Design Patterns Explained

Intent
Problem
Solution
Participants and
Collaborators
Consequences
Implementation
GoF Reference
Chapter 15 • The Decorator Pattern 251
The Decorator Pattern: Key Features
Attach additional responsibilities to an object dynamically.
The object that you want to use does the basic functions you require.
However, you may need to add some additional functionality to the object,
occurring before or after the object's base functionality. Note that the Java
foundation classes use the Decorator pattern extensively for I/O handling.
Allows for extending the functionality of an object without resorting to subclassing.
The ConcreteComponent is the class having function added to it by the
Decorators. Sometimes classes derived from ConcreteComponent
are used to provide the core functionality, in which case
Concrete-Component is no longer concrete, but rather abstract. The
Component defines the interface for all of these classes to use.
Functionality that is to be added resides in small objects. The advantage is
the ability to dynamically add this function before or after the functionality in
the ConcreteComponent. Note: While a decorator may add its
functionality before or after that which it decorates, the chain of instantiation
always ends with the ConcreteComponent.
Create an abstract class that represents both the original class and the new
functions to be added to the class. In the decorators, place the new function
calls before or after the trailing calls to get the correct order.
Pages 175-184.
Figure 15-7 Standard, simplified view of the Decorator pattern.