C# Design Patterns: The Flyweight Pattern

• By James W. Cooper
• Apr 25, 2003
• Sample Chapter is provided courtesy of Addison-Wesley Professional

This chapter is from the book
C# Design Patterns: A Tutorial

The Flyweight pattern is used to avoid the overhead of large numbers of very similar classes. There are cases in programming where it seems that you need to generate a very large number of small class instances to represent data. Sometimes you can greatly reduce the number of different classes that you need to instantiate if you can recognize that the instances are fundamentally the same except for a few parameters. If you can move those variables outside the class instance and pass them in as part of a method call, the number of separate instances can be greatly reduced by sharing them.

The Flyweight design pattern provides an approach for handling such classes. It refers to the instance's intrinsic data that makes the instance unique and the extrinsic data that is passed in as arguments. The Flyweight is appropriate for small, fine-grained classes like individual characters or icons on the screen. For example, you might be drawing a series of icons on the screen in a window, where each represents a person or data file as a folder, as shown in Figure 19-1.

Figure 19-1. A set of folders representing information about various people. Since these are so similar, they are candidates for the Flyweight pattern.

In this case, it does not make sense to have an individual class instance for each folder that remembers the person's name and the icon's screen position. Typically, these icons are one of a few similar images, and the position where they are drawn is calculated dynamically based on the window's size in any case.

In another example in Design Patterns, each character in a document is represented as a single instance of a character class, but the positions where the characters are drawn on the screen are kept as external data. In this case there only has to be one instance of each character, rather than one for each appearance of that character.