有关android技术英文文献翻译

1、英语原文AndroidApplicationFundamentalsAndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecodealongwithanydataandresourcefiles-intoanAndroidpackageanarchivean.apksuffix.Allthecodeinasingle.apkconsideredtobeoneapplicationandistheAndroid-powereddevicesusetoinstalltheappli

2、cation.Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:TheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.Bydefault,thesystemassignseachapplicationauniqueLinuxuserID(theIDisusedonlybythesystemandisunknowntotheapplication).Thesystemsetspe

3、rmissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.Eachprocesshasitsownvirtualmachine(VM),soanapplicationscoderunsinisolationfromotherapplications.Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplicationscomponents

4、needtobeexecuted,thenshutsdowntheprocesswhenitsnolongerneededorwhenthesystemmustrecovermemoryforotherapplications.Inthisway,theAndroidsystemimplementstheprincipleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenv

5、ironmentinwhichanapplicationcannotaccesspartsofthesystemforwhichitisnotgivenpermission.However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:ItspossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachother

6、sfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocessandsharethesameVM(theapplicationsmustalsobesignedwiththesamecertificate).Anapplicationcanrequestpermissiontoaccessdevicedatasuchastheuserscontacts,SMSmessages,themountablestorage(SDcard),camera,Blue

7、tooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Therestofthisdocumentintroducesyouto:Thecoreframeworkcomponentsthatdefineyourapplication.Themanifestwhichyoudeclarecomponentsandrequireddevicefeaturesfo

8、ryourapplication.Resourcesthatareseparatefromtheapplicationcodeandallowyourapplicationtogracefullyoptimizeitsbehaviorforavarietyofdeviceconfigurations.ApplicationComponentsApplicationcomponentsaretheessentialbuildingblocksofanAndroidapplication.Eachcomponentisadifferentpointthroughwhichthesystemcane

9、nteryourapplication.Notallcomponentsareactualentrypointsfortheuserandsomedependoneachother,buteachoneexistsasitsownentityandplaysaspecificrole-eachoneisauniquebuildingblockthathelpsdefineyourapplicationsoverallbehavior.Therearefourdifferenttypesofapplicationcomponents.Eachtypeservesadistinctpurposea

10、ndhasadistinctlifecyclethatdefineshowthecomponentiscreatedanddestroyed.Herearethefourtypesofapplicationcomponents:ActivitiesAnactivityrepresentsasinglescreenwithauserinterface.Forexample,anemailapplicationmighthaveoneactivitythatshowsalistofnewemails,anotheractivitytocomposeanemail,andanotheractivit

11、yforreadingemails.Althoughtheactivitiesworktogethertoformacohesiveuserexperienceintheemailapplication,eachoneisindependentoftheothers.Assuch,adifferentapplicationcanstartanyoneoftheseactivities(iftheemailapplicationallowsit).Forexample,acameraapplicationcanstarttheactivityintheemailapplicationthatco

12、mposesnewmail,inorderfortheusertoshareapicture.AnactivityisimplementedasasubclassoActivityandyoucanlearnmoreaboutitintheActivitiesdeveloperguide.ServicesAserviceisacomponentthatrunsinthebackgroundtoperformlong-runningoperationsortoperformworkforremoteprocesses.Aservicedoesnotprovideauserinterface.Fo

13、rexample,aservicemightplaymusicinthebackgroundwhiletheuserisinadifferentapplication,oritmightfetchdataoverthenetworkwithoutblockinguserinteractionwithanactivity.Anothercomponent,suchasanactivity,canstarttheserviceandletitrunorbindtoitinordertointeractwithit.AserviceisimplementedasasubclassofServicea

14、ndyoucanlearnmoreaboutitintheServicesdeveloperguide.ContentprovidersAcontentprovidermanagesasharedsetofapplicationdata.Youcanstorethedatainthe,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata(

15、ifthecontentproviderallowsit).Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuserscontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider(suchasContactsContract.Datatoreadandwriteinformationaboutaparticularperson.Contentprovidersarealsousefu

16、lforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,theNotePadsampleapplicationusesacontentprovidertosavenotes.AcontentproviderisimplementedasasubclassofContentProviderandmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,see

17、theContentProvidersdeveloperguide.BroadcastreceiversAbroadcastreceiverisacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystemforexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcasts-f

18、orexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdontdisplayauserinterface,theymaycreateastatusbarnotificationtoalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcastreceiverisjustagatewaytoothercomponentsa

19、ndisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.AbroadcastreceiverisimplementedasasubclassofBroadcastReceiverandeachbroadcastisdeliveredasanIntentobject.Formoreinformation,seetheBroadcastReceiverclass.AuniqueaspectoftheAndroidsystemdesigni

