数据结构+算法=程序
程序+设计模式=软件

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状态模式(State)

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官方定义

允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它的类。

分类

行为型模式

思想

将本来是针对对象的操作，委托给了这个对象的状态操作。运用状态模式可以将代码中的if，else代码块抽离出来，每个if，else下的代码块都封装为状态的某一个方法，所有状态都必须实现状态接口，即实现抽离出来的所有方法，由各自去执行在该状态下应该实现的逻辑，包括不合理的逻辑，可以抛出异常，这样可以规避大量的if，else嵌套。

UML类图

实现

定义状态机Content用来维护自身状态

比如Content代表一个糖果机，它有4种状态：售空、未投币、已投币、售出糖果，我们还可以很容易的为它附加一个中奖状态，稍后就可以看到这比不使用设计模式容易得多，否则会增加很多if，else判断。
糖果机包含5个操作：投币、退币、拉动遥控、出糖果、填充糖果。

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/// <summary>
/// 糖果机-状态机
/// </summary>
class GumballMachine
{
    public State.State soldOutState { get; private set; }
    public State.State noQuarterState { get; private set; }
    public State.State hasQuarterState { get; private set; }
    public State.State soldState { get; private set; }
    public State.State winnerState { get; private set; }
    public int count { get; private set; }
    State.State state;

    public GumballMachine(int numberGumballs)
    {
        soldOutState = new SoldOutState(this);
        noQuarterState = new NoQuarterState(this);
        hasQuarterState = new HasQuarterState(this);
        soldState = new SoldState(this);
        winnerState = new WinnerState(this);

        count = numberGumballs;
        state = count > 0 ? noQuarterState : soldOutState;
    }

    /// <summary>
    /// 糖果机投币操作
    /// </summary>
    public void InsertQuarter()
    {
        state.InsertQuarter();
    }

    /// <summary>
    /// 糖果机退币操作
    /// </summary>
    public void EjectQuarter()
    {
        state.EjectQuarter();
    }

    /// <summary>
    /// 糖果机摇杆操作
    /// </summary>
    public void TurnCrank()
    {
        if (state.TurnCrank())
            state.Dispense();
    }

    public void SetState(State.State state)
    {
        this.state = state;
    }

    public void ReleaseBall()
    {
        Console.WriteLine("1颗糖果从卡槽滚了出来..");
        count = Math.Max(count - 1, 0);
    }

    public void Refill(int number)
    {
        count = number;
        state = count > 0 ? noQuarterState : soldOutState;
    }

    public override string ToString()
    {
        return string.Format("虚拟糖果机#1001:\n糖果数量:{0}\n当前状态:{1}", count, state);
    }
}

定义状态接口

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/// <summary>
/// 对于糖果机的每种状态，都实现4种操作
/// </summary>
abstract class State
{
    protected GumballMachine gumballMachine { get; private set; }
    public State(GumballMachine gumballMachine)
    {
        this.gumballMachine = gumballMachine;
    }

    /// <summary>
    /// 投币
    /// </summary>
    public abstract void InsertQuarter();
    /// <summary>
    /// 退币
    /// </summary>
    public abstract void EjectQuarter();
    /// <summary>
    /// 搬动摇杆
    /// </summary>
    public abstract bool TurnCrank();
    /// <summary>
    /// 出糖果
    /// </summary>
    public abstract void Dispense();
}

为每种状态实现具体的操作

状态下会有很多无效操作，比如未投币就拉摇杆，未使用模式的情况下，我们肯定会在拉摇杆方法中用if，else进行判断，现在就不用了，由状态自身去判断。

售空状态

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/// <summary>
/// 糖果售完状态
/// </summary>
class SoldOutState : State
{
    public SoldOutState(GumballMachine machine) : base(machine)
    {

    }
    public override void Dispense()
    {
        Console.WriteLine("你必须先投币");
    }

    public override void EjectQuarter()
    {
        Console.WriteLine("无法退币");
    }

    public override void InsertQuarter()
    {
        Console.WriteLine("糖果已售空，无法投币");
    }

    public override bool TurnCrank()
    {
        Console.WriteLine("你还没有投币，拉动摇杆无反应");
        return false;
    }

    public override string ToString()
    {
        return "糖果已售空";
    }
}

未投币状态

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/// <summary>
/// 未投币状态
/// </summary>
class NoQuarterState : State
{
    public NoQuarterState(GumballMachine machine) : base(machine)
    {

