设计模式之装饰者模式

Decorator Design Pattern

definition Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. Frequency of use:  nbsp;medium

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UML class diagram

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participants

    The classes and/or objects participating in this 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.

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sample code in C#

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

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// Decorator pattern -- Structural example

using System; namespace DoFactory.GangOfFour.Decorator.Structural {    // MainApp test application    class MainApp   {      static void Main()     {        // 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();        // Wait for user       Console.Read();     }   }    // "Component"    abstract class Component   {      public abstract void Operation();   }    // "ConcreteComponent"    class ConcreteComponent : Component   {      public override void Operation()     {       Console.WriteLine("ConcreteComponent.Operation()");     }   }    // "Decorator"    abstract class Decorator : Component   {      protected Component component;      public void SetComponent(Component component)     {        this.component = component;     }      public override void Operation()     {        if (component != null)       {         component.Operation();       }     }   }    // "ConcreteDecoratorA"    class ConcreteDecoratorA : Decorator   {      private string addedState;      public override void Operation()     {        base.Operation();       addedState = "New State";       Console.WriteLine("ConcreteDecoratorA.Operation()");     }   }    // "ConcreteDecoratorB"    class ConcreteDecoratorB : Decorator   {      public override void Operation()     {        base.Operation();       AddedBehavior();       Console.WriteLine("ConcreteDecoratorB.Operation()");     }      void AddedBehavior()     {     }   } }

Output ConcreteComponent.Operation() ConcreteDecoratorA.Operation() ConcreteDecoratorB.Operation()

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

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// Decorator pattern -- Real World example

using System; using System.Collections; namespace DoFactory.GangOfFour.Decorator.RealWorld {    // MainApp test application    class MainApp   {      static void Main()     {        // Create book       Book book = new Book ("Worley", "Inside ASP.NET", 10);       book.Display();        // Create video       Video video = new Video ("Spielberg", "Jaws", 23, 92);       video.Display();        // Make video borrowable, then borrow and display       Console.WriteLine("/nMaking video borrowable:");       Borrowable borrowvideo = new Borrowable(video);       borrowvideo.BorrowItem("Customer #1");       borrowvideo.BorrowItem("Customer #2");       borrowvideo.Display();        // Wait for user       Console.Read();     }   }    // "Component"    abstract class LibraryItem   {      private int numCopies;      // Property      public int NumCopies     {        get{ return numCopies; }        set{ numCopies = value; }     }      public abstract void Display();   }    // "ConcreteComponent"    class Book : LibraryItem   {      private string author;      private string title;      // Constructor      public Book( string author, string title, int numCopies)     {        this.author = author;        this.title = title;        this.NumCopies = numCopies;     }      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   {      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;     }      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}/n", playTime);     }   }    // "Decorator"    abstract class Decorator : LibraryItem   {      protected LibraryItem libraryItem;      // Constructor      public Decorator(LibraryItem libraryItem)     {        this.libraryItem = libraryItem;     }      public override void Display()     {       libraryItem.Display();     }   }    // "ConcreteDecorator"    class Borrowable : Decorator   {      protected ArrayList borrowers = new ArrayList();      // Constructor      public Borrowable(LibraryItem libraryItem)       : base(libraryItem)     {     }      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: " + borrower);       }     }   } }

Output Book ------ Author: Worley Title: Inside ASP.NET # Copies: 10 Video ----- Director: Spielberg Title: Jaws # Copies: 23 Playtime: 92 Making video borrowable: Video ----- Director: Spielberg Title: Jaws # Copies: 21 Playtime: 92 borrower: Customer #1 borrower: Customer #2