datasnapxe-callbacks(回调)

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1、§Delphi 实验室: DataSnap XE - Callbacks(回调)   Abstract:"DelphiLabs"DataSnapXE"Callbacks"demoshowsthemostsimpleuseofcallbacks.BothclientandserverareDelphiVCLFormsapplications.Thistutorialcoversbroadcastingtoachannelandnotifyingaspecificcallback.    IntroductionTheob

2、jectiveofthistutorialistocreatethesimplestpossibleDataSnapDelphiclientandserverapplicationsthatusecallbacksforcommunication.Inthislabexercise,wearegoingtouseDelphiXEtobuildasimplecallbacksdemosystemconsistingofserverandclientapplications.Theserverapplicationwill

3、serveasacommunicationhubformultipleclientapplicationsrunninginthenetwork.Itisamorerealisticscenarioascomparedtosendingnotificationsdirectlyfromserverapplicationuserinterfacetoclients.Inmostscenarios,aserverapplicationwillnothaveanyuserinterface,socallbacksareagr

4、eatmechanismforclientstocommunicatewitheachother.MessageExchangePatternsThemostcommonmessageexchangepatterninclient/serverapplicationsis“request-response”.Oneapplication(“aclient”)issendingamessage(“arequest)toanotherapplicationrunninginthenetwork(“aserver”)andt

5、heserversendsbackamessage(“aresponse").Hideimageexpandview>>Inmanyrealworldapplications,itwouldalsobeusefultohavetheoppositesituation,whereitisaserverapplicationthatsendsamessage(“anotification”)toaclientapplication.Aserverapplicationmaywanttoinformaclientthatso

6、methinginterestinghashappenedontheserver.Thisiscalled“acallback”–asituationwhenserver“callsback”theclient.Hideimageexpandview>>    Imagineachatapplicationwheremultipleclientapplicationsconnectedtotheservercancommunicatewitheachother.Oneclientsendsamessagetothese

7、rverandthentheserverforwardsthismessagetooneormoreconnectedclientapplications.Hideimageexpandview>>Thepossibilityfortheservertoasynchronouslysendanotificationtooneormoreclientsisveryusefulinmanyscenarios.DataSnapCallbacksandChannels    InordertousecallbacksinDat

8、aSnapapplications,youneedtodefineacustomcallbackclassthatisinheritedfromtheabstract“TDBXCallback”classandoverrideoneofitsvirtual,abstract“Execute”methodswhicharecalle