(完整版)基于Android開發(fā)的外文文獻(xiàn)

1、AndroidAndroid,asasystem,isaJava-basedoperatingsystemthatrunsontheLinux2.6kernel.Thesystemisverylightweightandfullfeatured.AndroidapplicationsaredevelopedusingJavaandcanbeportedrathereasilytothenewplatform.IfyouhavenotyetdownloadedJavaorareunsureaboutwhichversionyouneed,Idetailtheinstallationofthede

2、velopmentenvironmentinChapter2.OtherfeaturesofAndroidincludeanaccelerated3-Dgraphicsengine(basedonhardwaresupport),databasesupportpoweredbySQLite,andanintegratedwebbrowser.IfyouarefamiliarwithJavaprogrammingorareanOOPdeveloperofanysort,youarelikelyusedtoprogrammaticuserinterface(UI)developmentthatis

3、,UIplacementwhichishandleddirectlywithintheprogramcode.Android,whilerecognizingandallowingforprogrammaticUIdevelopment,alsosupportsthenewer,XML-basedUIlayout.XMLUIlayoutisafairlynewconcepttotheaveragedesktopdeveloper.IwillcoverboththeXMLUIlayoutandtheprogrammaticUIdevelopmentinthesupportingchapterso

4、fthisbook.OneofthemoreexcitingandcompellingfeaturesofAndroidisthat,becauseofitsarchitecture,third-partyapplicationsincludingthosethatare“homegrown”areexecutedwiththesamesystempriorityasthosethatarebundledwiththecoresystem.Thisisamajordeparturefrommostsystems,whichgiveembeddedsystemappsagreaterexecut

5、ionprioritythanthethreadpriorityavailabletoappscreatedbythird-partydevelopers.Also,eachapplicationisexecutedwithinitsownthreadusingaverylightweightvirtualmachine.AsidefromtheverygenerousSDKandthewell-formedlibrariesthatareavailabletoustodevelopwith,themostexcitingfeatureforAndroiddevelopersisthatwen

6、owhaveaccesstoanythingtheoperatingsystemhasaccessto.Inotherwords,ifyouwanttocreateanapplicationthatdialsthephone,youhaveaccesstothephonesdialer;ifyouwanttocreateanapplicationthatutilizesthephonesinternalGPS(ifequipped),youhaveaccesstoit.Thepotentialfordeveloperstocreatedynamicandintriguingapplicatio

7、nsisnowwideopen.OntopofallthefeaturesthatareavailablefromtheAndroidsideoftheequation,Googlehasthrowninsomeverytantalizingfeaturesofitsown.DevelopersofAndroidapplicationswillbeabletotietheirapplicationsintoexistingGoogleofferingssuchasGoogleMapsandtheomnipresentGoogleSearch.Supposeyouwanttowriteanapp

8、licationthatpullsupaGooglemapofwhereanincomingcallisemanatingfrom,oryouwanttobeabletostorecommonsearchresultswithyourcontacts;thedoorsofpossibilityhavebeenflungwideopenwithAndroid.Chapter2beginsyourjourneytoAndroiddevelopment.Youwilllearnthehowsandwhysofusingspecificdevelopmentenvironmentsorintegrat

9、eddevelopmentenvironments(IDE),andyouwilldownloadandinstalltheJavaIDEEclipse.ApplicationComponentsAcentralfeatureofAndroidisthatoneapplicationcanmakeuseofelementsofotherapplications(providedthoseapplicationspermitit).Forexample,ifyourapplicationneedstodisplayascrollinglistofimagesandanotherapplicati

10、onhasdevelopedasuitablescrollerandmadeitavailabletoothers,youcancalluponthatscrollertodothework,ratherthandevelopyourown.Yourapplicationdoesntincorporatethecodeoftheotherapplicationorlinktoit.Rather,itsimplystartsupthatpieceoftheotherapplicationwhentheneedarises.Forthistowork,thesystemmustbeabletost

11、artanapplicationprocesswhenanypartofitisneeded,andinstantiatetheJavaobjectsforthatpart.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdonthaveasingleentrypointforeverythingintheapplication(nomain()function,forexample).Rather,theyhaveessentialcomponentsthatthesystemcaninstantiatean

