//负责状态管理的环境类 | The context class in charge of state management
public class Context {
    State CurrentState;   //此字段记录了当前状态 | This field records the current stage
    public void CurrentStateFunction( ) {   //当前状态下应执行的功能 | Functions to execute on current stage
        CurrentState.Handler( );   //通过当前状态的Handler方法执行功能 | Execute the functions via the Handler() of the current state.
    }
    public void SetState(State StateIn) {   //设置或修改当前State的方法 | Method to set or modify current state.
        CurrentState = StateIn;  //将当前状态修改为参数提供的状态实例 | Turn the current state into the instance from the parameter.
    }
}

//所有具体状态类的基类，定义了具体状态类应该包含的成员结构 | The base class for all classes of concrete states. It defines all the 
public abstract class State {
    //每个State必须知道由哪个Context实例管理 | Each state must be managed by a context instance.
    protected Context _Context=null;
    public State (Context ContextIn) {   //在构建State实例时须指定Context实例 | Must specify a context instance on construction.
        _Context = ContextIn;
    }
    //每个State的抽象功能，待派生具体类重写 | Abstract functions of a state to be overridden in derived classes of concrete states.
    public abstract void Handler( ); 
}

public class StateA : State {   //继承了抽象State类的具体类StateA | Concrete State A deriving from the abstract State class
    public StateA (Context ContextIn): base (ContextIn) { }   //派生类不继承构造函数，必须写明 | Constructors are not inherited.
    public override void Handler( ) {   //重写当前状态下的功能 | Override the functions of the current state.
        if(Input.GetKeyDown(KeyCode.Alpha2)) {   //在StateA状态下可按2跳至StateB | In StateA, you may press 2 to jump to StateB.
            Debug.Log("From StateA to StateB.");
            _Context.SetState(new StateB(_Context));   //跳转到下一个状态的语句 | Statement used to jump to the next state.
        }
    }
}

public class StateB : State {   //继承了抽象State类的具体类StateB | Concrete State B deriving from the abstract State class
    public StateB (Context ContextIn): base (ContextIn) { }
    public override void Handler( ) {
        if(Input.GetKeyDown(KeyCode.Alpha3)) {
            Debug.Log("From StateB to StateC.");
            _Context.SetState(new StateC(_Context));
        }
    }
}

public class StateC : State {   //继承了抽象State类的具体类StateC | Concrete State C deriving from the abstract State class
    public StateC (Context ContextIn): base (ContextIn) { }
    public override void Handler( ) {
        if(Input.GetKeyDown(KeyCode.Alpha1)) {
            Debug.Log("From StateC to StateA.");
            _Context.SetState(new StateA(_Context));
        }
    }
}

//客户端代码 | Client Code
public class test : MonoBehaviour
{
    //此Context类型的字段实例用于管理当前状态并实现状态跳转 |
    //The instance of this field whose type is Context is meant to manage the current state and switch between states.
    Context testContext=new Context( );

    void Start( ) {
        //创建一个StateA的实例，并指明其由testContext这个实例管理 |
        //Create an instance of StateA and specify that it is managed by the testContext instance.
        State TempState = new StateA(testContext);
        //通过调用testContext实例的SetState方法将刚创建的TempState设置为当前状态 |
        //Use the SetState( ) method of the testContext instance to set the current state to TempState.
        testContext.SetState(TempState);
    }
    void Update( ) {
        //在每一帧调用testContext实例的CurrentStateFunction方法，实质上执行的是在各实际状态类中定义的功能 |
        //Call the CurrentStateFunction( ) of the testContext instance and actually execute functions defined in the concrete states.
        testContext.CurrentStateFunction( );
    }
}