设计模式之享元模式

享元模式：

• 运用共享技术有效地支持大量细粒度的对象。

示例代码：

• Flyweight类，它是所有具体享元类的超类或接口，通过这个接口，Flyweight可以接受并作用于外部状态。

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

• ConcreteFlyweight是继承Flyweight超类或实现Flyweight接口，并为内部状态增加存储空间。

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

• UnsharedConcreteFlyweight是指那些不需要共享的Flyweight子类。因为Flyweight接口共享成为可能，但它并不强制共享。

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

• FlyweightFactory，是一个享元工厂，用来创建并管理Flyweight对象。它主要是用来确保合理地共享Flyweight，当用户请求一个Flyweight时，FlyweightFactory对象提供一个已创建的实例或者创建一个（ 如果不存在的话）。

class FlyweightFactory {
    private Hashtable flyweights = new Hashtable();
    
    //初始化工厂时，先生成三个实例
    public FlyweightFactory() {
        flyweights.Add("X", new ConcreteFlyweight());
        flyweights.Add("Y", new ConcreteFlyweight());
        flyweights.Add("Z", new ConcreteFlyweight());
    }
    
    //根据客户端请求，获得已生成的实例
    public Flyweight GetFlyweight(String key) {
        return ((Flyweight) flyweights[key]);
    }
}

• 客户端代码

static void Main(String[] args) {
    //代码外部状态
    int extrinsicstate = 22;
    
    FlyweightFactory f = new FlyweightFactory();
    
    Flyweight fx = f.GetFlyweight("X");
    fx.Operation(--extrinsicstate);
    
    Flyweight fy = f.GetFlyweight("y");
    fy.Operation(--extrinsicstate);
    
    Flyweight fz = f.GetFlyweight("z");
    fy.Operation(--extrinsicstate);
    
    Flyweight uf = new UnsharedConcreteFlyweight();
    
    uf.Operation(--extrinsicstate);
    
    Console.Read();
}

结果表示

具体Flyweight:21
具体Flyweight:20
具体Flyweight:19
不共享的具体Flyweight:18