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访问者

访问者在 C# 中的实现

访问者是一种行为设计模式 允许你在不修改已有代码的情况下向已有类层次结构中增加新的行为

阅读我们的文章访问者和双分派以了解为什么不能通过方法重载来简单地替换访问者

进一步了解访问者模式

在 C# 中使用模式

复杂度

流行度

使用示例 访问者不是常用的设计模式 因为它不仅复杂 应用范围也比较狭窄

概念示例

本例说明了访问者设计模式的结构并重点回答了下面的问题

  • 它由哪些类组成
  • 这些类扮演了哪些角色
  • 模式中的各个元素会以何种方式相互关联

Program.cs: 概念示例

using System;
using System.Collections.Generic;

namespace RefactoringGuru.DesignPatterns.Visitor.Conceptual
{
    // The Component interface declares an `accept` method that should take the
    // base visitor interface as an argument.
    public interface IComponent
    {
        void Accept(IVisitor visitor);
    }

    // Each Concrete Component must implement the `Accept` method in such a way
    // that it calls the visitor's method corresponding to the component's
    // class.
    public class ConcreteComponentA : IComponent
    {
        // Note that we're calling `VisitConcreteComponentA`, which matches the
        // current class name. This way we let the visitor know the class of the
        // component it works with.
        public void Accept(IVisitor visitor)
        {
            visitor.VisitConcreteComponentA(this);
        }

        // Concrete Components may have special methods that don't exist in
        // their base class or interface. The Visitor is still able to use these
        // methods since it's aware of the component's concrete class.
        public string ExclusiveMethodOfConcreteComponentA()
        {
            return "A";
        }
    }

    public class ConcreteComponentB : IComponent
    {
        // Same here: VisitConcreteComponentB => ConcreteComponentB
        public void Accept(IVisitor visitor)
        {
            visitor.VisitConcreteComponentB(this);
        }

        public string SpecialMethodOfConcreteComponentB()
        {
            return "B";
        }
    }

    // The Visitor Interface declares a set of visiting methods that correspond
    // to component classes. The signature of a visiting method allows the
    // visitor to identify the exact class of the component that it's dealing
    // with.
    public interface IVisitor
    {
        void VisitConcreteComponentA(ConcreteComponentA element);

        void VisitConcreteComponentB(ConcreteComponentB element);
    }

    // Concrete Visitors implement several versions of the same algorithm, which
    // can work with all concrete component classes.
    //
    // You can experience the biggest benefit of the Visitor pattern when using
    // it with a complex object structure, such as a Composite tree. In this
    // case, it might be helpful to store some intermediate state of the
    // algorithm while executing visitor's methods over various objects of the
    // structure.
    class ConcreteVisitor1 : IVisitor
    {
        public void VisitConcreteComponentA(ConcreteComponentA element)
        {
            Console.WriteLine(element.ExclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor1");
        }

        public void VisitConcreteComponentB(ConcreteComponentB element)
        {
            Console.WriteLine(element.SpecialMethodOfConcreteComponentB() + " + ConcreteVisitor1");
        }
    }

    class ConcreteVisitor2 : IVisitor
    {
        public void VisitConcreteComponentA(ConcreteComponentA element)
        {
            Console.WriteLine(element.ExclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor2");
        }

        public void VisitConcreteComponentB(ConcreteComponentB element)
        {
            Console.WriteLine(element.SpecialMethodOfConcreteComponentB() + " + ConcreteVisitor2");
        }
    }

    public class Client
    {
        // The client code can run visitor operations over any set of elements
        // without figuring out their concrete classes. The accept operation
        // directs a call to the appropriate operation in the visitor object.
        public static void ClientCode(List<IComponent> components, IVisitor visitor)
        {
            foreach (var component in components)
            {
                component.Accept(visitor);
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<IComponent> components = new List<IComponent>
            {
                new ConcreteComponentA(),
                new ConcreteComponentB()
            };

            Console.WriteLine("The client code works with all visitors via the base Visitor interface:");
            var visitor1 = new ConcreteVisitor1();
            Client.ClientCode(components,visitor1);

            Console.WriteLine();

            Console.WriteLine("It allows the same client code to work with different types of visitors:");
            var visitor2 = new ConcreteVisitor2();
            Client.ClientCode(components, visitor2);
        }
    }
}

Output.txt: 执行结果

The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1

It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2

访问者在其他编程语言中的实现

访问者在 Java 中的实现 访问者在 PHP 中的实现 访问者在 Python 中的实现 访问者在 Ruby 中的实现 访问者在 Swift 中的实现 访问者在 TypeScript 中的实现