12、drunasneeded.Therearefourtypesofcomponents:ActivitiesAnactivitypresentsavisualuserinterfaceforonefocusedendeavortheusercanundertake.Forexample,anactivitymightpresentalistofmenuitemsuserscanchoosefromoritmightdisplayphotographsalongwiththeircaptions.Atextmessagingapplicationmighthaveoneactivitythatsh

13、owsalistofcontactstosendmessagesto,asecondactivitytowritethemessagetothechosencontact,andotheractivitiestoreviewoldmessagesorchangesettings.Thoughtheyworktogethertoformacohesiveuserinterface,eachactivityisindependentoftheothers.EachoneisimplementedasasubclassoftheActivitybaseclass.Anapplicationmight

14、consistofjustoneactivityor,likethetextmessagingapplicationjustmentioned,itmaycontainseveral.Whattheactivitiesare,andhowmanytherearedepends,ofcourse,ontheapplicationanditsdesign.Typically,oneoftheactivitiesismarkedasthefirstonethatshouldbepresentedtotheuserwhentheapplicationislaunched.Movingfromoneac

15、tivitytoanotherisaccomplishedbyhavingthecurrentactivitystartthenextone.Eachactivityisgivenadefaultwindowtodrawin.Typically,thewindowfillsthescreen,butitmightbesmallerthanthescreenandfloatontopofotherwindows.Anactivitycanalsomakeuseofadditionalwindowsforexample,apop-updialogthatcallsforauserresponsei

16、nthemidstoftheactivity,orawindowthatpresentsuserswithvitalinformationwhentheyselectaparticularitemon-screen.ThevisualcontentofthewindowisprovidedbyahierarchyofviewsobjectsderivedfromthebaseViewclass.Eachviewcontrolsaparticularrectangularspacewithinthewindow.Parentviewscontainandorganizethelayoutofth

17、eirchildren.Leafviews(thoseatthebottomofthehierarchy)drawintherectanglestheycontrolandrespondtouseractionsdirectedatthatspace.Thus,viewsarewheretheactivitysinteractionwiththeusertakesplace.Forexample,aviewmightdisplayasmallimageandinitiateanactionwhentheusertapsthatimage.Androidhasanumberofready-mad

18、eviewsthatyoucanuseincludingbuttons,textfields,scrollbars,menuitems,checkboxes,andmore.AviewhierarchyisplacedwithinanactivityswindowbytheActivity.setContentView()method.ThecontentviewistheViewobjectattherootofthehierarchy.(SeetheseparateUserInterfacedocumentformoreinformationonviewsandthehierarchy.)

19、ServicesAservicedoesnthaveavisualuserinterface,butratherrunsinthebackgroundforanindefiniteperiodoftime.Forexample,aservicemightplaybackgroundmusicastheuserattendstoothermatters,oritmightfetchdataoverthenetworkorcalculatesomethingandprovidetheresulttoactivitiesthatneedit.EachserviceextendstheServiceb

20、aseclass.Aprimeexampleisamediaplayerplayingsongsfromaplaylist.Theplayerapplicationwouldprobablyhaveoneormoreactivitiesthatallowtheusertochoosesongsandstartplayingthem.However,themusicplaybackitselfwouldnotbehandledbyanactivitybecauseuserswillexpectthemusictokeepplayingevenaftertheyleavetheplayerandb

21、eginsomethingdifferent.Tokeepthemusicgoing,themediaplayeractivitycouldstartaservicetoruninthebackground.Thesystemwouldthenkeepthemusicplaybackservicerunningevenaftertheactivitythatstarteditleavesthescreen.Itspossibletoconnectto(bindto)anongoingservice(andstarttheserviceifitsnotalreadyrunning).Whilec

22、onnected,youcancommunicatewiththeservicethroughaninterfacethattheserviceexposes.Forthemusicservice,thisinterfacemightallowuserstopause,rewind,stop,andrestarttheplayback.Likeactivitiesandtheothercomponents,servicesruninthemainthreadoftheapplicationprocess.Sothattheywontblockothercomponentsortheuserin

