Flyweight

Flyweight

Use sharing to support large numbers of objects efficiently and to save RAM.

Problem

• Large numbers of objects should be supported efficiently.
• Creating large numbers of objects should be avoided.

When representing large text documents, for example, creating an object for each character in the document would result in a huge amount of objects that couldn't be processed efficiently.

For instance, every tree object has a copy of all the other dependent objects.

Source: GameProgrammingPatterns.com

Solution

Define Flyweight objects that

• store intrinsic (invariant) state that can be shared
  • intrinsic state is context independent, for example the character A.
• provide an interface through which extrinsic (variant) state can be passed in
  • extrinsic state depends on and varies with the flyweight's context and therefore can't be shared, for example the position of the character A in a GUI.

This enables clients to reuse / share Flyweight objects (instead of creating a new object each time) and pass in extrinsic state when they invoke a Flyweight operation. This greatly reduces the number of physically created objects.

Source: GameProgrammingPatterns.com

Common Structure

• Flyweight
  • declares an interface through which flyweights can receive and act on extrinsic state.
• ConcreteFlyweight
  • implements the Flyweight interface and holds the intrinsic state, which must be independent of the object's context.
• UnsharedConcreteFlyweight (not appropriate)
• FlyweightFactory
  • creates and manages flyweight objects
  • ensures that flyweights are shared properly. When a client requests a flyweight, the FlyweightFactory object supplies an exisiting instance or creates one, if none exists.
• Client
  • maintains a reference to flyweights.
  • computes or stores the extrinsic state of flyweights.

Collaboration

• Clients should not instantiate ConcreteFlyweights directly. Clients must obtain ConcreteFlyweight objects exclusively from the FlyweightFactory object to ensure they are shared properly.

Benefits

• Saves RAM, thus allowing a program to support much more objects.
• Don't repeat yourself (DRY) on data

Drawbacks

• Wastes CPU time on searching or calculating the context
• Increases overall code complexity by creating multiple additional classes.

Example

Definition

Usage

Comparison with other patterns

• Composite - Flyweight is often combined with Composite to implement a shared leaf nodes and save RAM.

  • A consequence of sharing is that flyweight leaf nodes cannot store a pointer to their parent. Rather, the parent pointer is passed to the flyweight as part of its extrinsic state. This has a major impact on how the objects in the hierarchy communicate with eath other.

• It's often best to implement State and Strategy objects as flyweights.

• Flyweight looks almost like Singleton. The key differences are

  1. Singleton object can be mutable. Flyweight objects are immutable.
  2. There should be only one Singleton instance, whereas Flyweight class can have multiple instances with a different intrinsic state.