享元模式

模式定义

享元模式(Flyweight Pattern)：运用共享技术有效地支持大量细粒度对象的复用。系统只使用少量的对象，而这些对象都很相似，状态变化很小，可以实现对象的多次复用。
目的：解决多次创建（大）对象引起的性能问题。

术语

1. 内部状态:存储在享元对象内部并且不会随环境改变而改变的状态，因此内部状态可以共享。
2. 外部状态是随环境改变而改变的、不可以共享的状态。
   享元对象的外部状态必须由客户端保存，并在享元对象被创建之后，在需要使用的时候再传入到享元对象内部。

UML类图

• 抽象享元类 Flyweight 享元类接口，通过它可以接受并作用于外部状态
• 具体享元类 ConcreteFlyweight 实现了抽象享元接口，实例称为享元对象（被多次复用的对象，客户端通过享元工厂创建）；
• 非共享具体享元类 UnsharedConcreteFlyweight 不能被共享的抽象享元类的子类（客户端直接new该对象）
• 享元工厂类 FlyweightFactory 用于创建并管理享元对象，它针对抽象享元类编程，将各种类型的具体享元对象存储在一个享元池中。
  根据业务直接从享元池中获取享元类对象，或创建新的享元类添加到享元池

代码结构

public static class FlyweightApp
	{
		public static void Run()
		{
			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("Y");
			fz.Operation(--extrinsicstate);

			UnsharedConcreteFlyweight fu = new UnsharedConcreteFlyweight();

			fu.Operation(--extrinsicstate);
		}
	}

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

	public class FlyweightFactory
	{
		private Hashtable flyweights = new Hashtable();

		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]);
		}
	}

情景模式

一服装店聘请一个男模和一个女模试穿衣服（请多了服装店付不起钱），但是有很多衣服需要试穿。

public static class FlyweightApp
	{
		public static void Run()
		{
			List<string> maleCloths = new List<string>() { "男鞋", "短裤", "衬衫" };
			List<string> femaleCloths = new List<string>() { "超短裤", "齐逼小短裙", "肉色袜裤" };

			ModelFactory modelFactory = new ModelFactory();
			Model male = modelFactory.GetFlyweight(Gender.male);
			Model female = modelFactory.GetFlyweight(Gender.female);

			foreach (string item in maleCloths)
			{
				male.Display(item);
			}

			foreach (string item in femaleCloths)
			{
				female.Display(item);
			}


		}
	}
	public enum Gender
	{
		male, female
	}

	public abstract class Model
	{
		public abstract void Display(string cloth);
	}
	public class MaleModel : Model
	{
		public override void Display(string cloth)
		{
			Console.WriteLine("我是帅气的男模，我穿的是{0}", cloth);
		}
	}
	public class FemaleModel : Model
	{
		public override void Display(string cloth)
		{
			Console.WriteLine("我是骚气的女模，我穿的是{0}", cloth);
		}
	}

	public class ModelFactory
	{
		private Hashtable flyweights = new Hashtable();

		public ModelFactory()
		{
			flyweights.Add(Gender.male, new MaleModel());
			flyweights.Add(Gender.female, new MaleModel());
		}
		public Model GetFlyweight(Gender key)
		{
			return ((Model)flyweights[key]);
		}
	}