设计模式总结通过命令模式.docx

设计模式总结通过命令模式.docx

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设计模式总结通过命令模式

注：

文档内容基本上来自于网上，并加上自己的理解而成。

有的觉得网友总结得非常好，就完全照搬下来，供学习之用。

然而，有的摘抄并没有加上原链接和出处，请谅解。

通过命令模式，通过在客户端和具体的命令之间添加一层Invoker，剪断了客户端和具体服务提供者之间的耦合，降低了两者之间的耦合度，同时也增加了灵活性，比如我们可以灵活的某一个请求的服务提供者，通过单独的服务提供者Command类，可以很方便的提供redo和undo的功能等等，这些都是命令模式的优势。

在软件系统中，“行为请求者”与“行为实现者”通常呈现一种“紧耦合”。

但在某些场合，比如要对行为进行“记录、撤销/重做、事务”等处理，这种无法抵御变化的紧耦合是不合适的。

在这种情况下，如何将“行为请求者”与“行为实现者”解耦？

将一组行为抽象为对象，实现二者之间的松耦合。

这就是命令模式（CommandPattern）

即命令模式的核心是要解决“行为请求者”和“行为实现”都之间的耦合，以达到灵活多变的效果。

目标：

客户只需要发命令，而不需要管命令是如何被执行的！

Commandpattern

FromWikipedia,thefreeencyclopedia

Thisarticleincludesa listofreferences,relatedreadingor externallinks,but itssourcesremainunclearbecauseitlacks inlinecitations. Please improve thisarticlebyintroducingmoreprecisecitations. （December2012）

In object-orientedprogramming,the commandpattern isa behavioural designpattern inwhich anobjectisusedtorepresentand encapsulate alltheinformationneededtocallamethodatalatertime.Thisinformationincludesthemethodname,theobjectthatownsthemethodandvaluesforthemethodparameters.

Fourtermsalwaysassociatedwiththecommandpatternare command, receiver, invoker and client. 

Aconcrete command objecthasa receiver objectandinvokesamethodofthereceiverinawaythatisspecifictothatreceiver'sclass（即在concretecommandObject里面有receiver的成员变量，并且有触发receiver动作的方法，这个方法是所有receiver所共有的接口级别的方法）.Thereceiverthendoesthework. 

Aconcretecommandobjectisseparatelypassedtoan invoker object,whichinvokesthecommand,andoptionallydoesbookkeepingaboutthecommandexecution.Anyconcretecommandobjectcanbepassedtothesameinvokerobject（即在Invoker有一个接受ConcreteCommand的方法，形如setCommand（XXX））.Bothaninvokerobjectandseveralconcretecommandobjectsareheldbya client object（Client创建具体的ConcreteCommand）.Theclientcontainsthedecisionmakingaboutwhichcommandstoexecuteatwhichpoints. Toexecuteacommand,itpassesthecommandobjecttotheinvokerobject. Seeexamplecodebelow.

Usingcommandobjectsmakesiteasiertoconstructgeneralcomponentsthatneedtodelegate,sequenceorexecutemethodcallsatatimeoftheirchoosingwithouttheneedtoknowtheclassofthemethodorthemethodparameters.Usinganinvokerobjectallowsbookkeepingaboutcommandexecutionstobeconvenientlyperformed,aswellasimplementingdifferentmodesforcommands,whicharemanagedbytheinvokerobject,withouttheneedfortheclienttobeawareoftheexistenceofbookkeepingormodes.

Contents

  [hide] 

∙1 Uses

∙2 Structure

∙3 Terminology

∙4 Example

∙4.1 C#

∙4.2 Java

∙4.3 Python

∙4.4 Scala

∙4.5 Javascript

∙5 Seealso

∙6 References

∙7 Externallinks

[edit]Uses

Commandobjectsareusefulforimplementing:

GUIbuttonsandmenuitems 

In Swing and BorlandDelphi programming,an Action isacommandobject.Inadditiontotheabilitytoperformthedesiredcommand,an Action mayhaveanassociatedicon,keyboardshortcut,tooltiptext,andsoon.AtoolbarbuttonormenuitemcomponentmaybecompletelyinitializedusingonlytheAction object.

Macro recording 

Ifalluseractionsarerepresentedbycommandobjects,aprogramcanrecordasequenceofactionssimplybykeepingalistofthecommandobjectsastheyareexecuted.Itcanthen"playback"thesameactionsbyexecutingthesamecommandobjectsagaininsequence.Iftheprogramembedsascriptingengine,eachcommandobjectcanimplementa toScript（） method,anduseractionscanthenbeeasilyrecordedasscripts.

MobileCode 

UsinglanguagessuchasJavawherecodecanbestreamed/slurpedfromonelocationtoanotherviaURLClassloadersandCodebasesthecommandscanenablenewbehaviortobedeliveredtoremotelocations（EJBCommand,MasterWorker）

Multi-level undo 

Ifalluseractionsinaprogramareimplementedascommandobjects,theprogramcankeepastackofthemostrecentlyexecutedcommands.Whentheuserwantstoundoacommand,theprogramsimplypopsthemostrecentcommandobjectandexecutesits undo（） method.

Networking 

Itispossibletosendwholecommandobjectsacrossthenetworktobeexecutedontheothermachines,forexampleplayeractionsincomputergames.

ParallelProcessing 

Wherethecommandsarewrittenastaskstoasharedresourceandexecutedbymanythreadsinparallel（possiblyonremotemachines-thisvariantisoftenreferredtoastheMaster/Workerpattern）

Progressbars 

Supposeaprogramhasasequenceofcommandsthatitexecutesinorder.Ifeachcommandobjecthasa getEstimatedDuration（） method,theprogramcaneasilyestimatethetotalduration.Itcanshowaprogressbarthatmeaningfullyreflectshowclosetheprogramistocompletingallthetasks.

