键盘大小改变的消息

在你的程序中加入监听[[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(keyboardWillShow:) 
                                                 name:UIKeyboardWillShowNotification
                                               object:nil];
切换输入法的时候会调用keyboardWillShow方法，用下面的方法就能取得输入键盘的高度（仔细看看API）：
- (void)keyboardWillShow:(id)sender {
    CGRect keyboardFrame;
    [[[((NSNotification *)sender) userInfo] objectForKey:UIKeyboardBoundsUserInfoKey] getValue:&keyboardFrame];
    keyboardHeight = CGRectGetHeight(keyboardFrame);
｝