结构型模式，装饰模式

Definition

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

UML class diagram

Participants

The classes and/or objects participating in the Decorator pattern are: Component   (LibraryItem) defines the interface for objects that can have responsibilities added to them dynamically. ConcreteComponent   (Book, Video) defines an object to which additional responsibilities can be attached. Decorator   (Decorator) maintains a reference to a Component object and defines an interface that conforms to Component's interface. ConcreteDecorator   (Borrowable) adds responsibilities to the component.

Sample code in C#
This structural code demonstrates the Decorator pattern which dynamically adds extra functionality to an existing object.

// Decorator pattern -- Structural example

using System;

// "Component"

abstract class Component
{
  // Methods
  abstract public void Operation();
}

// "ConcreteComponent"

class ConcreteComponent : Component
{
  // Methods
  override public void Operation()
  {
    Console.WriteLine("ConcreteComponent.Operation()");
  }
}

// "Decorator"

abstract class Decorator : Component
{
  // Fields
  protected Component component;

  // Methods
  public void SetComponent( Component component )
  {
    this.component = component;
  }

  override public void Operation()
  {
    if( component != null )
      component.Operation();
  }
}

// "ConcreteDecoratorA"

class ConcreteDecoratorA : Decorator
{
  // Fields
  private string addedState;

  // Methods
  override public void Operation()
  {
    base.Operation();
    addedState = "new state";
    Console.WriteLine("ConcreteDecoratorA.Operation()");
  }
}

// "ConcreteDecoratorB"

class ConcreteDecoratorB : Decorator
{
  // Methods
  override public void Operation()
  {
    base.Operation();
    AddedBehavior();
    Console.WriteLine("ConcreteDecoratorB.Operation()");
  }

  void AddedBehavior()
  {
  }
}

/// <summary>
/// Client test
/// </summary>
public class Client
{
  public static void Main( string[] args )
  {
    // Create ConcreteComponent and two Decorators
    ConcreteComponent c = new ConcreteComponent();
    ConcreteDecoratorA d1 = new ConcreteDecoratorA();
    ConcreteDecoratorB d2 = new ConcreteDecoratorB();

    // Link decorators
    d1.SetComponent( c );
    d2.SetComponent( d1 );

    d2.Operation();
  }
}

This real-world code demonstrates the Decorator pattern in which 'borrowable' functionality is added to existing library items (books and videos).

// Decorator pattern -- Real World example

using System;
using System.Collections;

// "Component"

abstract class LibraryItem
{
  // Fields
  private int numCopies;
  // Properties

  public int NumCopies
  {
    get{ return numCopies; }
    set{ numCopies = value; }
  }

  // Methods
  public abstract void Display();
}

// "ConcreteComponent"

class Book : LibraryItem
{
  // Fields
  private string author;
  private string title;

  // Constructors
  public Book(string author,string title,int numCopies)
  {
    this.author = author;
    this.title = title;
    this.NumCopies = numCopies;
  }

  // Methods
  public override void Display()
  {
    Console.WriteLine( "\nBook ------ " );
    Console.WriteLine( " Author: {0}", author );
    Console.WriteLine( " Title: {0}", title );
    Console.WriteLine( " # Copies: {0}", NumCopies );
  }
}

// "ConcreteComponent"

class Video : LibraryItem
{
  // Fields
  private string director;
  private string title;
  private int playTime;

  // Constructor
  public Video( string director, string title,
                      int numCopies, int playTime )
  {
    this.director = director;
    this.title = title;
    this.NumCopies = numCopies;
    this.playTime = playTime;
  }

  // Methods
  public override void Display()
  {
    Console.WriteLine( "\nVideo ----- " );
    Console.WriteLine( " Director: {0}", director );
    Console.WriteLine( " Title: {0}", title );
    Console.WriteLine( " # Copies: {0}", NumCopies );
    Console.WriteLine( " Playtime: {0}", playTime );
  }
}

// "Decorator"

abstract class Decorator : LibraryItem
{
  // Fields
  protected LibraryItem libraryItem;

  // Constructors
  public Decorator ( LibraryItem libraryItem )
  {
    this.libraryItem = libraryItem;
  }

  // Methods
  public override void Display()
  {
    libraryItem.Display();
  }
}

// "ConcreteDecorator"

class Borrowable : Decorator
{
  // Fields
  protected ArrayList borrowers = new ArrayList();

  // Constructors
  public Borrowable( LibraryItem libraryItem )
                            : base( libraryItem ) {}

  // Methods
  public void BorrowItem( string name )
  {
    borrowers.Add( name );
    libraryItem.NumCopies--;
  }

  public void ReturnItem( string name )
  {
    borrowers.Remove( name );
     libraryItem.NumCopies++;
  }

  public override void Display()
  {
    base.Display();
    foreach( string borrower in borrowers )
      Console.WriteLine( " borrower: {0}", borrower );
  }
}
 
/// <summary>
///  DecoratorApp test
/// </summary>
public class DecoratorApp
{
  public static void Main( string[] args )
  {
    // Create book and video and display
    Book book = new Book( "Schnell", "My Home", 10 );
    Video video = new Video( "Spielberg",
                          "Schindler's list", 23, 60 );
    book.Display();
    video.Display();

    // Make video borrowable, then borrow and display
    Console.WriteLine( "\nVideo made borrowable:" );
    Borrowable borrowvideo = new Borrowable( video );
    borrowvideo.BorrowItem( "Cindy Lopez" );
    borrowvideo.BorrowItem( "Samuel King" );

    borrowvideo.Display();

  }
}