Modelling Web Navigation by Statechart(3)

UML,OOP,面向对象

(a) (b)

Figure1.XORgroupingofstates2.3Statechart

Statechartis rstproposedbyHarelin1987[3,5].Itisavisualformalismtoextendstatediagramsformodelingofcomplexsystemsthatinvolveslargenumberofconcur-rentstates,synchronizationsandactiontriggers.Therearenumerousextensionstostatecharttosupportdifferentmod-elingneeds,wewillintroducethebasicandtherelevantpartofstatechartweusedinthispaperhere.

Astatechartcanbede nedasthebasicsetofele-ments:states,transitions,primitiveevents,primitivecon-ditionsandvariables,plustheextendedsetofevents,con-ditions,expressionsandlabelsandtheirinter-relations[5].isde nedasthesetofstates.Ahierarchyfunction

,isde nedtomapeachstatetoitssub-states.meansisabasicstate.Atypefunc-tionde neswhetherisacomposedAND/ORstateornot.isthesetofhistorysymbols.Itisrelatedtobyafunction.Thedefaultfunction,de nestheinitialstatesin.isde nedasthesetofvariablesandthesetofexpressions,,isde nedon.Thesetofprimitivecon-ditionsisde nedasandwitharethede nedsetofasconditionsthesetofprimitive

de nedonit.Similarly,eventsandeventsrespectively.

isde nedasthesetofactions.isthesetoflabelsontransitions.Thesetoftransitions,,isde nedas.Forclarity,statenameswillbeshowninboldandeventlabelswillbeshowninsansinthispaper.

Likeastatediagram,astatechartmodelstartsinaninitialstate.Whenaneventhappens,thesystemwilltransittothenextstatebyatransitiontriggered(labeled)bythatevent.Anoptionalactivitycanbeaddedtothetransitionlabel,in-dicatingthattheactivitywilltakeplacewhenthetransitionhappens.Thetriggeredactivitycaninturnreceivedbythesystemasanothereventtotriggerothertransitions.

StatecanbegroupedtogetherbyXORforef cientuseoftransitionarrows.Figure1showsthesavingsfromtheexpanded(a)tothemoreconcise(b).ThesystemcanbeexclusivelyinstateAOR

B.Concurrentstates,likeinde-Figure2.ANDgroupingof

states

Figure3.Abstractionofstates

pendentmodulesinasystem,canbemodeledbyorthogo-nalANDofsub-statesthatexistconcurrently(Figure2).Inthe gure,beingsub-statesofthecomposedstateA,A1,A2andA3maintainstheirownstate.

Abstractionofstatechartcanbedonebyhidingdetailsofsub-states.Figure3aisabstractedtoFigure3bbyhidingthedetailsoftheXORsub-states.Thestubbedtransitionarrowofe2representsthatthetransitionisavailablefromsomeofthesub-statesofAonly.Theselectconnectivecanbeusedtoreducethenumberoftransitionarrows.EventseinFigure4bisde nedasthedisjunctionofeventse1,e2inFigure4a.ereplacese1,e2bypointingtotheselectcon-nective,meaningthatAwill

transittoB1orB2dependingone1ore2happened.

Figure4.Selectconnective