3.swift修饰符

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

http://www.yiibai.com/swift/class_inheritance_and_initialization.html  Swift类的继承和构造过程

http://www.jianshu.com/p/b5c87824e33c  swift坑（oc对比）