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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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Implementing the Decorator Pattern
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The purpose of the Decorator pattern10 is to be able to add functionality dynamically to an object, making it appear as if the object used inheritance. The UML diagram of the Decorator pattern is illustrated in Figure 5-8.
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Figure 5-8. UML implementation of the Decorator pattern Yet again, this UML diagram looks strikingly similar to those of the Chain of Responsibility and Composite patterns. The Decorator pattern is different in that it uses all child nodes to process data, and the data structure can be created dynamically on the fly. The Decorator pattern can be realized using interfaces, abstract base classes, or even delegates as illustrated in the Chain of Responsibility pattern. If an interface or an abstract base class is used, you can define multiple methods. The different methods must be related, and all application logic related. This means that there should be no structural methods in the base type. To illustrate the Decorator pattern, consider the creation of a burger. A burger is essentially a bun, some meat, and other stuff. There can be cheeseburgers or regular hamburgers. All of the items that make up a burger are ingredients, and that would be the base interface of a Decorator implementation. Following is the base interface definition called Ingredient: public interface Ingredient { String GetIdentifier(); } The base type Ingredient doesn t contain any methods that relate to organization of the Ingredient implementations. The only method, GetIdentifier, relates to retrieving all of the ingredients of the chain. It s assumed that when one method of the Ingredient interface is called, that method invocation will be delegated to all of the other Ingredient implementations.
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10. Design Patterns: Elements of Reusable Object-Oriented Software, p. 175.
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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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The different implementations of the Decorator pattern are related and as a whole form a grouping. This is useful to know because it enables the definition of a helper class to do some of the heavy lifting as illustrated by the following source code: public abstract class Decorator : Ingredient { protected Ingredient _nextIngredient; public Decorator( Ingredient ingredient) { _nextIngredient = ingredient; } public virtual String GetIdentifier() { return _nextIngredient.GetIdentifier(); } } The abstract base class Decorator implements a default constructor that manages the next Ingredient reference. The method GetIdentifier provides a mechanism to call the next Ingredient. The Decorator type provides a default implementation or a helper class for all Ingredient implementations. The link to the next ingredient could have been managed by a proxy as illustrated by the delegate-based Chain of Responsibility pattern. That approach isn t advisable, as the Decorator pattern is intended to be a self-contained grouping that can be dynamically modified. If a container manages the individual elements, then the individual elements would need to know about the container and its siblings. The end result would be an overly complicated hierarchy that would need to include the Mediator pattern to manage the individual references. Following is the source code that illustrates how the individual ingredients are implemented: public class Bun : Ingredient { private String _description = "bun"; public Bun() { } public String GetIdentifier() { return _description; } } public class Lettuce : Decorator { private String _description = "lettuce"; public Lettuce( Ingredient component) : base (component) { } public override String GetIdentifier() { return _nextIngredient.GetIdentifier() + " " + _description; } }
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CHAPTER 5 IMPLEMENTING COMPONENT GROUPINGS
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public class Cheese : Decorator { private String _description = "cheese"; public Cheese( Ingredient component) : base( component) { } public override String GetIdentifier() { return _ nextIngredient.GetIdentifier() + " " + _description; } } public class Meat : Decorator { private String _description = "meat"; public Meat( Ingredient component) : base( component) { } public override String GetIdentifier() { return _nextIngredient.GetIdentifier() + " " + _description; } } The individual Ingredient implementations derive from the Decorator type, except for the Bun class. This is because the decorator has a complication in how it s created. Each ingredient references another ingredient, which is the default implementation technique as defined by the Decorator abstract base class. Where the default implementation technique can t be used is at the end of the list, since the end of the list doesn t have another ingredient. To understand the problem of the end of the list, consider the following source code, which illustrates programmatically how to create a hamburger: Ingredient hamburger = new Meat( new Lettuce( new Bun())); The hamburger is created by successively instantiating an ingredient and passing that instance to the constructor of another ingredient. Since Bun is the end of the list, there is no class to pass in. Of course, an option would be to pass in a null value to indicate an end of list. However, doing that is a hack on a hack. The Decorator pattern is very useful because it provides a neutral interface to what a client considers as a single object. In effect, it s as if a class has its functionality extended. The downside to the Decorator pattern is the instantiation and organization of the individual elements. When creating the hamburger, the new statement was used. Going back to 3 and the Factory pattern, recall that I explicitly said this is a bad idea, because it locks down the implementation. A better idea is to use the Builder pattern to preconstruct specific configurations of hamburgers. If the structure of the Decorator pattern is to change dynamically, then the Builder pattern implementation that created the structure initially should have methods to dynamically modify the structure. It isn t recommended that the structure be manipulated and created by multiple Builder or Factory patterns. Doing so will complicate the bookkeeping of the references, causing maintenance and extension problems. The Decorator pattern has the following distinguishing attributes:
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