结构型设计模式之享元模式(Flyweight)

结构
意图	运用共享技术有效地支持大量细粒度的对象。
适用性	一个应用程序使用了大量的对象。 完全由于使用大量的对象，造成很大的存储开销。 对象的大多数状态都可变为外部状态。 如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象。 应用程序不依赖于对象标识。由于F l y w e i g h t 对象可以被共享，对于概念上明显有别的对象，标识测试将返回真值。

 using System;
    using System.Collections;

    class FlyweightFactory 
    {
        private ArrayList pool = new ArrayList();

        // the flyweightfactory can crete all entries in the pool at startup 
        // (if the pool is small, and it is likely all will be used), or as 
        // needed, if the pool si large and it is likely some will never be used
        public FlyweightFactory()
        {
            pool.Add(new ConcreteEvenFlyweight());        
            pool.Add(new ConcreteUnevenFlyweight());            
        }

        public Flyweight GetFlyweight(int key)
        {
            // here we would determine if the flyweight identified by key 
            // exists, and if so return it. If not, we would create it. 
            // As in this demo we have implementation all the possible 
            // flyweights we wish to use, we retrun the suitable one. 
            int i = key % 2;
            return((Flyweight)pool[i]); 
        }
    }

    abstract class Flyweight 
    {
        abstract public void DoOperation(int extrinsicState);        
    }

    class UnsharedConcreteFlyweight : Flyweight
    {
        override public void DoOperation(int extrinsicState)
        {
            
        }
    }

    class ConcreteEvenFlyweight : Flyweight
    {
        override public void DoOperation(int extrinsicState)
        {
            Console.WriteLine("In ConcreteEvenFlyweight.DoOperation: {0}", extrinsicState);                        
        }
    }

    class ConcreteUnevenFlyweight : Flyweight
    {
        override public void DoOperation(int extrinsicState)
        {
            Console.WriteLine("In ConcreteUnevenFlyweight.DoOperation: {0}", extrinsicState);            
        }    
    }

    /// <summary>
    ///    Summary description for Client.
    /// </summary>
    public class Client
    {
        public static int Main(string[] args)
        {
            int[] data = {1,2,3,4,5,6,7,8};
            
            FlyweightFactory f = new FlyweightFactory();
            
            int extrinsicState = 3;
            foreach (int i in data)
            {
                Flyweight flyweight = f.GetFlyweight(i);
                  flyweight.DoOperation(extrinsicState);
            }
            
            return 0;
        }
    }