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名稱	Flyweight
結(jié)構(gòu)
意圖	運(yùn)用共享技術(shù)有效地支持大量細(xì)粒度的對(duì)象。
適用性	一個(gè)應(yīng)用程序使用了大量的對(duì)象。完全由于使用大量的對(duì)象，造成很大的存儲(chǔ)開(kāi)銷。對(duì)象的大多數(shù)狀態(tài)都可變?yōu)橥獠繝顟B(tài)。如果刪除對(duì)象的外部狀態(tài)，那么可以用相對(duì)較少的共享對(duì)象取代很多組對(duì)象。應(yīng)用程序不依賴于對(duì)象標(biāo)識(shí)。由于F l y w e i g h t 對(duì)象可以被共享，對(duì)于概念上明顯有別的對(duì)象，標(biāo)識(shí)測(cè)試將返回真值。

Code Example
將內(nèi)部狀態(tài)（不會(huì)發(fā)生變化的狀態(tài)）與外部狀態(tài)（可能發(fā)生變化的狀態(tài)）分離，這樣不會(huì)發(fā)生變化的部分可以抽取出來(lái)共享，而可能發(fā)生變化的部分由客戶維護(hù)。

//?Flyweight?pattern?--?Structural?example??

using?System;

using?System.Collections;

namespace?DoFactory.GangOfFour.Flyweight.Structural

{

??//?MainApp?test?application?

??class?MainApp

??{

????static?void?Main()

????{

??????//?Arbitrary?extrinsic?state?

??????int?extrinsicstate?=?22;

????

??????FlyweightFactory?f?=?new?FlyweightFactory();

??????//?Work?with?different?flyweight?instances?

??????Flyweight?fx?=?f.GetFlyweight("X");

??????fx.Operation(--extrinsicstate);

??????Flyweight?fy?=?f.GetFlyweight("Y");

??????fy.Operation(--extrinsicstate);

??????Flyweight?fz?=?f.GetFlyweight("Z");

??????fz.Operation(--extrinsicstate);

??????UnsharedConcreteFlyweight?fu?=?new?

????????UnsharedConcreteFlyweight();

??????fu.Operation(--extrinsicstate);

??????//?Wait?for?user?

??????Console.Read();

????}

??}

??//?"FlyweightFactory"?

??class?FlyweightFactory?

??{

????private?Hashtable?flyweights?=?new?Hashtable();

????//?Constructor?

????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]);?

????}

??}

??//?"Flyweight"?

??abstract?class?Flyweight?

??{

????public?abstract?void?Operation(int?extrinsicstate);

??}

??//?"ConcreteFlyweight"?

??class?ConcreteFlyweight?:?Flyweight

??{

????public?override?void?Operation(int?extrinsicstate)

????{

??????Console.WriteLine("ConcreteFlyweight:?"?+?extrinsicstate);

????}

??}

??//?"UnsharedConcreteFlyweight"?

??class?UnsharedConcreteFlyweight?:?Flyweight

??{

????public?override?void?Operation(int?extrinsicstate)

????{

??????Console.WriteLine("UnsharedConcreteFlyweight:?"?+?

????????extrinsicstate);

????}

??}

}

享元模式應(yīng)當(dāng)在什么情況下使用

當(dāng)以下所有的條件都滿足時(shí)，可以考慮使用享元模式：

一個(gè)系統(tǒng)有大量的對(duì)象。
這些對(duì)象耗費(fèi)大量的內(nèi)存。
這些對(duì)象的狀態(tài)中的大部分都可以外部化。
這些對(duì)象可以按照內(nèi)蘊(yùn)狀態(tài)分成很多的組，當(dāng)把外蘊(yùn)對(duì)象從對(duì)象中剔除時(shí)，每一個(gè)組都可以僅用一個(gè)對(duì)象代替。
軟件系統(tǒng)不依賴于這些對(duì)象的身份，換言之，這些對(duì)象可以是不可分辨的。

滿足以上的這些條件的系統(tǒng)可以使用享元對(duì)象。

最后，使用享元模式需要維護(hù)一個(gè)記錄了系統(tǒng)已有的所有享元的表，而這需要耗費(fèi)資源。因此，應(yīng)當(dāng)在有足夠多的享元實(shí)例可供共享時(shí)才值得使用享元模式。

八、?享元模式的優(yōu)點(diǎn)和缺點(diǎn)

享元模式的優(yōu)點(diǎn)在于它大幅度地降低內(nèi)存中對(duì)象的數(shù)量。但是，它做到這一點(diǎn)所付出的代價(jià)也是很高的：

享元模式使得系統(tǒng)更加復(fù)雜。為了使對(duì)象可以共享，需要將一些狀態(tài)外部化，這使得程序的邏輯復(fù)雜化。
享元模式將享元對(duì)象的狀態(tài)外部化，而讀取外部狀態(tài)使得運(yùn)行時(shí)間稍微變長(zhǎng)。

享元模式的應(yīng)用

享元模式在編輯器系統(tǒng)中大量使用。一個(gè)文本編輯器往往會(huì)提供很多種字體，而通常的做法就是將每一個(gè)字母做成一個(gè)享元對(duì)象。享元對(duì)象的內(nèi)蘊(yùn)狀態(tài)就是這個(gè)字母，而字母在文本中的位置和字模風(fēng)格等其他信息則是外蘊(yùn)狀態(tài)。比如，字母a可能出現(xiàn)在文本的很多地方，雖然這些字母a的位置和字模風(fēng)格不同，但是所有這些地方使用的都是同一個(gè)字母對(duì)象。這樣一來(lái)，字母對(duì)象就可以在整個(gè)系統(tǒng)中共享。

//?Flyweight?pattern?--?Real?World?example??

using?System;

using?System.Collections;

namespace?DoFactory.GangOfFour.Flyweight.RealWorld

{

??//?MainApp?test?application?

??class?MainApp

??{

????static?void?Main()

????{

??????//?Build?a?document?with?text?

??????string?document?=?"AAZZBBZB";

??????char[]?chars?=?document.ToCharArray();

??????CharacterFactory?f?=?new?CharacterFactory();

??????//?extrinsic?state?

??????int?pointSize?=?10;

??????//?For?each?character?use?a?flyweight?object?

??????foreach?(char?c?in?chars)

??????{

????????pointSize++;

????????Character?character?=?f.GetCharacter(c);

????????character.Display(pointSize);

??????}

??????//?Wait?for?user?

??????Console.Read();

????}

??}

??//?"FlyweightFactory"?

??class?CharacterFactory

??{

????private?Hashtable?characters?=?new?Hashtable();

????public?Character?GetCharacter(char?key)

????{

??????//?Uses?"lazy?initialization"?

??????Character?character?=?characters[key]?as?Character;

??????if?(character?==?null)

??????{

????????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;

????}

??}

??//?"Flyweight"?

??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);

??}

??//?"ConcreteFlyweight"?

??class?CharacterA?:?Character

??{

????//?Constructor?

????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?+?")");

????}

??}

??//?"ConcreteFlyweight"?

??class?CharacterB?:?Character

??{

????//?Constructor?

????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.?

??//?"ConcreteFlyweight"?

??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?+?")");

????}

??}

}

Output

A (pointsize 11)
A (pointsize 12)
Z (pointsize 13)
Z (pointsize 14)
B (pointsize 15)
B (pointsize 16)
Z (pointsize 17)
B (pointsize 18)

posted on 2006-01-03 15:50 夢(mèng)在天涯閱讀(745) 評(píng)論(0) 編輯收藏引用所屬分類: Design pattern

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八、?享元模式的優(yōu)點(diǎn)和缺點(diǎn)

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