame);
   if (maxY > y - 2) {
   sub.center = CGPointMake(CGRectGetWidth(self.frame)/2.0, sub.center.y - maxY + y - 2);
   }
   }
   [UIView commitAnimations];
   }
   
   - (void)keyboardShow:(NSNotification *)notif {
   if (self.hidden == YES) {
   return;
   }
   }
   
   - (void)keyboardWillHide:(NSNotification *)notif {
   if (self.hidden == YES) {
   return;
   }
   [UIView beginAnimations:nil context:nil];
   [UIView setAnimationDuration:0.25];
   NSArray *subviews = [self subviews];
   for (UIView *sub in subviews) {
   if (sub.center.y < CGRectGetHeight(self.frame)/2.0) {
   sub.center = CGPointMake(CGRectGetWidth(self.frame)/2.0, CGRectGetHeight(self.frame)/2.0);
   }
   }
   [UIView commitAnimations];
   }
   
   - (void)keyboardHide:(NSNotification *)notif {
   if (self.hidden == YES) {
   return;
   }
   }