Structural and decorative Modes

Definition

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

UML class digoal

Participant

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();   } }