设计模式(15)---享元模式

一、定义

享元模式：运用共享技术有效地支持大量细粒度的对象。

解释：需要大量重复new一个对象时，使用享元模式可以让你减少对象的初始化，从而减小内存开销。太苍白了，理解的不到位，希望自己以后补充吧。

补充：当一个对象的状态可在外部初始化，同时系统中需要大量重复创建该对象时，享元模式在创建对象的工厂方法中使用一个哈希表，用于保存其外部化状态和对象本身。在系统中，使用对象时首先判断其是否存在，存在则拿来直接使用，不存在需要重新创建并保存到哈希表中。

 

二、UML类图及基本代码

基本代码：

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

    public class ConcreteFlyweight : Flyweight
    {
        // 内部状态
        private string intrinsicstate;

        // 构造函数
        public ConcreteFlyweight(string innerState)
        {
            this.intrinsicstate = innerState;
        }

        /// <summary>
        /// 享元类的实例方法
        /// </summary>
        /// <param name="extrinsicstate">外部状态</param>
        public override void Operation(int extrinsicstate)
        {
            Console.WriteLine("具体实现类: intrinsicstate {0}, extrinsicstate {1}", intrinsicstate, extrinsicstate);
        }
    }

public class FlyweightFactory
    {
        // 最好使用泛型Dictionary<string,Flyweighy>
        //public Dictionary<string, Flyweight> flyweights = new Dictionary<string, Flyweight>();
        public Hashtable flyweights = new Hashtable();

        public FlyweightFactory()
        {
            flyweights.Add("A", new ConcreteFlyweight("A"));
            flyweights.Add("B", new ConcreteFlyweight("B"));
            flyweights.Add("C", new ConcreteFlyweight("C"));
        }

        public Flyweight GetFlyweight(string key)
        {
            // 更好的实现如下
            //Flyweight flyweight = flyweights[key] as Flyweight;
            //if (flyweight == null)
            //{
            // Console.WriteLine("驻留池中不存在字符串" + key);
            // flyweight = new ConcreteFlyweight(key);
            //}
            //return flyweight;

            return flyweights[key] as Flyweight;
        }
    }

客户端调用：

class Program
    {
        static void Main(string[] args)
        {
            // 定义外部状态，例如字母的位置等信息
            int externalstate = 10;
            // 初始化享元工厂
            FlyweightFactory factory = new FlyweightFactory();

            // 判断是否已经创建了字母A，如果已经创建就直接使用创建的对象A
            Flyweight fa = factory.GetFlyweight("A");
            if (fa != null)
            {
                // 把外部状态作为享元对象的方法调用参数
                fa.Operation(--externalstate);
            }
            // 判断是否已经创建了字母B
            Flyweight fb = factory.GetFlyweight("B");
            if (fb != null)
            {
                fb.Operation(--externalstate);
            }

            // 判断是否已经创建了字母D
            Flyweight fd = factory.GetFlyweight("D");
            if (fd != null)
            {
                fd.Operation(--externalstate);
            }
            else
            {
                Console.WriteLine("驻留池中不存在字符串D");
                // 这时候就需要创建一个对象并放入驻留池中
                ConcreteFlyweight d = new ConcreteFlyweight("D");
                factory.flyweights.Add("D", d);
            }

            Console.Read();
        }
    }

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