Flyweight An Object Structural Design Pattern

1. FlyweightAn Object Structural Design Pattern JM Imbrescia

2. Intent • “Use Sharing to support large numbers of fine-grained objects efficiently.” • Simply put, a method for storing a small number of complex objects that are used repeatedly.

3. Motivation(Why use the Flyweight Pattern?) • If you have an application that would benefit from using objects throughout your design. • Document Editors • 50 Decks of cards

4. Definitions • Flyweight • A shared object that can be used in multiple contexts simultaneously. • Intrinsic • State information that is independent of the flyweights context. Shareable Information. • Extrinsic • State information that depends on the context of the flyweight and cannot be shared.

5. Applicability • Use the Flyweight when all of the following are true: • Application has a large number of objects. • Storage costs are high because of the large quantity of objects. • Most object state can be made extrinsic. • Many groups of objects may be replaced by relatively few once you remove their extrinsic state. • The application doesn’t depend on object identity.

6. Participants • Flyweight • ConcreteFlyweight • UnsharedConcreeteFlyweight • FlyweightFactory • Client

7. Image from the book thanks to: http://www.cs.jhu.edu/~scott/oos/lectures/PatternDiagrams

8. Consequences • Flyweights introduce runtime costs, these costs need to be offset by storage space savings. Savings come from: • Fewer total instances due to sharing. • Increasing intrinsic space per object. • Computing extrinsic state instead of storing it.

9. Related Patterns • Often combined with the Composite pattern. • State and Strategy objects are often implemented as flyweights.