热门问题
时间线
聊天
视角

代理模式

来自维基百科,自由的百科全书

代理模式
Remove ads

代理模式(英语:proxy pattern),是程式设计中的一种设计模式。所谓的代理者是指一个类别可以作为其它东西的介面。代理者可以作任何东西的介面:网路连接、记忆体中的大物件、档案或其它昂贵或无法复制的资源。

Thumb
代理模式的统一建模语言(UML)

概述

著名的代理模式例子为参照计数(英语:reference counting)指标物件。当一个复杂物件的多份副本须存在时,代理模式可以结合享元模式以减少记忆体用量。典型作法是建立一个复杂物件及多个代理者,每个代理者会参照到原本的复杂物件。而作用在代理者的运算会转送到原本物件。一旦所有的代理者都不存在时,复杂物件会被移除。

结构

Thumb
代理模式的样例UML类图和序列图[1]

在上面的UML类图中,Proxy类实现了Subject接口,使它充当Subject对象的代替者。它维护一个引用(realSubject)至被代替的对象(RealSubject),使其可以转发给它(realSubject.operation())。

序列图展示了运行时交互:The Client对象通过合作于Proxy对象,来控制到RealSubject对象的访问。在这个例子中,Proxy转发请求至RealSubject,它办理这个请求。

示例

Java

以下Java范例解释"虚拟代理"模式。ProxyImage 类别用来存取远端方法。

import java.util.*;
 
interface Image {
    public void displayImage();
}

//on System A 
class RealImage implements Image {
    private String filename;
    public RealImage(String filename) { 
        this.filename = filename;
        loadImageFromDisk();
    }

    private void loadImageFromDisk() {
        System.out.println("Loading   " + filename);
    }

    public void displayImage() { 
        System.out.println("Displaying " + filename); 
    }
}

//on System B 
class ProxyImage implements Image {
    private String filename;
    private Image image;
 
    public ProxyImage(String filename) { 
        this.filename = filename; 
    }
    public void displayImage() {
        if(image == null)
              image = new RealImage(filename);
        image.displayImage();
    }
}
 
class ProxyExample {
    public static void main(String[] args) {
        Image image1 = new ProxyImage("HiRes_10MB_Photo1");
        Image image2 = new ProxyImage("HiRes_10MB_Photo2");     
        
        image1.displayImage(); // loading necessary
        image2.displayImage(); // loading necessary
    }
}

程式的输出为:

 Loading    HiRes_10MB_Photo1
 Displaying HiRes_10MB_Photo1
 Loading    HiRes_10MB_Photo2
 Displaying HiRes_10MB_Photo2
Remove ads

C#

C#的例子:

using System;

namespace Proxy {
    interface IImage {
        void Display();
    }

    class RealImage : IImage {
        public RealImage(string fileName) {
            FileName = fileName;
            LoadFromFile();
        }

        private void LoadFromFile() {
            Console.WriteLine("Loading " + FileName);
        }

        public String FileName { get; private set; }

        public void Display() {
            Console.WriteLine("Displaying " + FileName);
        }
    }

    class ProxyImage : IImage {
        public ProxyImage(string fileName) {
            FileName = fileName;
        }

        public String FileName { get; private set; }

        private IImage image;

        public void Display() {
            if (image == null)
                image = new RealImage(FileName);
            image.Display();
        }
    }
    class Program {
        static void Main(string[] args) {
            IImage image = new ProxyImage("HiRes_Image");
            for (int i = 0; i < 10; i++)
                image.Display();
        }
    }
}

程式的输出为:

 Loading HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
 Displaying HiRes_Image
Remove ads

Python

下面是Python的例子:

from abc import ABC, abstractmethod

class Image(ABC):
    @abstractmethod
    def display(self): pass

class RealImage(Image):
    def __init__(self, filename):
        self.filename = filename
        self.load_from_disk()
    def load_from_disk(self):
        print("Loading    ", self.filename)
    def display(self):
        print("Displaying ", self.filename)

class ProxyImage(Image):
    def __init__(self, filename):
        self.filename = filename
        self.image = None
    def display(self):
        if self.image is None:
            self.image = RealImage(self.filename)
        self.image.display()

def example():
    image1 = ProxyImage("HiRes_10MB_Photo1")
    image2 = ProxyImage("HiRes_10MB_Photo2")
    image1.display()
    image2.display()
    image1.display()
    image2.display()

其执行:

>>> example()
Loading     HiRes_10MB_Photo1
Displaying  HiRes_10MB_Photo1
Loading     HiRes_10MB_Photo2
Displaying  HiRes_10MB_Photo2
Displaying  HiRes_10MB_Photo1
Displaying  HiRes_10MB_Photo2
Remove ads

另见

引用

外部链接

Loading related searches...

Wikiwand - on

Seamless Wikipedia browsing. On steroids.

Remove ads