The Visitor design pattern represents an operation to be performed on the elements of an object structure. This pattern lets you define a new operation (ObjectStructure) without changing the classes of the elements on which it operates.

簡單來說~ 此模式讓你能定義一個新的操作來訪問對象結構~ 無須動到原本的結構~

學習目標: 訪問者模式的概念及實務

學習難度: ☆☆☆

using System;

using System.Collections.Generic;

namespace ConsoleApp1
{

    //為對象結構中的每個 ConcreteElement 類聲明一個訪問操作。

    public abstract class Visitor
    {
        public abstract void VisitConcreteProductA(ProductA concreteProductA);
    }

    //實現訪問者聲明的每個操作。每個操作都實現了為結構中的相應類或對象定義的算法片段。

    public class ConcreteVisitor1 : Visitor
    {
        public override void VisitConcreteProductA(ProductA concreteProductA)
        {
            Console.WriteLine(concretePlayerA.GetType().Name + " visited by "+this.GetType().Name);
        }
    }


    //定義一個以訪問者為參數的 Accept 操作
    public abstract class Product
    {
        public abstract void Accept(Visitor visitor);
    }


    //實現以訪問者為參數的 Accept 操作
    public class ProductA : Product
    {
        public override void Accept(Visitor visitor)
        {
            visitor.VisitConcreteProductA(this);
        }
    }

    //可以提供高級界面以允許訪問者訪問其元素，例如列表或集合

    public class ProductManager
    {
        List<Product> products = new List<Product>();

        public void Attach(Product product)
        {
            products.Add(product);
        }

        public void Detach(Product product)
        {
            products.Remove(product);
        }

        public void Accept(Visitor visitor)
        {
            foreach (Product product in products)
            {
                product.Accept(visitor);
            }
        }
    }

    public class MainProgram
    {

        public static void Main(string[] args)
        {
            ProductManager productManager = new ProductManager();

            Product productA = new ProductA();

            productManager.Attach(productA);

            ConcreteVisitor1 visitor1 = new ConcreteVisitor1();

            productManager.Accept(visitor1);
        }
    }

}

參考資料:

https://www.dofactory.com/net/visitor-design-pattern