Intent

Make instances of classes on the fly to improve performance efficiently, like individual characters or icons on the screen.

Where to use & benefits

• Need to instantiate a large amount of small and fine-grained classes.
• Need icons to represent object.
• An object extrinsic state can be shared by classes.
• Reduce the number of objects created, decrease memory footprint and increase performance.
• Increase runtime cost associated with transferring, finding, or computing extrinsic data.

Structure

Participants

Flyweight
declares an interface through which flyweights can receive and act on extrinsic state.

ConcreteFlyweight
implements the Flyweight interface and adds storage for intrinsic state, if any. A ConcreteFlyweight object must be sharable. Any state it stores must be intrinsic, that is, it must be independent of the ConcreteFlyweight object's context.

UnsharedConcreteFlyweight
not all Flyweight subclasses need to be shared. The Flyweight interface enables sharing, but it doesn't enforce it. It is common for UnsharedConcreteFlyweight objects to have ConcreteFlyweight objects as children at some level in the flyweight object structure (as the Row and Column classes have).

FlyweightFactory
creates and manages flyweight objects and ensures that flyweight are shared properly. When a client requests a flyweight, the FlyweightFactory objects supplies an existing instance or creates one, if none exists.

Client
maintains a reference to flyweight(s) and computes or stores the extrinsic state of flyweight(s).

Example

The Flyweight pattern is not implemented in the demonstartion application.