http://blog.csdn.net/yongyinmg/article/details/39673345 required修饰符
http://www.open-open.com/lib/view/open1420702464031.html swift初始化的安全方式
http://m.blog.csdn.net/article/details?id=51150952 swift @available #available
原文：http://www.cnblogs.com/yaoyao0110/p/5486146.html