訪問者模式(Visitor)

• 表示一個作用於某物件結構中的各元素之操作。它使你可以在不改變各元素之類別的前提下，定義作用於這些元素的新操作。
• 適用於資料結構相對穩定的系統
• 它把資料結構和作用於結構上的操作之間的耦合解脫開，使得操作集合可以相對自由地演化。
• 其目的是要把處理從資料結構分離出來。
• 其缺點係不易增加新的資料結構

using System;
using System.Collections.Generic;

namespace DPExample
{
    // 狀態
    abstract class Action
    {
        protected string name;
        protected string Name
        {
            get { return name; }
            set { name = value; }
        }

        // 得到男人結論或反應
        public abstract void GetManConclusion(Man concreteElementA);

        // 得到女人結論或反應
        public abstract void GetWomanConclusion(Woman concreteElementB);
    }

    // 人
    abstract class Person
    {
        protected string name;
        protected string Name
        {
            get { return name; }
            set { name = value; }
        }

        // 接受
        public abstract void Accept(Action visitor);
    }

    // 具體狀態
    class Success : Action
    {
        public override void GetManConclusion (Man concreteElementA)
        {
            Console.WriteLine ("{0}{1}時，背後多半有一個偉大的女人。", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void GetWomanConclusion(Woman concreteElementB)
        {
            Console.WriteLine ("{0}{1}時，背後多半有一個不成功的女人。", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    class Failing : Action
    {
        public override void GetManConclusion (Man concreteElementA)
        {
            Console.WriteLine ("{0}{1}時，悶頭喝酒，誰也不用勸。", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void GetWomanConclusion(Woman concreteElementB)
        {
            Console.WriteLine ("{0}{1}時，眼淚汪汪，誰也勸不了。", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    class Amativeness : Action
    {
        public override void GetManConclusion (Man concreteElementA)
        {
            Console.WriteLine ("{0}{1}時，凡事不懂也要裝懂。", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void GetWomanConclusion(Woman concreteElementB)
        {
            Console.WriteLine ("{0}{1}時，遇事懂也裝作不懂。", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    class Marriage : Action
    {
        public override void GetManConclusion(Man concreteElementA)
        {
            Console.WriteLine ("{0}{1}時，感慨道：戀愛遊戲終結時，'有妻徒刑' 遙無期。", concreteElementA.GetType().Name, this.GetType().Name);
        }

        public override void GetWomanConclusion(Woman concreteElementB)
        {
            Console.WriteLine ("{0}{1}時，欣慰曰：愛情長跑路漫漫，婚姻保險保平安。", concreteElementB.GetType().Name, this.GetType().Name);
        }
    }

    // 男人
    class Man : Person
    {
        public override void Accept (Action visitor)
        {
            visitor.GetManConclusion(this);
        }
    }

    // 女人
    class Woman : Person
    {
        public override void Accept (Action visitor)
        {
            visitor.GetWomanConclusion(this);
        }
    }

    // 物件結構
    class ObjectStructure
    {
        private IList elements = new List();

        public void Attach (Person element)
        {
            elements.Add (element);
        }

        public void Detach (Person element)
        {
            elements.Remove (element);
        }

        public void Display (Action visitor)
        {
            foreach (Person e in elements)
                e.Accept (visitor);
        }
    }
}

using System;

namespace DPExample
{
 public class Program
 {
  static void Main()
        {
            ObjectStructure o = new ObjectStructure();
            o.Attach (new Man());
            o.Attach (new Woman());

            Success v1 = new Success();
            o.Display (v1);

            Failing v2 = new Failing();
            o.Display (v2);

            Amativeness v3 = new Amativeness();
            o.Display (v3);

            Marriage v4 = new Marriage();
            o.Display (v4);
  }

 }
}