用设计模式固化你的C#程序(5-2)

【译注：以下是state模式完整示例

C#示例：

using System;

abstract class State

{

public virtual void AddNickel(VendingMachine vm) { }

public virtual void AddDime(VendingMachine vm) { }

public virtual void AddQuarter(VendingMachine vm) { }

protected virtual void ChangeState(VendingMachine vm, State s)

{

vm.ChangeState(s);

}

}

class VendingMachine

{

private State state;

public VendingMachine()

{

Console.WriteLine("The Vending Machine is now online: product costs 25c");

state = Start.Instance();

}

public void ChangeState(State to)

{

state = to;

}

public void Vend()

{

// 发饮料

Console.WriteLine("Dispensing product...Thank you!");

}

public void AddNickel()

{

state.AddNickel(this);

}

public void AddDime()

{

state.AddDime(this);

}

public void AddQuarter()

{

state.AddQuarter(this);

}

}

class Start : State

{

private static State state = new Start();

private Start()

{

}

public static State Instance()

{

// singleton逻辑

Console.WriteLine("Credit: 0c");

return state;

}

public override void AddNickel(VendingMachine vm)

{

ChangeState(vm, Five.Instance());

}

public override void AddDime(VendingMachine vm)

{

ChangeState(vm, Ten.Instance());

}

public override void AddQuarter(VendingMachine vm)

{

vm.Vend();

}

}

class Five : State

{

private static State state = new Five();

private Five()

{

}

public static State Instance()

{

// singleton 逻辑

Console.WriteLine("Credit: 5c");

return state;

}

public override void AddNickel(VendingMachine vm)

{

ChangeState(vm, Ten.Instance());

}

public override void AddDime(VendingMachine vm)

{

ChangeState(vm, Fifteen.Instance());

}

public override void AddQuarter(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance()); // no change returned :-)

}

}

class Ten : State

{

private static State state = new Ten();

private Ten()

{

}

public static State Instance()

{

// singleton 逻辑

Console.WriteLine("Credit: 10c");

return state;

}

public override void AddNickel(VendingMachine vm)

{

ChangeState(vm, Fifteen.Instance());

}

public override void AddDime(VendingMachine vm)

{

ChangeState(vm, Twenty.Instance());

}

public override void AddQuarter(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance()); // no change returned :-)

}

}

class Fifteen : State

{

private static State state = new Fifteen();

private Fifteen()

{

}

public static State Instance()

{

// singleton 逻辑

Console.WriteLine("Credit: 15c");

return state;

}

public override void AddNickel(VendingMachine vm)

{

ChangeState(vm, Twenty.Instance());

}

public override void AddDime(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance());

}

public override void AddQuarter(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance()); // no change returned :-)

}

}

class Twenty : State

{

private static State state = new Twenty();

private Twenty()

{

}

public static State Instance()

{

// singleton 逻辑

Console.WriteLine("Credit: 20c");

return state;

}

public override void AddNickel(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance());

}

public override void AddDime(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance());

}

public override void AddQuarter(VendingMachine vm)

{

vm.Vend();

ChangeState(vm, Start.Instance()); // no change returned :-)

}

}

class Application

{

public static void Main()

{

int coin = 0;

string input = null;

VendingMachine vm = new VendingMachine();

while (true)

{

Console.Write("Insert a coin (5, 10, 25): ");

input = Console.ReadLine();

coin = Int32.Parse(input);

switch (coin)

{

case 5:

vm.AddNickel();

break;

case 10:

vm.AddDime();

break;

case 25:

vm.AddQuarter();

break;

default:

break;

}

}

}

}

/*以下是某次运行时输出结果：

The Vending Machine is now online: product costs 25c

Credit: 0c

Insert a coin <5, 10, 25>: 5

Credit: 5c

Insert a coin <5, 10, 25>: 10

Credit: 15c

Insert a coin <5, 10, 25>: 5

Credit: 20c

Insert a coin <5, 10, 25>: 5

Dispensing product...Thank you!

*/