基于Android开发的外文文献.docx
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基于Android开发的外文文献.docx
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基于基于Android开发的外文文献开发的外文文献AndroidAndroid,asasystem,isaJava-basedoperatingsystemthatrunsontheLinux2.6kernel.Thesystemisverylightweightandfullfeatured.AndroidapplicationsaredevelopedusingJavaandcanbeportedrathereasilytothenewplatform.IfyouhavenotyetdownloadedJavaorareunsureaboutwhichversionyouneed,IdetailtheinstallationofthedevelopmentenvironmentinChapter2.OtherfeaturesofAndroidincludeanaccelerated3-Dgraphicsengine(basedonhardwaresupport),databasesupportpoweredbySQLite,andanintegratedwebbrowser.IfyouarefamiliarwithJavaprogrammingorareanOOPdeveloperofanysort,youarelikelyusedtoprogrammaticuserinterface(UI)developmentthatis,UIplacementwhichishandleddirectlywithintheprogramcode.Android,whilerecognizingandallowingforprogrammaticUIdevelopment,alsosupportsthenewer,XML-basedUIlayout.XMLUIlayoutisafairlynewconcepttotheaveragedesktopdeveloper.IwillcoverboththeXMLUIlayoutandtheprogrammaticUIdevelopmentinthesupportingchaptersofthisbook.OneofthemoreexcitingandcompellingfeaturesofAndroidisthat,becauseofitsarchitecture,third-partyapplicationsincludingthosethatare“homegrown”areexecutedwiththesamesystempriorityasthosethatarebundledwiththecoresystem.Thisisamajordeparturefrommostsystems,whichgiveembeddedsystemappsagreaterexecutionprioritythanthethreadpriorityavailabletoappscreatedbythird-partydevelopers.Also,eachapplicationisexecutedwithinitsownthreadusingaverylightweightvirtualmachine.AsidefromtheverygenerousSDKandthewell-formedlibrariesthatareavailabletoustodevelopwith,themostexcitingfeatureforAndroiddevelopersisthatwenowhaveaccesstoanythingtheoperatingsystemhasaccessto.Inotherwords,ifyouwanttocreateanapplicationthatdialsthephone,youhaveaccesstothephonesdialer;ifyouwanttocreateanapplicationthatutilizesthephonesinternalGPS(ifequipped),youhaveaccesstoit.Thepotentialfordeveloperstocreatedynamicandintriguingapplicationsisnowwideopen.OntopofallthefeaturesthatareavailablefromtheAndroidsideoftheequation,Googlehasthrowninsomeverytantalizingfeaturesofitsown.DevelopersofAndroidapplicationswillbeabletotietheirapplicationsintoexistingGoogleofferingssuchasGoogleMapsandtheomnipresentGoogleSearch.SupposeyouwanttowriteanapplicationthatpullsupaGooglemapofwhereanincomingcallisemanatingfrom,oryouwanttobeabletostorecommonsearchresultswithyourcontacts;thedoorsofpossibilityhavebeenflungwideopenwithAndroid.Chapter2beginsyourjourneytoAndroiddevelopment.Youwilllearnthehowsandwhysofusingspecificdevelopmentenvironmentsorintegrateddevelopmentenvironments(IDE),andyouwilldownloadandinstalltheJavaIDEEclipse.ApplicationComponentsAcentralfeatureofAndroidisthatoneapplicationcanmakeuseofelementsofotherapplications(providedthoseapplicationspermitit).Forexample,ifyourapplicationneedstodisplayascrollinglistofimagesandanotherapplicationhasdevelopedasuitablescrollerandmadeitavailabletoothers,youcancalluponthatscrollertodothework,ratherthandevelopyourown.Yourapplicationdoesntincorporatethecodeoftheotherapplicationorlinktoit.Rather,itsimplystartsupthatpieceoftheotherapplicationwhentheneedarises.Forthistowork,thesystemmustbeabletostartanapplicationprocesswhenanypartofitisneeded,andinstantiatetheJavaobjectsforthatpart.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdonthaveasingleentrypointforeverythingintheapplication(nomain()function,forexample).Rather,theyhaveessentialcomponentsthatthesystemcaninstantiateandrunasneeded.Therearefourtypesofcomponents:
ActivitiesAnactivitypresentsavisualuserinterfaceforonefocusedendeavortheusercanundertake.Forexample,anactivitymightpresentalistofmenuitemsuserscanchoosefromoritmightdisplayphotographsalongwiththeircaptions.Atextmessagingapplicationmighthaveoneactivitythatshowsalistofcontactstosendmessagesto,asecondactivitytowritethemessagetothechosencontact,andotheractivitiestoreviewoldmessagesorchangesettings.Thoughtheyworktogethertoformacohesiveuserinterface,eachactivityisindependentoftheothers.EachoneisimplementedasasubclassoftheActivitybaseclass.Anapplicationmightconsistofjustoneactivityor,likethetextmessagingapplicationjustmentioned,itmaycontainseveral.Whattheactivitiesare,andhowmanytherearedepends,ofcourse,ontheapplicationanditsdesign.Typically,oneoftheactivitiesismarkedasthefirstonethatshouldbepresentedtotheuserwhentheapplicationislaunched.Movingfromoneactivitytoanotherisaccomplishedbyhavingthecurrentactivitystartthenextone.Eachactivityisgivenadefaultwindowtodrawin.Typically,thewindowfillsthescreen,butitmightbesmallerthanthescreenandfloatontopofotherwindows.Anactivitycanalsomakeuseofadditionalwindowsforexample,apop-updialogthatcallsforauserresponseinthemidstoftheactivity,orawindowthatpresentsuserswithvitalinformationwhentheyselectaparticularitemon-screen.ThevisualcontentofthewindowisprovidedbyahierarchyofviewsobjectsderivedfromthebaseViewclass.Eachviewcontrolsaparticularrectangularspacewithinthewindow.Parentviewscontainandorganizethelayoutoftheirchildren.Leafviews(thoseatthebottomofthehierarchy)drawintherectanglestheycontrolandrespondtouseractionsdirectedatthatspace.Thus,viewsarewheretheactivitysinteractionwiththeusertakesplace.Forexample,aviewmightdisplayasmallimageandinitiateanactionwhentheusertapsthatimage.Androidhasanumberofready-madeviewsthatyoucanuseincludingbuttons,textfields,scrollbars,menuitems,checkboxes,andmore.AviewhierarchyisplacedwithinanactivityswindowbytheActivity.setContentView()method.ThecontentviewistheViewobjectattherootofthehierarchy.(SeetheseparateUserInterfacedocumentformoreinformationonviewsandthehierarchy.)ServicesAservicedoesnthaveavisualuserinterface,butratherrunsinthebackgroundforanindefiniteperiodoftime.Forexample,aservicemightplaybackgroundmusicastheuserattendstoothermatters,oritmightfetchdataoverthenetworkorcalculatesomethingandprovidetheresulttoactivitiesthatneedit.EachserviceextendstheServicebaseclass.Aprimeexampleisamediaplayerplayingsongsfromaplaylist.Theplayerapplicationwouldprobablyhaveoneormoreactivitiesthatallowtheusertochoosesongsandstartplayingthem.However,themusicplaybackitselfwouldnotbehandledbyanactivitybecauseuserswillexpectthemusictokeepplayingevenaftertheyleavetheplayerandbeginsomethingdifferent.Tokeepthemusicgoing,themediaplayeractivitycouldstartaservicetoruninthebackground.Thesystemwouldthenkeepthemusicplaybackservicerunningevenaftertheactivitythatstarteditleavesthescreen.Itspossibletoconnectto(bindto)anongoingservice(andstarttheserviceifitsnotalreadyrunning).Whileconnected,youcancommunicatewiththeservicethroughaninterfacethattheserviceexposes.Forthemusicservice,thisinterfacemightallowuserstopause,rewind,stop,andrestarttheplayback.Likeactivitiesandtheothercomponents,servicesruninthemainthreadoftheapplicationprocess.Sothattheywontblockothercomponentsortheuserinterface,theyoftenspawnanotherthreadfortime-consumingtasks(likemusicplayback).SeeProcessesandThreads,later.BroadcastreceiversAbroadcastreceiverisacomponentthatdoesnothingbutreceiveandreacttobroadcastannouncements.Manybroadcastsoriginateinsystemcodeforexample,announcementsthatthetimezonehaschanged,thatthebatteryislow,thatapicturehasbeentaken,orthattheuserchangedalanguagepreference.Applicationscanalsoinitiatebroadcastsforexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Anapplicationcanhaveanynumberofbroadcastreceiverstorespondtoanyannouncementsitconsidersimportant.AllreceiversextendtheBroadcastReceiverbaseclass.Broadcastreceiversdonotdisplayauserinterface.However,theymaystartanactivityinresponsetotheinformationtheyreceive,ortheymayusetheNotificationManagertoalerttheuser.Notificationscangettheusersattentioninvariouswaysflashingthebacklight,vibratingthedevice,playingasound,andsoon.Theytypicallyplaceapersistenticoninthestatusbar,whichuserscanopentogetthemessage.ContentprovidersAcontentprovidermakesaspecificsetoftheapplicationsdataavailabletootherapplications.Thedatacanbestoredinthefilesystem,inanSQLitedatabase,orinanyothermannerthatmakessense.ThecontentproviderextendstheContentProviderbaseclasstoimplementastandardsetofmethodsthatenableotherapplicationstoretrieveandstoredataofthetypeitcontrols.However,applicationsdonotcallthesemethodsdirectly.RathertheyuseaContentResolverobjectandcallitsmethodsinstead.AContentResolvercantalktoanycontentprovider;itcooperateswiththeprovidertomanageanyinterprocesscommunicationthatsinvolved.SeetheseparateContentProvidersdocumentformoreinformationonusingcontentproviders.Whenevertheresarequestthatshouldbehandledbyaparticularcomponent,Androidmakessurethattheapplicationprocessofthecomponentisrunning,startingitifnecessary,andthatanappropriateinstanceofthecomponentisavailable,creatingtheinstan
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