20、sthatanyapplicationcanstartanotherapplicationscomponent.Forexample,ifyouwanttheouseptureaphotowiththedevicecamera,theresprobablyanotherapplicationthatdoesthatandyourapplicationcanuseit,insteadofdevelopinganactivitytocaptureaphotoyourself.Youdontneedtoincorporateorevenlinktothecodefromthecameraapplic

21、ation.Instead,youcansimplystarttheactivityinthecameraapplicationthatcapturesaphoto.Whencomplete,thephotoisevenreturnedtoyourapplicationsoyoucanuseit.Totheuser,itseemsasifthecameraisactuallyapartofyourapplication.Whenthesystemstartsacomponent,itstartstheprocessforthatapplication(ifitsnotalreadyrunnin

22、g)andinstantiatestheclassesneededforthecomponent.Forexample,ifyourapplicationstartstheactivityinthecameraapplicationthatcapturesaphoto,thatactivityrunsintheprocessthatbelongstothecameraapplication,notinyourapplicationsprocess.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdonthave

23、asingleentrypoint(theresnomain()function,forexample).Becausethesystemrunseachapplicationinaseparateprocesswitthatrestrictaccesstootherapplications,yourapplicationcannotdirectlyactivateacomponentfromanotherapplication.TheAndroidsystem,however,can.So,toactivateacomponentinanotherapplication,youmustdel

24、iveramessagetothesystemthatspecifiesyountenttostartaparticularcomponent.Thesystemthenactivatesthecomponentforyou.ActivatingComponentsThreeofthefourcomponenttypes-activities,services,andbroadcastreceiversareactivatedbyanasynchronousmessagecalledantent.Intentsbindindividualcomponentstoeachotheratrunti

25、me(youcanthinkofthemasthemessengersthatrequestanactionfromothercomponents),whetherthecomponentbelongstoyourapplicationoranother.AnintentiscreatedwithanIntentobject,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecifictypeofcomponentanintentcanbeeitherexplicitorimplicit,respectively.Forac

26、tivitiesandservices,anintentdefinestheactiontoperform(forexample,tovieworsendsomething)andmayspecifytheURIofthedatatoacton(amongotherthingsthatthecomponentbeingstartedmightneedtoknow).Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivityto

27、receivearesult,inwhichcase,theactivityalsoreturnstheresultinanIntent(forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyouthereturnintentincludesaURIpointingtothechosencontact).Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast(forexample,abroadc

28、asttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicatesbatteryislow).Theothercomponenttype,contentprovider,isnotactivatedbyintents.Rather,itisactivatedwhentargetedbyarequestfronaContentResolverThecontentresolverhandlesalldirecttransactionswiththecontentprovidersothatthecomponen

29、tthatsperformingtransactionswiththeproviderdoesntneedtoandinsteadcallsmethodsontheContentResolverobject.Thisleavesalayerofabstractionbetweenthecontentproviderandthecomponentrequestinginformation(forsecurity).Thereareseparatemethodsforactivatingeachtypeofcomponent:?Youcanstartanactivity(orgiveitsomet

30、hingnewtodo)bypassinganIntenttostartActivity()orstartActivityForResult()(whenyouwanttheactivitytoreturnaresult).?Youcanstartaservice(orgivenewinstructionstoanongoingservice)bypassinganIntenttostartService().OryoucanbindtotheservicebypassinganIntenttobindService().?Youcaninitiateabroadcastbypassingal

31、ntenttomethodslikesendBroadcast()sendOrderedBroadcast,)orsendStickyBroadcast()?Youcanperformaquerytoacontentproviderbycallingquery()onaContentResolverFormoreinformationaboutusingintents,seethententsandIntentFiltersdocument.Moreinformationaboutactivatingspecificcomponentsisalsoprovidedinthefollowingd

32、ocuments:Activities,ServicesBroadcastReceiverandContentProvidersDeclaringcomponentsTheprimarytaskofthemanifestistoinformthesystemabouttheapplicationscomponents.Forexample,amanifestdeclareanactivityasfollows:/activity.Intheelement,theandroid:iconattributepointstoresourcesforaniconthatidentifiestheapp

33、lication.Intheelement,theandroid:nameattributespecifiesthefUlyqualifiedclassnameoftheActivitysubclassandtheandroid:labelattributesspecifiesastringtouseastheuser-visiblelabelfortheactivity.Youmustdeclareallapplicationcomponentsthisway:elementsforactivitieselementsforserviceselementsforbroadcastreceiv

