flyweight - Harsh4999/Design-Patterns GitHub Wiki

Flyweight

Use

• It allows us to share an object in multiple contexts. Instead of sharing entire object, which may not be feasible, we divide object in 2 parts: Intrinsic & Extrinsic state
• We create objects which only intrinsic state and share them in multiple contexts
• We use a factory class to initiate an object

Uml

• Flyweight= Interface for client which define methods and operation which accept extrinsic state to be passed to it
• Concrete Flyweight = Implementation of flyweight, its implemented with only intrinsic state in it
• Unshared Concrete Flyweight = Objects of these are not shared, its objects will have all the states
• Flyweight Factory = Used by the client provides instances of flyweights, it takes care of sharing
• Client = It computes and stores the extrinsic state of used flyweights.

Implementation

• Identify intrinsic and extrinsic state of our object
• We create an interface of shared flyweight to provide common methods that accept extrinsic state
• In unshared flyweight we ignore the extrinsic state as we have all state within object
• Flyweight factory will cache flyweights and also provide methods to get them client will be using this to get flyweight
• In client we either maintain extrinsic state or compute it on runtime when using flyweight.
• We can divide the Flyweight into 2 parts one which will be consistent and be same all over (Intrinsic) and other part will be changing as per situation and will be provided by client (Extrinsic).
• In concrete flyweight we will accept extrinsic flyweight from client and will have intrinsic flyweight inside and then combine both of them in order to use object
• Its really useful when we have large information loaded
• A factory is always needed to access flyweight instance
• Eg: Integer wrapper class