设计模式-访问者 2022-03-07

访问者模式(Visitor Pattern)是一种行为型设计模式，它能将算法与其所作用的对象隔离开来。

解释

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访问者模式建议将新行为放入一个名为访问者的独立类中，而不是试图将其整合到已有类中。现在，需要执行操作的原始对象将作为参数被传递给访问者中的方法，让方法能访问对象所包含的一切必要数据。

访问者模式 UML

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适用场景

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1. 如果你需要对一个复杂对象结构(例如对象树)中的所有元素执行某些操作，可使用访问者模式
2. 可使用访问者模式来清理辅助行为的业务逻辑
3. 当某个行为仅在类层次结构中的一些类中有意义，而在其他类中没有意义时，可使用该模式

代码示例

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using System;
namespace VisitorPatternDemo 
{
    // 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

            // 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
        }
    }
}