装饰者模式。
1：什么时候用？
当你想通过继承父类来增加新功能时候，考虑用装饰者模式作为替代。在软件开发中，提倡少用继承，多用组合。
在swift中实现装饰者模式，通常用两种方式：extension 和 delegation
The Decorator pattern dynamically adds behaviors and responsibilities to an object without modifying its code. It’s an alternative to subclassing where you modify a class’s behavior by wrapping it with another object.
Swift extensions are slightly different from the classic definition of a decorator, because a extension doesn’t hold an instance of the class it extends.