    }
    public override void Dispense()
    {
        Console.WriteLine("你必须先投币");
    }

    public override void EjectQuarter()
    {
        Console.WriteLine("你还没有投币，无法退币");
    }

    public override void InsertQuarter()
    {
        Console.WriteLine("你投入了硬币");
        gumballMachine.SetState(gumballMachine.hasQuarterState);
    }

    public override bool TurnCrank()
    {
        Console.WriteLine("你还没有投币，拉动摇杆无反应");
        return false;
    }

    public override string ToString()
    {
        return "等待投币";
    }
}

投币状态

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/// <summary>
/// 投完币状态
/// </summary>
class HasQuarterState : State
{
    public HasQuarterState(GumballMachine machine) : base(machine)
    {

    }

    public override void Dispense()
    {
        Console.WriteLine("没有糖果放出来");
    }

    public override void EjectQuarter()
    {
        Console.WriteLine("硬币已退出");
        gumballMachine.SetState(gumballMachine.noQuarterState);
    }

    public override void InsertQuarter()
    {
        Console.WriteLine("你已经投过币了，请不要投2次");
    }

    /// <summary>
    /// 拉动摇杆的时候，10%概率出现中奖状态
    /// </summary>
    /// <returns></returns>
    public override bool TurnCrank()
    {
        Console.WriteLine("你转动了摇杆..");
        Random rd = new Random();
        if (rd.Next(10) == 0 && gumballMachine.count > 1)
            gumballMachine.SetState(gumballMachine.winnerState);
        else
            gumballMachine.SetState(gumballMachine.soldState);
        return true;
    }

    public override string ToString()
    {
        return "机器已投币";
    }
}

售出状态

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/// <summary>
/// 售出状态
/// </summary>
class SoldState : State
{
    public SoldState(GumballMachine machine) : base(machine)
    {

    }
    public override void Dispense()
    {
        gumballMachine.ReleaseBall();
        if (gumballMachine.count > 0)
            gumballMachine.SetState(gumballMachine.noQuarterState);
        else
        {
            Console.WriteLine("糖果已售空！");
            gumballMachine.SetState(gumballMachine.soldOutState);
        }
    }

    public override void EjectQuarter()
    {
        Console.WriteLine("对不起，你已经拉动摇杆了，无法退币");
    }

    public override void InsertQuarter()
    {
        Console.WriteLine("请等待，我们正在为你放出糖果");
    }

    public override bool TurnCrank()
    {
        Console.WriteLine("拉动摇杆2次也不会出来第2颗糖果！");
        return false;
    }

    public override string ToString()
    {
        return "正在出糖果";
    }
}

中奖状态

可以出来2颗糖果

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class WinnerState : State
{
    public WinnerState(GumballMachine machine) : base(machine)
    {

    }
    public override void Dispense()
    {
        Console.WriteLine("恭喜中奖了，你可以获得2颗糖果");
        gumballMachine.ReleaseBall();
        if (gumballMachine.count > 0)
        {
            gumballMachine.ReleaseBall();
            if (gumballMachine.count > 0)
                gumballMachine.SetState(gumballMachine.noQuarterState);
            else
            {
                Console.WriteLine("糖果已售空！");
                gumballMachine.SetState(gumballMachine.soldOutState);
            }
        }                
        else
        {
            Console.WriteLine("糖果已售空！");
            gumballMachine.SetState(gumballMachine.soldOutState);
        }
    }

    public override void EjectQuarter()
    {
        Console.WriteLine("对不起，你已经拉动摇杆了，无法退币");
    }

    public override void InsertQuarter()
    {
        Console.WriteLine("请等待，我们正在为你放出糖果");
    }

    public override bool TurnCrank()
    {
        Console.WriteLine("拉动摇杆2次也不会出来第2颗糖果！");
        return false;
    }

    public override string ToString()
    {
        return "中奖了，正在出糖果";
    }
}

测试用例

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static void Main(string[] args)
{
    GumballMachine gumballMachine = new GumballMachine(5);

    Console.WriteLine(gumballMachine);

    gumballMachine.InsertQuarter();
    gumballMachine.TurnCrank();

    Console.WriteLine(gumballMachine);

    gumballMachine.InsertQuarter();
    gumballMachine.TurnCrank();
    gumballMachine.InsertQuarter();
    gumballMachine.TurnCrank();

    Console.WriteLine(gumballMachine);

    Console.ReadKey();
}

输出结果