Decorator Design Pattern
 definition UML diagram participants | sample code in C# |
definition
|  |
medium
UML class diagram
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.
sample code in C#
This structural code demonstrates the Decorator pattern which dynamically adds extra functionality to an existing object.
Hide code
// Decorator pattern -- Structural example
|
using System;
namespace DoFactory.GangOfFour.Decorator.Structural
{
/// <summary>
/// MainApp startup class for Structural
/// Decorator Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
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.ReadKey();
}
}
/// <summary>
/// The 'Component' abstract class
/// </summary>
abstract class Component
{
public abstract void Operation();
}
/// <summary>
/// The 'ConcreteComponent' class
/// </summary>
class ConcreteComponent : Component
{
public override void Operation()
{
Console.WriteLine("ConcreteComponent.Operation()");
}
}
/// <summary>
/// The 'Decorator' abstract class
/// </summary>
abstract class Decorator : Component
{
protected Component component;
public void SetComponent(Component component)
{
this.component = component;
}
public override void Operation()
{
if (component != null)
{
component.Operation();
}
}
}
/// <summary>
/// The 'ConcreteDecoratorA' class
/// </summary>
class ConcreteDecoratorA : Decorator
{
public override void Operation()
{
base.Operation();
Console.WriteLine("ConcreteDecoratorA.Operation()");
}
}
/// <summary>
/// The 'ConcreteDecoratorB' class
/// </summary>
class ConcreteDecoratorB : Decorator
{
public override void Operation()
{
base.Operation();
AddedBehavior();
Console.WriteLine("ConcreteDecoratorB.Operation()");
}
void AddedBehavior()
{
}
}
}
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Output
ConcreteComponent.Operation()
ConcreteDecoratorA.Operation() ConcreteDecoratorB.Operation() |
This real-world code demonstrates the Decorator pattern in which 'borrowable' functionality is added to existing library items (books and videos).
Hide code
// Decorator pattern -- Real World example
|
using System;
using System.Collections.Generic;
namespace DoFactory.GangOfFour.Decorator.RealWorld
{
/// <summary>
/// MainApp startup class for Real-World
/// Decorator Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
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.ReadKey();
}
}
/// <summary>
/// The 'Component' abstract class
/// </summary>
abstract class LibraryItem
{
private int _numCopies;
// Property
public int NumCopies
{
get { return _numCopies; }
set { _numCopies = value; }
}
public abstract void Display();
}
/// <summary>
/// The 'ConcreteComponent' class
/// </summary>
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);
}
}
/// <summary>
/// The 'ConcreteComponent' class
/// </summary>
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);
}
}
/// <summary>
/// The 'Decorator' abstract class
/// </summary>
abstract class Decorator : LibraryItem
{
protected LibraryItem libraryItem;
// Constructor
public Decorator(LibraryItem libraryItem)
{
this.libraryItem = libraryItem;
}
public override void Display()
{
libraryItem.Display();
}
}
/// <summary>
/// The 'ConcreteDecorator' class
/// </summary>
class Borrowable : Decorator
{
protected List<string> borrowers = new List<string>();
// 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 |
This .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc.
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