A sample of how to implement the drag keyboard down effect like Apple's SMS app. 一个能让iOS的键盘可以想Apple的短信程序一样工作的示例程序。 The application is buggy and far from complete. If you know a better implementation, pls join in to make it better. 这个程序目前还不是很完善，如果你有更好的实现方式请加入进来。