一、享元模式简介（Brief Introduction）
享元模式（Flyweight Pattern）,运用共享技术有效支持大量细粒度的对象。
Use sharing to support large numbers of fine-grained objects efficiently.
享元模式可以避免大量非常相似类的开销。在程序设计中有时需要生成大量细粒度的类实例来表示数据。如果发现这些实例除了几个参数外基本伤都是相同的，有时就能够受大幅度第减少需要实例化的类的数量。如果能把这些参数移到类实例外面，在方法调用时将他们传递进来，就可以通过共享大幅度地减少单个实例的数目。
享元对象的内部状态与外部状态：
内部状态，在享元对象的内部并且不会随环境改变而改变的共享部分。
外部状态，随环境改变而改变的，不可以共享的状态。
二、解决的问题（What To Solve）
如果一个应用程序使用了大量的对象，而大量的这些对象造成了很大的存储开销，这时可以考虑使用享元模式。
当对象的大多数状态是外部状态，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，这时也可以考虑使用享元模式。
三、享元模式分析（Analysis）
1、享元模式结构
FlyweightFactory类:享元工厂，用来创建和管理Flyweight对象。如果请求的Flyweight对象存在，怎返回已经存在的对象。否则新创建一个新的对象返回。
Flyweight类：享元抽象类，通过这个接口，Flyweight可以接受并作用与外部状态。
UnsharedConcreteFlyweight类：不需要共享的Flyweight子类。Flyweight接口并不强制共享。
ConcreteFlyweight类：实现享元抽象类，为内部状态添加存储空间。
2、代码
| 1、享元工厂类FlyweightFactory |
| public class FlyweightFactory { public Hashtable flyweights = new Hashtable();
public FlyweightFactory() { flyweights.Add("A", new ConcreteFlyweight()); flyweights.Add("B", new ConcreteFlyweight()); flyweights.Add("C", new ConcreteFlyweight()); }
public Flyweight GetFlyweight(string key) { return flyweights[key] as Flyweight; } } |
| 2、享元抽象类Flyweight 及其具体实现类UnsharedConcreteFlyweight 和ConcreteFlyweight |
| public abstract class Flyweight { public abstract void Operation(int extrinsicstate); }
public class UnsharedConcreteFlyweight:Flyweight { public override void Operation(int extrinsicstate) { Console.WriteLine("{0}:{1}",this.GetType().Name,extrinsicstate); } }
public class ConcreteFlyweight:Flyweight { public override void Operation(int extrinsicstate) { Console.WriteLine("{0}:{1}", this.GetType().Name, extrinsicstate); } } |
| 3、客户端代码 |
| static void Main(string[] args) { // Arbitrary extrinsic state int extrinsicstate = 20; FlyweightFactory factory = new FlyweightFactory();
// Work with different flyweight instances Flyweight fx = factory.GetFlyweight("A"); fx.Operation(--extrinsicstate);
Flyweight fy = factory.GetFlyweight("B"); fy.Operation(--extrinsicstate);
Flyweight fz = factory.GetFlyweight("C"); fz.Operation(--extrinsicstate);
UnsharedConcreteFlyweight fu = new UnsharedConcreteFlyweight(); fu.Operation(--extrinsicstate);
Console.ReadKey(); } |
3、实例运行结果
四．享元模式实例分析（Example）
1、场景
一个文档Document中只有少数字符需要共享。结构如下图所示
CharacterFactory类，享元工厂，用来创建和管理Charactor对象。如果请求的Charactor对象存在，怎返回已经存在的对象。否则新创建一个新的对象返回。
Character类：享元抽象类，通过这个接口，Character可以接受并作用与外部状态。
CharacterA /CharacterB/CharacterC 类：实现享元抽象类，为内部状态添加存储空间。
2、代码
| 1、字符工厂类CharacterFactory |
| class CharacterFactory { private Dictionary<char, Character> _characters = new Dictionary<char,Character>(); public Character GetCharacter(char key) { // Uses "lazy initialization" Character character = null; if (_characters.ContainsKey(key)) { character = _characters[key]; } else { switch (key) { case 'A': character = new CharacterA(); break; case 'B': character = new CharacterB(); break; //... case 'Z': character = new CharacterZ(); break; } _characters.Add(key, character); } return character; } } |
| 2、抽象数据对象类DataObject及其具体实现类CustomersData |
| /// <summary> /// The 'Flyweight' abstract class /// </summary> abstract class Character { protected char symbol; protected int width; protected int height; protected int ascent; protected int descent; protected int pointSize; public abstract void Display(int pointSize); } /// <summary> /// A 'ConcreteFlyweight' class /// </summary> class CharacterA : Character { public CharacterA() { this.symbol = 'A'; this.height = 100; this.width = 120; this.ascent = 70; this.descent = 0; } public override void Display(int pointSize) { this.pointSize = pointSize; Console.WriteLine(this.symbol + " (pointsize " + this.pointSize +")"); } } /// <summary> /// A 'ConcreteFlyweight' class /// </summary> class CharacterB : Character { public CharacterB() { this.symbol = 'B'; this.height = 100; this.width = 140; this.ascent = 72; this.descent = 0; } public override void Display(int pointSize) { this.pointSize = pointSize; Console.WriteLine(this.symbol + " (pointsize " + this.pointSize +")"); } } // ... C, D, E, etc. /// <summary> /// A 'ConcreteFlyweight' class /// </summary> class CharacterZ : Character { // Constructor public CharacterZ() { this.symbol = 'Z'; this.height = 100; this.width = 100; this.ascent = 68; this.descent = 0; } public override void Display(int pointSize) { this.pointSize = pointSize; Console.WriteLine(this.symbol +" (pointsize " + this.pointSize +")"); } } |
| 3、客户端代码 |
| static void Main(string[] args) { // Build a document with text string document = "AAZZBBZB"; char[] chars = document.ToCharArray(); CharacterFactory factory = new CharacterFactory(); // extrinsic state int pointSize = 10; // For each character use a flyweight object foreach (char c in chars) { pointSize++; Character character = factory.GetCharacter(c); character.Display(pointSize); } Console.ReadKey(); } |
3、实例运行结果
五、总结（Summary）
本文对享元模式（Flyweight Pattern）的概念、设计结构图、代码、使用场景、进行了描述。以一个享元模式实例进行了说明。如果一个应用程序使用了大量的对象，而大量的这些对象造成了很大的存储开销，这时可以考虑使用享元模式。
本文转自灵动生活博客园博客，原文链接： http://www.cnblogs.com/ywqu/archive/2010/01/21/1653087.html ，如需转载请自行联系原作者
