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The Decorator Pattern in Action


In this third part of a six-part series on the Decorator pattern, we move from last week's conceptual example to something more concrete: using the pattern to decorate a paper doll. This article is excerpted from chapter four of ActionScript 3.0 Design Patterns Object Oriented Programming Techniques, written by William B. Sanders and Chandima Cumaranatunge (O'Reilly, 2007 ISBN: 0596528469). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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December 11, 2008
TABLE OF CONTENTS:
  1. · The Decorator Pattern in Action
  2. · Decorator Class: Dressing the Dolls
  3. · Concrete decorator classes
  4. · Implementing the Paper Doll Decorator

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The Decorator Pattern in Action - Decorator Class: Dressing the Dolls
(Page 2 of 4 )

Example 4-8 is the abstract Decorator class, Dresser, which extends the component class, Model. The major contribution here is simply re-implementing thegetDressed()method with a reference to the inheritedwhatToWearproperty.

Example 4-8. Dresser.as

package
{
   //Abstract class
   public class Dresser extends Model
   {
     
override public function getDressed():String
      {
         return whatToWear;
      }
   }
}

All of the concrete decorators will be subclassed for this class.

The Concrete Classes

Once the two main abstract classes in the Decorator pattern have been established, you’re all set to create the concrete ones. In the previous minimalist Decorator example, you saw the output using the trace() statement. Instead of trace() statements, both the concrete component and decorators need to be formatted for later parsing, so a tilde character (~) has been added as a demarcation point. Because all the strings from both the concrete component and decorators are grouped together into a single large string, the tilde serves as cutting point.

Concrete component class

The concrete component class is the only concrete class that extends directly from the abstract concrete class. All the other classes in this application extend from the abstract decorator class. Using a simple constructor function, Sue(), the class assigns a value to the whatToWear variable. This is enough to identify the class as an instance of the main abstract component class,Model, and to establish a unique name. All decorations use the concrete component as the target for the decorations. Save Example 4-9 asSue.as.

Example 4-9. Sue.as

package
{
   public class Sue extends Model
   {
     
public function Sue() 
      {
         whatToWear="~sue";
      }
   }
}

If you want to create more concrete component classes, all you need to do is create a similar class with a different name and value for thewhatToWear variable. With this structure, you have no limit to the number of new concrete components you can add.


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