flyweight

Flyweight

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

Structure

The class diagram below shows how a flyweight is commonly implemented:

classDiagram

class Client

class Flyweight {
    Operation(extrinsicState)
}

class ConcreteFlyweight {
    Operation(extrinsicState)
}

class UnsharedConcreteFlyweight {
    Operation(extrinsicState)
}

class FlyweightFactory {
    GetFlyweight(key)
}

FlyweightFactory --> Flyweight : has-many

Flyweight <|-- ConcreteFlyweight : implements
Flyweight <|-- UnsharedConcreteFlyweight : implements
Client --> ConcreteFlyweight : interacts-with
Client --> UnsharedConcreteFlyweight : interacts-with
Client --> FlyweightFactory : uses-a

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In this structure, the Client uses a FlyweightFactory to get a flyweight by its key. The factory can decide whether/when to return a ConcreteFlyweight or a UnsharedConcreteFlyweight based on some decisions regarding intrinsic and extrinsic states. From the client's point of view, though, the way to interact with the flyweight is the same.

Intrinsic and extrinsic states

Intrinsic state is the internal state of a flyweight. Imagine a text editor where each character is represented by a flyweight, for instance. Intrinsic state in this case would be the charCode of the character. Extrinsic state is all the context-dependent information that transcends the boundaries of an object, making it unique at the same time. A character with a different font is a good example of an object with extrinsic state.

Working example

As the working example for this pattern, the LandscapeDesign App app was created. It's a command line tool that allows users to create their landscapes, using trees, houses and colored squares. It implements the Flyweight pattern was used in this project as a way of limiting the memory usage when holding references to symbols drawn in the screen. Check out LandscapeDesign for implementation details.