23、terface,theyoftenspawnanotherthreadfortime-consumingtasks(likemusicplayback).SeeProcessesandThreads,later.BroadcastreceiversAbroadcastreceiverisacomponentthatdoesnothingbutreceiveandreacttobroadcastannouncements.Manybroadcastsoriginateinsystemcodeforexample,announcementsthatthetimezonehaschanged,tha

24、tthebatteryislow,thatapicturehasbeentaken,orthattheuserchangedalanguagepreference.Applicationscanalsoinitiatebroadcastsforexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Anapplicationcanhaveanynumberofbroadcastreceiverstorespondtoanyannouncementsi

25、tconsidersimportant.AllreceiversextendtheBroadcastReceiverbaseclass.Broadcastreceiversdonotdisplayauserinterface.However,theymaystartanactivityinresponsetotheinformationtheyreceive,ortheymayusetheNotificationManagertoalerttheuser.Notificationscangettheusersattentioninvariouswaysflashingthebacklight,

26、vibratingthedevice,playingasound,andsoon.Theytypicallyplaceapersistenticoninthestatusbar,whichuserscanopentogetthemessage.ContentprovidersAcontentprovidermakesaspecificsetoftheapplicationsdataavailabletootherapplications.Thedatacanbestoredinthefilesystem,inanSQLitedatabase,orinanyothermannerthatmake

27、ssense.ThecontentproviderextendstheContentProviderbaseclasstoimplementastandardsetofmethodsthatenableotherapplicationstoretrieveandstoredataofthetypeitcontrols.However,applicationsdonotcallthesemethodsdirectly.RathertheyuseaContentResolverobjectandcallitsmethodsinstead.AContentResolvercantalktoanyco

28、ntentprovider;itcooperateswiththeprovidertomanageanyinterprocesscommunicationthatsinvolved.SeetheseparateContentProvidersdocumentformoreinformationonusingcontentproviders.Whenevertheresarequestthatshouldbehandledbyaparticularcomponent,Androidmakessurethattheapplicationprocessofthecomponentisrunning,

29、startingitifnecessary,andthatanappropriateinstanceofthecomponentisavailable,creatingtheinstanceifnecessary.KeySkills&ConceptsCreatingnewAndroidprojectsWorkingwithViewsUsingaTextViewModifyingthemain.xmlfileCreatingYourFirstAndroidProjectinEclipseTostartyourfirstAndroidproject,openEclipse.WhenyouopenE

30、clipseforthefirsttime,itopenstoanemptydevelopmentenvironment(seeFigure5-1),whichiswhereyouwanttobegin.Yourfirsttaskistosetupandnametheworkspaceforyourapplication.ChooseFile|New|AndroidProject,whichwilllaunchtheNewAndroidProjectwizard.CAUTIONDonotselectJavaProjectfromtheNewmenu.WhileAndroidapplicatio

31、nsarewritteninJava,andyouaredoingallofyourdevelopmentinJavaprojects,thisoptionwillcreateastandardJavaapplication.SelectingAndroidProjectenablesyoutocreateAndroid-specificapplications.IfyoudonotseetheoptionforAndroidProject,thisindicatesthattheAndroidpluginforEclipsewasnotfullyorcorrectlyinstalled.Re

32、viewtheprocedureinChapter3forinstallingtheAndroidpluginforEclipsetocorrectthis.TheNewAndroidProjectwizardcreatestwothingsforyouAshellapplicationthattiesintotheAndroidSDK,usingtheandroid.jarfile,andtiestheprojectintotheAndroidEmulator.ThisallowsyoutocodeusingalloftheAndroidlibrariesandpackages,andals

33、oletsyoudebugyourapplicationsintheproperenvironment.Yourfirstshellfilesforthenewproject.Theseshellfilescontainsomeofthevitalapplicationblocksuponwhichyouwillbebuildingyourprograms.InmuchthesamewayascreatingaMicrosoft.NETapplicationinVisualStudiogeneratessomeWindows-createdprogramcodeinyourfiles,usingtheAndroidProjectwizardinEclipsegeneratesyourinitialprogramfilesandsomeAndroid-createdcode.Inaddition,theNewAndroidProjectwizardcontainsafewoptions,shownnext,thatyoumustsettoinitiateyourAndroidproject.FortheProjectNamefield,forpurposesofthisexample,usethetitleHelloWorldText.Thisnamesufficientlyd