Design Patterns 设计模式.docx

Design Patterns 设计模式.docx

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DesignPatterns设计模式

2012/11/19Monday

设计模式分类标准

1设计模式有两种分类标准，按用途来分：

2按scope来分，主要分成类模式和对象模式两类。

a）类模式处理类和其子类之间的关系，这种关系一般是通过继承实现的，因此在编译时就已经确定。

b）对象模式处理对象间的关系，这种关系可在运行时修改，更灵活。

c）因为大部分的模式都用到了继承，因此只有少数几个模式被认为是类模式。

Classpatternsdealwithrelationshipsbetweenclassesandtheirsubclasses.Theserelationshipsareestablishedthroughinheritance,sotheyarestatic—fixedatcompile-time.Objectpatternsdealwithobjectrelationships,whichcanbechangedatrun-timeandaremoredynamic.Almostallpatternsuseinheritancetosomeextent.Sotheonlypatternslabeled"classpatterns"arethosethatfocusonclassrelationships.NotethatmostpatternsareintheObjectscope.

3Anobjectpackagesbothdataandtheproceduresthatoperateonthatdata.Theproceduresaretypicallycalledmethodsoroperations.Anobjectperformsanoperationwhenitreceivesarequest（ormessage）fromaclient.

4Requestsaretheonlywaytogetanobjecttoexecuteanoperation.Operationsaretheonlywaytochangeanobject'sinternaldata.Becauseoftheserestrictions,theobject'sinternalstateissaidtobeencapsulated;itcannotbeaccesseddirectly,anditsrepresentationisinvisiblefromoutsidetheobject.

5面向对象设计的难点：

decomposingasystemintoobjects，可考虑的因素太多，又没有唯一合适的标准。

6Manyobjectsinadesigncomefromtheanalysismodel.Butobject-orienteddesignsoftenendupwithclassesthathavenocounterpartsintherealworld.Someofthesearelow-levelclasseslikearrays.Othersaremuchhigher-level.Forexample,theComposite（183）patternintroducesanabstractionfortreatingobjectsuniformlythatdoesn'thaveaphysicalcounterpart.Strictmodelingoftherealworldleadstoasystemthatreflectstoday'srealitiesbutnotnecessarilytomorrow's.Theabstractionsthatemergeduringdesignarekeytomakingadesignflexible.

7Everyoperationdeclaredbyanobjectspecifiestheoperation'sname,theobjectsittakesasparameters,andtheoperation'sreturnvalue.Thisisknownastheoperation'ssignature.Thesetofallsignaturesdefinedbyanobject'soperationsiscalledtheinterfacetotheobject.

8Atypeisanameusedtodenoteaparticularinterface.

9Therun-timeassociationofarequesttoanobjectandoneofitsoperationsisknownasdynamicbinding.

10,dynamicbindingletsyousubstituteobjectsthathaveidenticalinterfacesforeachotheratrun-time.Thissubstitutabilityisknownaspolymorphism,andit'sakeyconceptinobject-orientedsystems.

11Anobject'simplementationisdefinedbyitsclass.Theclassspecifiestheobject'sinternaldataandrepresentationanddefinestheoperationstheobjectcanperform.

12Objectsarecreatedbyinstantiatingaclass.Theobjectissaidtobeaninstanceoftheclass.Theprocessofinstantiatingaclassallocatesstoragefortheobject'sinternaldata（madeupofinstancevariables）andassociatestheoperationswiththesedata.

13Adashedarrowheadlineindicatesaclassthatinstantiatesobjectsofanotherclass.Thearrowpointstotheclassoftheinstantiatedobjects.

14接口继承与类继承差别特别大。

sdefineshowtheobjectisimplemented.Theclassdefinestheobject'sinternalstateandtheimplementationofitsoperations.Incontrast,anobject'stypeonlyreferstoitsinterface—thesetofrequeststowhichitcanrespond.Anobjectcanhavemanytypes,andobjectsofdifferentclassescanhavethesametype.

15Becauseaclassdefinestheoperationsanobjectcanperform,italsodefinestheobject'stype.

16Classinheritance是一种mechanismforcodeandrepresentationsharing.interfaceinheritance（orsubtyping）describeswhenanobjectcanbeusedinplaceofanother.

17Programtoaninterface,notanimplementation.

18Don'tdeclarevariablestobeinstancesofparticularconcreteclasses.Instead,commitonlytoaninterfacedefinedbyanabstractclass.Youwillfindthistobeacommonthemeofthedesignpatternsinthisbook.

19面向对象设计中复用的两种基本方法：

classinheritance、objectcomposition.

20white-boxreuse：

Reusebysubclassing。

theinternalsofparentclassesareoftenvisibletosubclasses.

21Objectcompositionrequiresthattheobjectsbeingcomposedhavewell-definedinterfaces.Thisstyleofreuseiscalledblack-boxreuse,becausenointernaldetailsofobjectsarevisible.

22inheritanceexposesasubclasstodetailsofitsparent'simplementation,"inheritancebreaksencapsulation"

23Onecureforthisistoinheritonlyfromabstractclasses,sincetheyusuallyprovidelittleornoimplementation.

24Objectcompositionisdefineddynamicallyatrun-timethroughobjectsacquiringreferencestootherobjects.

25使用objectcompositionhelpsyoukeepeachclassencapsulatedandfocusedononetask。

Yourclassesandclasshierarchieswillremainsmallandwillbelesslikelytogrowintounmanageablemonsters.

