一句话Design Pattern：Strategy

定义：The strategy pattern defines a family of algorithm, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

一句话：action不是object里面hardcode的一部分而是一个object instance，所以action可以runtime换成你需要的。

类比：跟赛车游戏里面给汽车换零件的道理一样：同一个功能不同的实现，需要的时候换一个。