结构型模式 代理模式

Definition

Provide a surrogate or placeholder for another object to control access to it.

UML class diagram

Participants

The classes and/or objects participating in the Proxy pattern are: Proxy   (MathProxy) maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same. provides an interface identical to Subject's so that a proxy can be substituted for for the real subject. controls access to the real subject and may be responsible for creating and deleting it. other responsibilites depend on the kind of proxy: remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space. virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real images's extent. protection proxies check that the caller has the access permissions required to perform a request. Subject   (IMath) defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected. RealSubject   (Math) defines the real object that the proxy represents.

Sample code in C#

This structural code demonstrates the Proxy pattern which provides a representative object (proxy) that controls access to another similar object. 
   

// Proxy pattern -- Structural example

using System;

// "Subject"

abstract class Subject
{
  // Methods
  abstract public void Request();
}

// "RealSubject"

class RealSubject : Subject
{
  // Methods
  override public void Request()
  {
    Console.WriteLine("Called RealSubject.Request()");
  }
}

// "Proxy"

class Proxy : Subject
{
  // Fields
  RealSubject realSubject;

  // Methods
  override public void Request()
  {
    // Uses "lazy initialization"
    if( realSubject == null )
      realSubject = new RealSubject();

    realSubject.Request();
  }
}

/// <summary>
/// Client test
/// </summary>
public class Client
{
  public static void Main( string[] args )
  {
    // Create proxy and request a service
    Proxy p = new Proxy();
    p.Request();

  }
}

This real-world code demonstrates the Remote Proxy pattern which provides a representative object that controls access to another object in a different AppDomain. 

   

// Proxy pattern -- Real World example

using System;
using System.Runtime.Remoting;

// "Subject"

public interface IMath
{
  // Methods
  double Add( double x, double y );
  double Sub( double x, double y );
  double Mul( double x, double y );
  double Div( double x, double y );
}

// "RealSubject"

class Math : MarshalByRefObject, IMath
{
  // Methods
  public double Add( double x, double y ){ return x + y; }
  public double Sub( double x, double y ){ return x - y; }
  public double Mul( double x, double y ){ return x * y; }
  public double Div( double x, double y ){ return x / y; }
}

// Remote "Proxy Object"

class MathProxy : IMath
{
  // Fields
  Math math;
  // Constructors
  public MathProxy()
  {
    // Create Math instance in a different AppDomain
    AppDomain ad = System.AppDomain.CreateDomain(
                                 "MathDomain",null, null );
    ObjectHandle o = 
      ad.CreateInstance("Proxy_RealWorld", "Math", false,
      System.Reflection.BindingFlags.CreateInstance, 
      null, null, null,null,null );
    math = (Math) o.Unwrap();
  }

  // Methods
  public double Add( double x, double y )
  {
    return math.Add(x,y);
  }

  public double Sub( double x, double y )
  {
    return math.Sub(x,y);
  }

  public double Mul( double x, double y )
  {
    return math.Mul(x,y);
  }

  public double Div( double x, double y )
  {
    return math.Div(x,y);
  }
}

/// <summary>
/// ProxyApp test
/// </summary>
public class ProxyApp
{
  public static void Main( string[] args )
  {
    // Create math proxy
    MathProxy p = new MathProxy();

    // Do the math
    Console.WriteLine( "4 + 2 = {0}", p.Add( 4, 2 ) );
    Console.WriteLine( "4 - 2 = {0}", p.Sub( 4, 2 ) );
    Console.WriteLine( "4 * 2 = {0}", p.Mul( 4, 2 ) );
    Console.WriteLine( "4 / 2 = {0}", p.Div( 4, 2 ) );
  }
}