代理模式

代理模式（英語：proxy pattern），是程式設計中的一種設計模式。所謂的代理者是指一個類別可以作為其它東西的介面。代理者可以作任何東西的介面：網絡連接、記憶體中的大物件、檔案或其它昂貴或無法複製的資源。

代理模式的統一建模語言(UML)圖

概述

著名的代理模式例子為參照計數（英語：reference counting）指標物件。當一個複雜物件的多份副本須存在時，代理模式可以結合享元模式以減少記憶體用量。典型作法是建立一個複雜物件及多個代理者，每個代理者會參照到原本的複雜物件。而作用在代理者的運算會轉送到原本物件。一旦所有的代理者都不存在時，複雜物件會被移除。

結構

代理模式的樣例UML類圖和序列圖^[1]。

在上面的UML類圖中，Proxy類實現了Subject介面，使它充當Subject對象的代替者。它維護一個參照（realSubject）至被代替的對象（RealSubject），使其可以轉發給它（realSubject.operation()）。

序列圖展示了執行時互動：The Client對象通過合作於Proxy對象，來控制到RealSubject對象的訪問。在這個例子中，Proxy轉發請求至RealSubject，它辦理這個請求。

範例

C++

下面是C++的例子：

import std;

using std::string;
using std::unique_ptr;

// Subject Interface
class Image {
public:
    virtual void display() const = 0;
    virtual ~Image() = default;
};

// Real Subject (expensive to load)
class RealImage : public Image {
private:
    string filename;
public:
    explicit RealImage(const string& file):
        filename{file} {
        loadFromDisk();
    }

    void display() const override {
        std::println("Displaying image: {}", filename);
    }

private:
    void loadFromDisk() const {
        std::println("Loading image from disk: {}", filename);
    }
};

// Proxy (controls access to RealImage)
class ProxyImage : public Image {
private:
    string filename;
    mutable unique_ptr<RealImage> realImage; // lazily created
public:
    explicit ProxyImage(const string& file):
        filename{file} {}

    void display() const override {
        if (!realImage) {
            std::println("(Proxy) Loading image on demand...");
            realImage = std::make_unique<RealImage>(filename);
        }
        realImage->display();
    }
};

// Client code
int main() {
    Image* img = new ProxyImage("photo.png");

    // First display (should load)
    img->display();

    // Second display (should use cached image)
    img->display();

    delete img;
    return 0;
}

Java

以下Java範例解釋惰性載入模式中的「虛擬代理」方法。ProxyImage 類別用來存取遠端方法。

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
    }
}

程式的輸出為：

 Loading    HiRes_10MB_Photo1
 Displaying HiRes_10MB_Photo1
 Loading    HiRes_10MB_Photo2
 Displaying HiRes_10MB_Photo2

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();
        }
    }
}

程式的輸出為：

 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

Python

下面是Python的上述類似記憶化反向代理的例子：

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()

其執行：

>>> 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

另見

• 合成模式
• 修飾模式
• 惰性初始化