State Pattern 状态模式

Definition

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
当一个对象内在状态改变时允许其改变行为，这个对象看起来像改变了其类。

Participants

The classes and objects participating in this pattern are:

Context (Account)

• defines the interface of interest to clients
• maintains an instance of a ConcreteState subclass that defines the current state.

State (State)

• defines an interface for encapsulating the behavior associated with a particular state of the Context.

Concrete State (RedState, SilverState, GoldState)

• each subclass implements a behavior associated with a state of Context