34、erselementsforcontentprovidersActivities,services,andcontentprovidersthatyouincludeinyoursourcebutdonotdeclareinthemanifestarenotvisibletothesystemand,consequently,canneverrun.However,broadcastreceiverscanbeeitherdeclaredinthemanifestorcreateddynamicallyincode(asBroadcastReceiverobjects)andregistere

35、dwiththesystembycallingregisterReceiver()DeclaringcomponentcapabilitiesAsdiscussedabove,inActivatingComponents,youcanuseanIntenttostartactivities,services,andbroadcastreceivers.Youcandosobyexplicitlynamingthetargetcomponent(usingthecomponentclassname)intheintent.However,therealpowerofintentsliesinth

36、econceptofintentactions.Withintentactions,yousimplydescribethetypeofactionyouwanttoperform(andoptionally,thedatauponwhichyoudliketoperformtheaction)andallcthesystemtofindacomponentonthedevicethatcanperformtheactionandstartit.Iftherearemultiplecomponentsthatcanperformtheactiondescribedbytheintent,the

37、ntheuserselectswhichonetouse.Thewaythesystemidentifiesthecomponentsthatcanrespondtoanintentisbycomparingtheintentreceivedtotheintentfiltersprovidedinthemanifestotherapplicationsonthedevice.Whenyoudeclareacomponentinyourapplicationsmanifest,youcanoptionallyincludeintentfiltersthatdeclarethecapabiliti

38、esofthecomponentsoitcanrespondtointentsfromotherapplications.Youcandeclareanintentilterforyourcomponentbyaddinganelementasachildofthecomponentsdeclarationelement.Forexample,anemailapplicationwithanactivityforcomposinganewemailmightdeclareanintentfilterinitsmanifestentrytorespondtosendintents(inorder

39、tosendemail).Anactivityinyourapplicationcanthencreateanintentwiththe”sene(ACTION_SEND),whichthesystemmatchestotheemailapplicationssend”actilaunchesitwhenyouinvoketheintentwithstartActivity().Formoreaboutcreatingintentfilters,seethententsandIntentFiltersdocument.DeclaringapplicationrequirementsTherea

40、reavarietyofdevicespoweredbyAndroidandnotallofthemprovidethesamefeaturesandcapabilities.Inordertopreventyourapplicationfrombeinginstalledondevicesthatlackfeaturesneededbyyourapplication,itsimportantthatyouclearlydefineaprothetypesofdevicesyourapplicationsupportsbydeclaringdeviceandsoftwarerequiremen

41、tsinyourmanifestfile.Mostofthesedeclarationsareinformationalonlyandthesystemdoesnotreadthem,butexternalservicessuchasGooglePlaydoreadtheminordertoprovidefilteringforuserswhentheysearchforapplicationsfromtheirdevice.Forexample,ifyourapplicationrequiresacameraandusesAPIsintroducedinAndroid2.1(APILevel

42、7),youshoulddeclaretheseasrequirementsinyourmanifestfile.Thatway,devicesthatdonothaveacameraandhaveanAndroidversionowerthan2.1cannotinstallyourapplicationfromGooglePlay.However,youcanalsodeclarethatyourapplicationusesthecamera,butdoesnotrequireit.Inthatcase,yourapplicationmustperformacheckatruntimet

43、odetermineifthedevicehasacameraanddisableanyfeaturesthatusethecameraifoneisnotavailable.Herearesomeoftheimportantdevicecharacteristicsthatyoushouldconsiderasyoudesignanddevelopyourapplication:ScreensizeanddensityInordertocategorizedevicesbytheirscreentype,Androiddefinestwocharacteristicsforeachdevic

44、e:screensize(thephysicaldimensionsofthescreen)andscreendensity(thephysicaldensityofthepixelsonthescreen,ordpi-dotsperinch).Tosimplifyallthedifferenttypesofscreenconfigurations,theAndroidsystemgeneralizesthemintoselectgroupsthatmakethemeasiertotarget.Thescreensizesare:small,normal,large,andextralarge

45、.Thescreendensitiesare:lowdensity,mediumdensity,highdensity,andextrahighdensity.Bydefault,yourapplicationiscompatiblewithallscreensizesanddensities,becausetheAndroidsystemmakestheappropriateadjustmentstoyourUIlayoutandimageresources.However,youshouldcreatespecializedlayoutsforcertainscreensizesandpr

46、ovidespecializedimagesforcertaindensities,usingalternativelayoutresources,andbydeclaringinyourmanifestexactlywhichscreensizesyourapplicationsupportswiththeelement.Formoreinformation,seetheSupportingMultipleScreensdocument.InputconfigurationsManydevicesprovideadifferenttypeofuserinputmechanism,suchas

