享元模式（Flyweight Pattern）,運用共享技術有效支援大量細粒度的對象。
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類的開銷。在程式設計中有時需要生成大量細粒度的類執行個體來表示資料。如果發現這些執行個體除了幾個參數外基本傷都是相同的，有時就能夠受大幅度第減少需要執行個體化的類的數量。如果能把這些參數移到類執行個體外面，在方法調用時将他們傳遞進來，就可以通過共享大幅度地減少單個執行個體的數目。
享元對象的内部狀态與外部狀态：
内部狀态，在享元對象的内部并且不會随環境改變而改變的共享部分。
外部狀态，随環境改變而改變的，不可以共享的狀态。
如果一個應用程式使用了大量的對象，而大量的這些對象造成了很大的存儲開銷，這時可以考慮使用享元模式。
當對象的大多數狀态是外部狀态，如果删除對象的外部狀态，那麼可以用相對較少的共享對象取代很多組對象，這時也可以考慮使用享元模式。
FlyweightFactory類:享元工廠，用來建立和管理Flyweight對象。如果請求的Flyweight對象存在，怎傳回已經存在的對象。否則新建立一個新的對象傳回。
Flyweight類：享元抽象類，通過這個接口，Flyweight可以接受并作用與外部狀态。
UnsharedConcreteFlyweight類：不需要共享的Flyweight子類。Flyweight接口并不強制共享。
ConcreteFlyweight類：實作享元抽象類，為内部狀态添加存儲空間。
1、享元工廠類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、享元抽象類Flyweight 及其具體實作類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
Console.WriteLine("{0}:{1}", this.GetType().Name, extrinsicstate);
3、用戶端代碼
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();
一個文檔Document中隻有少數字元需要共享。結構如下圖所示
CharacterFactory類，享元工廠，用來建立和管理Charactor對象。如果請求的Charactor對象存在，怎傳回已經存在的對象。否則新建立一個新的對象傳回。
Character類：享元抽象類，通過這個接口，Character可以接受并作用與外部狀态。
CharacterA /CharacterB/CharacterC 類：實作享元抽象類，為内部狀态添加存儲空間。
1、字元工廠類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、抽象資料對象類DataObject及其具體實作類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);
/// A 'ConcreteFlyweight' class
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 +")");
class CharacterB : Character
public CharacterB()
this.symbol = 'B';
this.width = 140;
this.ascent = 72;
// ... C, D, E, etc.
class CharacterZ : Character
// Constructor
public CharacterZ()
this.symbol = 'Z';
this.width = 100;
this.ascent = 68;
Console.WriteLine(this.symbol +" (pointsize " + this.pointSize +")");
// 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);
本文對享元模式（Flyweight Pattern）的概念、設計結構圖、代碼、使用場景、進行了描述。以一個享元模式執行個體進行了說明。如果一個應用程式使用了大量的對象，而大量的這些對象造成了很大的存儲開銷，這時可以考慮使用享元模式。
本文轉自靈動生活部落格園部落格，原文連結： http://www.cnblogs.com/ywqu/archive/2010/01/21/1653087.html ，如需轉載請自行聯系原作者