Design Patterns 设计模式

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1、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.Theserelationshipsareestablishedthroughinheritan

2、ce,sotheyarestatic—fixedatcompile-time.Objectpatternsdealwithobjectrelationships,whichcanbechangedatrun-timeandaremoredynamic.Almostallpatternsuseinheritancetosomeextent.Sotheonlypatternslabeled"classpatterns"arethosethatfocusonclassrelationships.NotethatmostpatternsareintheObjectscope.3Anobje

3、ctpackagesbothdataandtheproceduresthatoperateonthatdata.Theproceduresaretypicallycalledmethodsoroperations.Anobjectperformsanoperationwhenitreceivesarequest(ormessage)fromaclient.1Requestsaretheonlywaytogetanobjecttoexecuteanoperation.Operationsaretheonlywaytochangeanobject'sinternaldata.Becau

4、seoftheserestrictions,theobject'sinternalstateissaidtobeencapsulated;itcannotbeaccesseddirectly,anditsrepresentationisinvisiblefromoutsidetheobject.2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。3Manyobjectsinadesigncomefromtheanalysismodel.Butobject-orienteddesignsoftenendupwith

5、classesthathavenocounterpartsintherealworld.Someofthesearelow-levelclasseslikearrays.Othersaremuchhigher-level.Forexample,theComposite(183)patternintroducesanabstractionfortreatingobjectsuniformlythatdoesn'thaveaphysicalcounterpart.Strictmodelingoftherealworldleadstoasystemthatreflectstoday'sr

6、ealitiesbutnotnecessarilytomorrow's.Theabstractionsthatemergeduringdesignarekeytomakingadesignflexible.4Everyoperationdeclaredbyanobjectspecifiestheoperation'sname,theobjectsittakesasparameters,andtheoperation'sreturnvalue.Thisisknownastheoperation'ssignature.Thesetofallsignaturesdefinedbyanob

7、ject'soperationsiscalledtheinterfacetotheobject.5Atypeisanameusedtodenoteaparticularinterface.6Therun-timeassociationofarequesttoanobjectandoneofitsoperationsisknownasdynamicbinding.7,dynamicbindingletsyousubstituteobjectsthathaveidenti