26另外，,adesignbasedonobjectcompositionwillhavemoreobjects（iffewerclasses）,andthesystem'sbehaviorwilldependontheirinterrelationshipsinsteadofbeingdefinedinoneclass.

27Favorobjectcompositionoverclassinheritance.

28继承和组合合作的方式：

Ideally,youshouldn'thavetocreatenewcomponentstoachievereuse.Youshouldbeabletogetallthefunctionalityyouneedjustbyassemblingexistingcomponentsthroughobjectcomposition.Butthisisrarelythecase,becausethesetofavailablecomponentsisneverquiterichenoughinpractice.Reusebyinheritancemakesiteasiertomakenewcomponentsthatcanbecomposedwitholdones.Inheritanceandobjectcompositionthusworktogether.

29Delegation使得composition像inheritance一样拥有强大的复用功能。

30代理的基本原理areceivingobjectdelegatesoperationstoitsdelegate.Thisisanalogoustosubclassesdeferringrequeststoparentclasses.Butwithinheritance,aninheritedoperationcanalwaysrefertothereceivingobjectthroughthethismembervariableinC++andselfinSmalltalk.Toachievethesameeffectwithdelegation,thereceiverpassesitselftothedelegatetoletthedelegatedoperationrefertothereceiver.

31ThefollowingdiagramdepictstheWindowclassdelegatingitsAreaoperationtoaRectangleinstance.

Aplainarrowheadlineindicatesthataclasskeepsareferencetoaninstanceofanotherclass.Thereferencehasanoptionalname,"rectangle"inthiscase.

32Themainadvantageofdelegationisthatitmakesiteasytocomposebehaviorsatrun-timeandtochangethewaythey'recomposed.Ourwindowcanbecomecircularatrun-timesimplybyreplacingitsRectangleinstancewithaCircleinstance,assumingRectangleandCirclehavethesametype.

33Delegation的缺点，与composition在带来灵活性的同时有其固有的缺点一样：

比起morestaticsoftware，它们更难理解。

34Delegationisagooddesignchoiceonlywhenitsimplifiesmorethanitcomplicates.Delegationworksbestwhenit'susedinhighlystylizedways—thatis,instandardpatterns.

35Delegationisanextremeexampleofobjectcomposition.Itshowsthatyoucanalwaysreplaceinheritancewithobjectcompositionasamechanismforcodereuse.

36parameterizedtypes、generics、templates。

defineatypewithoutspecifyingalltheothertypesituses。

Theunspecifiedtypesaresuppliedasparametersatthepointofuse.ThelanguageimplementationwillcreateacustomizedversionoftheListclasstemplateforeachtypeofelement.会创建新类型

37Parameterizedtypesgiveusathirdway（inadditiontoclassinheritanceandobjectcomposition）tocomposebehaviorinobject-orientedsystems.Manydesignscanbeimplementedusinganyofthesethreetechniques.

2012/11/20Tue

1三种复用方式的比较：

Thereareimportantdifferencesbetweenthesetechniques.Objectcompositionletsyouchangethebehaviorbeingcomposedatrun-time,butitalsorequiresindirectionandcanbelessefficient.Inheritanceletsyouprovidedefaultimplementationsforoperationsandletssubclassesoverridethem.Parameterizedtypesletyouchangethetypesthataclasscanuse.Butneitherinheritancenorparameterizedtypescanchangeatrun-time.Whichapproachisbestdependsonyourdesignandimplementationconstraints.

2面对对象程序运行时的程序结构与其代码结构通常没什么关系。

3Acquaintance和Aggregation。

Aggregation关系中两个对象的生命周期一般是相同的。

Acquaintance中则对象相互间并不怎么了解，这种关系又称为association或者using

4DesigningforChange

5需要重设计的几种情况：

a）Creatinganobjectbyspecifyingaclassexplicitly.

b）Dependenceonspecificoperations.

c）Dependenceonhardwareandsoftwareplatform.

d）Dependenceonobjectrepresentationsorimplementations.Clientsthatknowhowanobjectisrepresented,stored,located,orimplementedmightneedtobechangedwhentheobjectchanges.

e）Algorithmicdependencies.

f）Tightcoupling.

g）Extendingfunctionalitybysubclassing.

h）Inabilitytoalterclassesconveniently.比如不能改代码

6Atoolkitisasetofrelatedandreusableclassesdesignedtoprovideuseful,general-purposefunctionality.如C++I/Ostreamlibrary

7ApplicationPrograms比如字处理程序的设计主要考虑internalreuse,maintainability,andextension

8设计Toolkit比设计ApplicationPrograms难多了。

因为不同平台下要都能用

9Aframeworkisasetofcooperatingclassesthatmakeupareusabledesignforaspecificclassofsoftware

10Youcustomizeaframeworktoaparticularapplicationbycreatingapplication-specificsubclassesofabstractclassesfromtheframework.Theframeworkdictatesthearchitectureofyourapplication.Frameworksthusemphasizedesignreuseovercodereuse,thoughaframeworkwillusuallyincludeconcretesubclassesyoucanputtoworkimmediately.

11Framework和Toolkit之间的比较：

Reuseonthislevelleadstoaninversionofcontrolbetweentheapplicationandthesoftwareonwhichit'sbased.Whenyouuseatoolkit（oraconventionalsubroutinelibraryforthatmatter）,youwritethemainbodyoftheapplicationandcallthecodeyouwanttoreuse.Whenyouuseaframework,youreusethemainbodyandwritethecodeitcalls.You'llhavetowriteoperationswithparticul