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Programming - Flyweight Structural Design Pattern

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

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

• Flyweightand
  • declares an interface through which flyweights can receive and act on extrinsic state.
• ConcreteFlyweightand and
  • implements the Flyweight interface and adds storage for intrinsic state, if any. A ConcreteFlyweight object must be sharable. Any state it stores must be intrinsic, that is, it must be independent of the ConcreteFlyweight object's context.
• UnsharedConcreteFlyweight
  • not all Flyweight subclasses need to be shared. The Flyweight interface enables sharing, but it doesn't enforce it. It is common for UnsharedConcreteFlyweight objects to have ConcreteFlyweight objects as children at some level in the flyweight object structure (as the Row and Column classes have).
• FlyweightFactoryand and
  • creates and manages flyweight objects
  • ensures that flyweight are shared properly. When a client requests a flyweight, the FlyweightFactory objects supplies an existing instance or creates one, if none exists.
• Clientand and
  • maintains a reference to flyweight(s).
  • computes or stores the extrinsic state of flyweight(s).

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Flyweight.Structural
{
// MainApp test application

class MainApp
{
static void Main()
{
// Arbitrary extrinsic state
int extrinsicstate = 22;

FlyweightFactory f = new FlyweightFactory();

// Work with different flyweight instances
Flyweight fx = f.GetFlyweight("X");
fx.Operation(--extrinsicstate);

Flyweight fy = f.GetFlyweight("Y");
fy.Operation(--extrinsicstate);

Flyweight fz = f.GetFlyweight("Z");
fz.Operation(--extrinsicstate);

UnsharedConcreteFlyweight uf = new
UnsharedConcreteFlyweight();

uf.Operation(--extrinsicstate);

// Wait for user
Console.Read();
}
}

// "FlyweightFactory"

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]);
}
}

// "Flyweight"

abstract class Flyweight
{
public abstract void Operation(int extrinsicstate);
}

// "ConcreteFlyweight"

class ConcreteFlyweight : Flyweight
{
public override void Operation(int extrinsicstate)
{
Console.WriteLine("ConcreteFlyweight: " + extrinsicstate);
}
}

// "UnsharedConcreteFlyweight"

class UnsharedConcreteFlyweight : Flyweight
{
public override void Operation(int extrinsicstate)
{
Console.WriteLine("UnsharedConcreteFlyweight: " +
extrinsicstate);
}
}
}

Output expected:

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

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