GoF定義
「定義一個能夠實行在一個物件架構中對於所有元素的操作。訪問者讓你可以定義一個新的操作，而不必更動到被操作元素的類別介面」

Visitor會去參訪所有有被收入的單位，以這個電腦的例子來說，這個Visitor實作的參訪對象就是Computer本身擁有的設備(Keyboard跟Monitor)跟自己本身

IComputerPart.cs

namespace LinXuan.DesignPattern.Example.Visitor 
{ 
    public interface IComputerPart 
    { 
        public void Accept(IComputerPartVisitor computerPartVisitor); 
    } 
}

ComputerPartItem.cs

namespace LinXuan.DesignPattern.Example.Visitor
{
    public class Computer : IComputerPart
    {
        IComputerPart[] parts;

        public Computer()
        {
            parts = new IComputerPart[] { new Keyboard(), new Monitor() };
        }

        public void Accept(IComputerPartVisitor computerPartVisitor)
        {
            for (int i = 0; i < parts.Length; i++)
            {
                parts[i].Accept(computerPartVisitor);
            }

            computerPartVisitor.Visit(this);
        }

        public override string ToString()
        {
            return "Computer 1036";
        }
    }



    public class Keyboard : IComputerPart
    {
        public void Accept(IComputerPartVisitor computerPartVisitor)
        {
            computerPartVisitor.Visit(this);
        }

        public override string ToString()
        {
            return "Keyboard 410";
        }
    }



    public class Monitor : IComputerPart
    {
        public void Accept(IComputerPartVisitor computerPartVisitor)
        {
            computerPartVisitor.Visit(this);
        }

        public override string ToString()
        {
            return "Monitor 4833";
        }
    }
}

IComputerPartVisitor.cs

namespace LinXuan.DesignPattern.Example.Visitor
{
    public interface IComputerPartVisitor
    {
        public void Visit(Keyboard keyboard);
        public void Visit(Monitor monitor);
        public void Visit(Computer computer);
    }
}

ComputerPartDisplayVisitor.cs

using UnityEngine;

namespace LinXuan.DesignPattern.Example.Visitor
{
    public class ComputerPartDisplayVisitor : IComputerPartVisitor
    {
        public void Visit(Keyboard keyboard)
        {
            Debug.Log("Visiting " + keyboard.ToString());
        }

        public void Visit(Monitor monitor)
        {
            Debug.Log("Visiting " + monitor.ToString());
        }

        public void Visit(Computer computer)
        {
            Debug.Log("Visiting " + computer.ToString());
        }
    }
}

VisitorTest.cs

using UnityEngine; 
namespace LinXuan.DesignPattern.Example.Visitor 
{ 
    public class VisitorTest : MonoBehaviour 
    { 
        private void Start() 
        { 
            IComputerPart computer = new Computer(); 
            computer.Accept(new ComputerPartDisplayVisitor()); 
        } 
    } 
}

輸出如下

參考資料

設計模式與遊戲開發的完美結合(暢銷回饋版)
Turn Based Strategy Framework
流離之歌
Design Patterns - Visitor Pattern