Flyweight Design Pattern -- Design Pattern

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

Participants

    The classes and/or objects participating in this pattern are:

• Flyweight   (Character)

       Declares an interface through which flyweights can receive and act on extrinsic state.

• Concrete Flyweight(CharacterA, CharacterB... CharacterZ)

        Implements the Flyweight interface and add storage for intrinsic state,

if any. A Concrete Flyweight object must be sharable. Any state it stores must be intrinsic,

that is, it must be independent of the Concrete Flyweight object's context.

• Un-shared Concrete Flyweight( not used )

       Not all Flyweight sub classes need to be shared. The Flyweight interface enables sharing, but it doesn't enforce it. It is common for Un-shared Concrete Flyweight objects to have Concrete Flyweight objects as children at some level in the flyweight object structure (as the Row and Column classes have).

• Flyweight Factory(Character Factory)

      Creates and manages flyweight objects  Ensures that flyweight is shared properly.

      When a client requests a flyweight, the Flyweight Factory objects assets an existing instance or creates one, if none exists.

•      Client(Flyweight App)

     Maintains a reference to flyweight(s).

    Computes or stores the extrinsic state of flyweight(s).

Example for Fly weight Design 

using System; using System.Collections; namespace Flyweight.Structural {   /// <summary>   /// MainApp startup class for Structural   /// Flyweight Design Pattern.   /// </summary>   class MainApp   {     /// <summary>     /// Entry point into console application.     /// </summary>     static void Main()     {       // Arbitrary extrinsic state       int extrinsicstate = 22;       FlyweightFactory factory = new FlyweightFactory();        // Work with different flyweight instances       Flyweight fx = factory.GetFlyweight("X");       fx.Operation(--extrinsicstate);        Flyweight fy = factory.GetFlyweight("Y");       fy.Operation(--extrinsicstate);        Flyweight fz = factory.GetFlyweight("Z");       fz.Operation(--extrinsicstate);        UnsharedConcreteFlyweight fu = new         UnsharedConcreteFlyweight();        fu.Operation(--extrinsicstate);        // Wait for user       Console.ReadKey();     }   }    /// <summary>   /// The 'FlyweightFactory' class   /// </summary>   class FlyweightFactory   {     private Hashtable flyweights = new Hashtable();     // Constructor     public FlyweightFactory()     {       flyweights.Add("X", new ConcreteFlyweight());       flyweights.Add("Y", new ConcreteFlyweight());       flyweights.Add("Z", new ConcreteFlyweight());     }      public Flyweight GetFlyweight(string key)     {       return ((Flyweight)flyweights[key]);     }   }    /// <summary>   /// The 'Flyweight' abstract class   /// </summary>   abstract class Flyweight   {     public abstract void Operation(int extrinsicstate);   }   /// <summary>   /// The 'ConcreteFlyweight' class   /// </summary>   class ConcreteFlyweight : Flyweight   {     public override void Operation(int extrinsicstate)     {       Console.WriteLine("ConcreteFlyweight: " + extrinsicstate);     }   }    /// <summary>   /// The 'UnsharedConcreteFlyweight' class   /// </summary>   class UnsharedConcreteFlyweight : Flyweight   {     public override void Operation(int extrinsicstate)     {       Console.WriteLine("UnsharedConcreteFlyweight: " +         extrinsicstate);     }   } }

Output ConcreteFlyweight: 21 ConcreteFlyweight: 20 ConcreteFlyweight: 19 UnsharedConcreteFlyweight: 18