Threadpools 

Atypical,general-purposethreadpoolclassmighthaveapublic addTask（） methodthataddsaworkitemtoaninternalqueueoftaskswaitingtobedone.Itmaintainsapoolofthreadsthatexecutecommandsfromthequeue.Theitemsinthequeuearecommandobjects.Typicallytheseobjectsimplementacommoninterfacesuchas java.lang.Runnable thatallowsthethreadpooltoexecutethecommandeventhoughthethreadpoolclassitselfwaswrittenwithoutanyknowledgeofthespecifictasksforwhichitwouldbeused.

Transactional behavior 

Similartoundo,adatabaseengineorsoftwareinstallermaykeepalistofoperationsthathavebeenorwillbeperformed.Shouldoneofthemfail,allotherscanbereversedordiscarded（usuallycalled rollback）.Forexample,iftwodatabasetablesthatrefertoeachothermustbeupdated,andthesecondupdatefails,thetransactioncanberolledback,sothatthefirsttabledoesnotnowcontainaninvalidreference.

Wizards 

Oftenawizardpresentsseveralpagesofconfigurationforasingleactionthathappensonlywhentheuserclicksthe"Finish"buttononthelastpage.Inthesecases,anaturalwaytoseparateuserinterfacecodefromapplicationcodeistoimplementthewizardusingacommandobject.Thecommandobjectiscreatedwhenthewizardisfirstdisplayed.EachwizardpagestoresitsGUIchangesinthecommandobject,sotheobjectispopulatedastheuserprogresses."Finish"simplytriggersacallto execute（）.Thisway,thecommandclasscontainsnouserinterfacecode.

[edit]Structure

∙UPDATE:

TheexplanationfortheReceiverblockaboveshouldbe"Theactualworktobedonebythecommand（action）"

[edit]Terminology

Theterminologyusedtodescribecommandpatternimplementationsisnotconsistentandcanthereforebeconfusing.Thisistheresultof ambiguity,theuseof synonyms,andimplementationsthatmayobscuretheoriginalpatternbygoingwellbeyondit.

1.Ambiguity.

2.Theterm command isambiguous.Forexample, moveup,moveup mayrefertoasingle（moveup）commandthatshouldbeexecutedtwice,oritmayrefertotwocommands,eachofwhichhappenstodothesamething（moveup）.Iftheformercommandisaddedtwicetoanundostack,bothitemsonthestackrefertothesamecommandinstance.Thismaybeappropriatewhenacommandcanalwaysbeundonethesameway（e.g.movedown）.Boththe GangofFour andthe Javaexamplebelow usethisinterpretationoftheterm command.Ontheotherhand,ifthelattercommandsareaddedtoanundostack,thestackreferstotwoseparateobjects.Thismaybeappropriatewheneachobjectonthestackmustcontaininformationthatallowsthecommandtobeundone.Forexample,toundoa deleteselection command,theobjectmaycontainacopyofthedeletedtextsothatitcanbere-inserted,ifthe deleteselection commandmustbeundone.Notethatusingaseparateobjectforeachinvocationofacommandisalsoanexampleofthe chainofresponsibilitypattern.

3.Theterm execute isalsoambiguous.Itmayrefertorunningthecodeidentifiedbythecommandobject's execute method.However,inMicrosoft's WindowsPresentationFoundation acommandisconsideredtohavebeenexecutedwhenthecommand's execute methodhasbeeninvoked,butthatdoesnotnecessarilymeanthattheapplicationcodehasrun.Thatoccursonlyaftersomefurthereventprocessing.

4.Synonymsand homonyms.

5.Client,Source,Invoker:

thebutton,toolbarbutton,ormenuitemclicked,theshortcutkeypressedbytheuser.

6.CommandObject,RoutedCommandObject,ActionObject:

asingletonobject（e.g.thereisonlyoneCopyCommandobject）,whichknowsaboutshortcutkeys,buttonimages,commandtext,etc.relatedtothecommand.Asource/invokerobjectcallstheCommand/Actionobject'sexecute/performActionmethod.TheCommand/Actionobjectnotifiestheappropriatesource/invokerobjectswhentheavailabilityofacommand/actionhaschanged.Thisallowsbuttonsandmenuitemstobecomeinactive（grayedout）whenacommand/actioncannotbeexecuted/performed.

7.Receiver,TargetObject:

theobjectthatisabouttobecopied,pasted,moved,etc.Thereceiverobjectownsthemethodthatiscalledbythecommand's execute method.Thereceiveristypicallyalsothetargetobject.Forexample,ifthereceiverobjectisa cursor andthemethodiscalled moveUp,thenonewouldexpectthatthecursoristhetargetofthemoveUpaction.Ontheotherhand,ifthecodeisdefinedbythecommandobjectitself,thetargetobjectwillbeadifferentobjectentirely.

8.CommandObject,routedeventargs,eventobject:

theobjectthatispassedfromthesourcetotheCommand/Actionobject,totheTargetobjecttothecodethatdoesthework.Eachbuttonclickorshortcutkeyresultsinanewcommand/eventobject.Someimplementationsaddmoreinformationtothecommand/eventobjectasitisbeingpassedfromoneobject（e.g.CopyCommand）toanother（e.g.documentsection）.Otherimplementationsputcommand/eventobjectsinothereventobjects（likeaboxinsideabiggerbox）astheymovealongthe