47、ahardwarekeyboard,atrackball,orafive-waynavigationpad.Ifyourapplicationrequiresaparticularkindofinputhardware,thenyoushoulddeclareitinyourmanifestwiththeelement.However,itisrarethatanapplicationshouldrequireacertaininputconfiguration.DevicefeaturesTherearemanyhardwareandsoftwarefeaturesthatmayormayn

48、otexistonagivenAndroid-powereddevice,suchasacamera,alightsensor,bluetooth,acertainversionofOpenGL,orthefidelityofthetouchscreen.YoushouldneverassumethatacertainfeatureisavailableonallAndroid-powereddevices(otherthantheavailabilityofthestandardAndroidlibrary),soyoushoulddeclareanyfeaturesusedbyyourap

49、plicationwiththeelement.PlatformVersionDifferentAndroid-powereddevicesoftenrundifferentversionsoftheAndroidplatform,suchasAndroid1.6orAndroid2.3.EachsuccessiveversionoftenincludesadditionalAPIsnotavailableinthepreviousversion.InordertoindicatewhichsetofAPIsareavailable,eachplatformversionspecifiesan

50、APILevel(forexample,Android1.0isAPILevel1andAndroid2.3isAPILevel9).IfyouuseanyAPIsthatwereaddedtotheplatformafterversion1.0,youshoulddeclaretheminimumAPILevelinwhichthoseAPIswereintroducedusingtheelement.Itsimportantthatyoudeclareallsuchrequirementsforyourapplication,because,whenyoudistributeyourapp

51、licationonGooglePlay,thestoreusesthesedeclarationstofilterwhichapplicationsareavailableoneachdevice.Assuch,yourapplicationshouldbeavailableonlytodevicesthatmeetallyourapplicationrequirements.FormoreinformationabouthowGooglePlayfiltersapplicationsbasedonthese(andother)requirements,seethiltersonGoogle

52、Playdocument.ApplicationResourcesAnAndroidapplicationiscomposedofmorethanjustcode-itrequiresresourcesthatareseparatefromthesourcecode,suchasimages,audiofiles,andanythingrelatingtothevisualpresentationoftheapplication.Forexample,youshoulddefineanimations,menus,styles,colors,andthelayoutofactivityuser

53、interfaceswithXMLfiles.Usingapplicationresourcesmakesiteasytoupdatevariouscharacteristicsofyourapplicationwithoutmodifyingcodeandbyprovidingsetsofalternativeresources-enablesyoutooptimizeyourapplicationforavarietyofdeviceconfigurations(suchasdifferentlanguagesandscreensizes).Foreveryresourcethatyoui

54、ncludeinyourAndroidproject,theSDKbuildtoolsdefineauniqueintegerID,whichyoucanusetoreferencetheresourcefromyourapplicationcodeorfromotherresourcesdefinedinXML.Forexample,ifyourapplicationcontainsanimagelogo.png(savedintheres/drawable/directory),theSDKtoolsgeneratearesourceIDnamedR.drawable.logQwhichy

55、oucanusetoreferencetheimageandinsertitinyouruserinterface.Oneofthemostimportantaspectsofprovidingresourcesseparatefromyoursourcecodeistheabilityforyoutoprovidealternativeresourcesfordifferentdeviceconfigurations.Forexample,bydefiningUIstringsinXML,youcantranslatethestringsintootherlanguagesandsaveth

56、osestringsinseparatefiles.Then,basedonalanguagealifierthatyouappendtotheresourcedirectorysname(suchares/values-fr/forFrenchstringvalues)andtheuserslanguagesetting,theAndroidsystemappliestheappropriatelanguagestringstoyourUI.Androidsupportsmanydifferentqualifiersforyouralternativeresources.Thequalifi

57、erisashortstringthatyouincludeinthenameofyourresourcedirectoriesinordertodefinethedeviceconfigurationforwhichthoseresourcesshouldbeused.Asanotherexample,youshouldoftencreatedifferentlayoutsforyouractivities,dependingonthedevicesscreenorientationandsize.Forexample,whenthedevicescreenisinportraitorientation(tall),youmightwantalayoutwithbuttonstobevertical,butwhenthescreenisinlandscapeorientation(wide),thebuttonsshouldbealignedhorizontally.Tochangethelayoutdependingontheorientation,youcandefinetwodifferentlayoutsandapplytheappropriatequalifiertoeachlayoutsdirectoryname.Then,thesy