Flyweight Pattern

# Flyweight

- Purpose)
- Structure)
- Applications)
- Consequences)

## Purpose

- Use sharing to support large numbers of fine-grained objects efficiently.

## Structure

- 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; it doesn't enforce it. It's 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. ensures that flyweights are shared properly. When a client requests a flyweight, the FlyweightFactory object supplies an existing instance or creates one, if none exists.
- Client : maintains a reference to flyweight(s). computes or stores the extrinsic state of flyweight(s).

## Applications

- An application uses a large number of objects.
- Storage costs are high because of the sheer quantity of objects.
- Most object state can be made extrinsic.
- Many groups of objects may be replaced by relatively few shared objects once extrinsic state is removed.
- The application doesn't depend on object identity. Since flyweight objects may be shared, identity tests will return true for conceptually distinct objects.

## Consequences

- the reduction in the total number of instances that comes from sharing
- the amount of intrinsic state per object
- whether extrinsic state is computed or stored.