设计模式-代理 2022-02-25

代理模式(Proxy Pattern)是一种结构型设计模式，让你能够提供对象的替代品或其占位符。代理控制着对于原对象的访问，并允许在将请求提交给对象前后进行一些处理。

代理模式 UML

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适用场景

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1. 延迟初始化。如果你有一个偶尔使用的重量级服务对象，一直保持该对象运行会消耗系统资源时，可使用代理模式
2. 访问控制。如果你只希望特定客户端使用服务对象，这里的对象可以是操作系统中非常重要的部分，而客户端则是各种已启动的程序(包括恶意程序)，此时可使用代理模式

代码示例

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using System;
namespace ProxyPatternDemo 
{
    public interface ISubject
    {
        void Request();
    }
    
    class RealSubject : ISubject
    {
        public void Request()
        {
            Console.WriteLine("RealSubject: Handling Request.");
        }
    }
    
    class Proxy : ISubject
    {
        private RealSubject _realSubject;
        
        public Proxy(RealSubject realSubject)
        {
            this._realSubject = realSubject;
        }
        
        public void Request()
        {
            if (this.CheckAccess())
            {
                this._realSubject.Request();

                this.LogAccess();
            }
        }
        
        public bool CheckAccess()
        {
            Console.WriteLine("Proxy: Checking access prior to firing a real request.");

            return true;
        }
        
        public void LogAccess()
        {
            Console.WriteLine("Proxy: Logging the time of request.");
        }
    }
    
    public class Client
    {
        public void ClientCode(ISubject subject)
        {
            // ...
            
            subject.Request();
            
            // ...
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            Client client = new Client();
            
            Console.WriteLine("Client: Executing the client code with a real subject:");
            RealSubject realSubject = new RealSubject();
            client.ClientCode(realSubject);

            Console.WriteLine();

            Console.WriteLine("Client: Executing the same client code with a proxy:");
            Proxy proxy = new Proxy(realSubject);
            client.ClientCode(proxy);
            // output

            // Client: Executing the client code with a real subject:
            // RealSubject: Handling Request.

            // Client: Executing the same client code with a proxy:
            // Proxy: Checking access prior to firing a real request.
            // RealSubject: Handling Request.
            // Proxy: Logging the time of request.
        }
    }
}