Flyweight%20An%20Object%20Structural%20Design%20Pattern

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FlyweightAn Object Structural Design Pattern

• JM Imbrescia

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Intent

• Use Sharing to support large numbers of
  fine-grained objects efficiently.
• Simply put, a method for storing a small number
  of complex objects that are used repeatedly.

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Motivation(Why use the Flyweight Pattern?)

• If you have an application that would benefit
  from using objects throughout your design.
• Document Editors
• 50 Decks of cards

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Definitions

• Flyweight
• A shared object that can be used in multiple
  contexts simultaneously.
• Intrinsic
• State information that is independent of the
  flyweights context. Shareable Information.
• Extrinsic
• State information that depends on the context of
  the flyweight and cannot be shared.

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Applicability

• Use the Flyweight when all of the following are
  true
• Application has a large number of objects.
• Storage costs are high because of the large
  quantity of objects.
• Most object state can be made extrinsic.
• Many groups of objects may be replaced by
  relatively few once you remove their extrinsic
  state.
• The application doesnt depend on object
  identity.

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Participants

• Flyweight
• ConcreteFlyweight
• UnsharedConcreeteFlyweight
• FlyweightFactory
• Client

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Image from the book thanks to http//www.cs.jhu.e
du/scott/oos/lectures/PatternDiagrams
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Consequences

• Flyweights introduce runtime costs, these costs
  need to be offset by storage space savings.
  Savings come from
• Fewer total instances due to sharing.
• Increasing intrinsic space per object.
• Computing extrinsic state instead of storing it.

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Related Patterns

• Often combined with the Composite pattern.
• State and Strategy objects are often implemented
  as flyweights.