4.7. Summary

The Decorator design pattern excels in adding features to core objects without having to fundamentally change those objects. Like the ornaments on a Christmas tree, the ornaments change the appearance of the tree, but the tree itself is not changed at all. Anything from the appearance of a web site to the contents of an online shopping cart can be structured using a Decorator pattern.

The Decorator should be considered a core design pattern when your project has to be updated with the addition of new objects and new features for those objects. We like to think of the Decorator as a "100-year" pattern. Imagining concrete components as types of people and decorations as the clothes they wear, it's not too difficult to envision a web site being updated with different kinds of people spanning a century—from a blacksmith at the beginning of the 20^th century to a nanotechnologist at the beginning of the 21^st century. Likewise, all kinds of people can be styled with a range of clothing over the same time span, from a blacksmith's leather apron in 1900 to an astronaut's spacesuit in 2000. However, the programmer who is thinking ahead from one century to the next is able to use a Decorator pattern, and doesn't have to change the core component at all. He just adds more concrete components and decorators as needed.

At the same time that the Decorator pattern is open to adding new features without changing the structure of the program, both components and decorators can be deleted when they're no longer needed. Further, changes to existing components and decorators are easy to make without altering anything other than their specifics, such as a string's label or a number's value.

The Decorator pattern, though, has certain drawbacks. For example, in this chapter, some readers may have been thinking that they could have programmed the same exact functionality for some of the sample applications using a fraction of the code and far fewer classes. That's definitely true, and for specific applications, the Decorator design pattern may be like swatting a fly with an elephant gun. However, like all design patterns, the developer has to be judicious in selecting which, if any pattern, he wants to employ. That decision needs to be made not just on the current size of the application, though. When starting any project where the Decorator pattern is considered, you have to ask yourself: are the concrete components and decorators going to grow and change or are they going to be fairly static? So, even though your application may begin as an acorn, you have to envision the oak tree as a possibility and plan accordingly.