.NET Design Patterns series: Part 1 The Decorator pattern
A brief introduction to software design patterns
Hello and welcome to my first article in a series on design patterns. A design pattern is simply a documented solution to a common software design problem. This idea is extremely useful in the world of software development. When designing and writing code, chances are whatever your task may be, someone has already accomplished what you've been tasked to do. In fact, many developers have probably accomplished the task at hand. Why reinvent the wheel? There are many benefits to practicing and using design patterns.
Industry standards
Design patterns are deeply ingrained in the world of software development. As such, you can look at tried and tested design patterns as "best practices". If you practice and understand solutions that are accepted by the software development community, your software solutions will improve as a result.
Efficient Communication
Design patterns are defined and described using a unified set of terms. As you practice design patterns, you will master these terms and in effect, you will be able to communicate much more effiecently with other developers. You will spend less time trying to describe a potential design when you understand how to describe the concepts clearly by using a unified terminology. As a result, other junior developers on your team will be more likely to try to learn design patterns which will make your overall team more efficient.
Brief history of design patterns
Kent Beck and Ward Cunningham began experimentation with design patterns in the context of computer science in the year 1987. Most of the works on design patterns have been based on their works.
Popular books on design patterns
- Design Patterns: Elements of Reusable Object-Oriented Software
- Patterns of Enterprise Application Architecture.
- Pattern-Oriented Software Architecture, Volume 1: A System of Patterns
- Pattern-Oriented Software Architecture, Volume 2: Patterns for Concurrent and Networked Objects
Pattern #1: The Decorator pattern, Adding new behavior to existing objects
I've decided to start this series with patterned called the Decorator pattern. I suppose it's best to start with the formal definition of the pattern.
The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclass for extending functionality.
Too much inheritance can be a bad thing
Inheritance is an important part of object oriented design. However, just like anything else, too much subclassing can be a bad thing. The idea behind a great object oriented application is to design your objects with change and reuse in mind. If you find yourself creating 20+ subclasses of an abstract class, you may want to step back and look at your design. With each new subclass that you design, there's a another class that you must maintain and change. Remember, to always design your classes to be Open for extension, closed for modification. This makes for easier maintienence and flexible designs.
The Decorator pattern allows you to add behavior to existing classes at run time through object composition. This means we create a class to "decorate" another class with new functionality. The class to be decorated would be passed into the decorator object's constructor and the reference would be stored as a field. This process can be repeated to achieve new functionality when calling the common contract of the class to be decorated.
The article example
Imagine the idea of an article. There are many different types of articles. There are news paper articles, blog articles, code project articles, magazine articles. The list goes on and on. In the age of electronic articles, it would be handy to write an article class and provide a post method. The post method would contain any external site API calls required to publish the article to it's destination. When posting an article, you may want to accomplish other tasks at the same time, such as advertise the article. Perhaps you'd like to advertise your new article via Twitter.
OOP Principals: Open for extension and closed for modification
You could add the code to tweet your new article right into your the article base class, however, this would go against our Object Oriented design princiapl "Open for extension, closed for modification". Wouldn't it be better to add this new behavior to our articles at run time? The decorator pattern allows us to do just that!
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Examine the following class diagram.
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And here is the code..
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Article.cs
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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
/// <summary>
/// Defines the methods and properties required for an article to be posted on the web.
/// </summary>
public abstract class Article
{
#region "properties"
public string Title { get; set; }
public string Url { get; set; }
public string Body { get; set; }
public string Author { get; set; }
#endregion
#region "methods"
/// <summary>
/// Provides a method to "Post" the article to it's destination.
/// </summary>
public abstract void Post();
#endregion
}
}
Decorator.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
/// <summary>
/// This interface takes an
/// </summary>
public abstract class Decorator : Article
{
//Holds a reference to the componoent to be decorated
//in this case an article.
protected Article _article;
/// <summary>
/// A emthod to provide the article to be extended.
/// </summary>
/// <param name="article">The article in which to add new behavior</param>
public void SetArticle(Article article)
{
_article = article;
}
/// <summary>
/// Implementation of the base classes' post method which calls the
/// referenced article's post method as well.
/// </summary>
public override void Post()
{
if (_article != null)
{
_article.Post();
}
}
}
}
CodeProjectArticle.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
/// <summary>
/// Represents an article to be posed to codeproject.com
/// </summary>
public class CodeProjectArticle : Article
{
public CodeProjectArticle() { }
/// <summary>
/// This would normally contain any calls to post the article to the codeproject.com automatically
/// </summary>
public override void Post()
{
Console.WriteLine("Posting the article {0} to {1}", this.Title, this.Url);
Console.WriteLine();
}
}
}
Tweetable.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
/// <summary>
/// This interface takes an
/// </summary>
public class Tweetable : Decorator
{
//Holds a reference to the componoent to be decorated
//in this case an article.
protected Article _article;
public Tweetable(){ }
/// <summary>
/// A emthod to provide the article to be extended.
/// </summary>
/// <param name="article">The article in which to add new behavior</param>
public Tweetable(Article article)
{
_article = article;
}
/// <summary>
/// Implementation of the base classes' post method which calls the
/// referenced article's post method as well.
/// </summary>
public override void Post()
{
base.Post();
if (_article != null)
{
_article.Post();
}
TweetArticle();
}
public void TweetArticle()
{
if (_article != null)
{
Console.WriteLine("Tweeting about your recent article posting entitled {0}", _article.Title);
Console.WriteLine();
}
}
}
}
BlogPost.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
public class BlogPost : Tweetable
{
protected Article _article;
public BlogPost() { }
public BlogPost(Article article)
{
_article = article;
}
public override void Post()
{
base.Post();
Console.WriteLine("Posting a personal blog called {0} to my personal blog site {1}.", this.Title, this.Url);
Console.WriteLine();
}
}
}
Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace DecoratorPatternApplication
{
public class Program
{
public static void Main(string[] args)
{
//Create an instance of our code project article
CodeProjectArticle codeProjectArticle = new CodeProjectArticle
{
Title = "Code Project Article: Why learn Assembly language?",
Author = "Buddy James",
Body = "All of the article's body would go here.",
Url = "http://www.codeproject.com/Articles/89460/Why-Learn-Assembly-Language"
};
codeProjectArticle.Post();
BlogPost personalBlogPost = new BlogPost
{
Title = "The Decorator pattern. How to extend the behavior of your existing objects",
Author = "Buddy James",
Body = "All of the article's contents would go here.",
Url = "http://www.refactorthis.net"
};
//Lets post a tweet when we post our blog article
Tweetable TweetBlogPost = new Tweetable(personalBlogPost);
TweetBlogPost.Post();
Console.ReadLine();
}
}
}
I've attached the entire solution so that you can see how it works. You can download the solution Here DecoratorPatternApplication.zip (78.94 kb) Notice that the post and tweet methods contain calls to Console.WriteLine(). Obviously in a real scenario these methods would contain calls to actually "post" or "publish" the article.
I hope you've enjoyed my first of many articles on design patterns.
Thanks and keep reading!
~/Buddy James
