外文翻译-安卓应用基础

0外文原文AndroidApplicationFundamentalsAndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecodealongwithanydataandresourcefilesintoanAndroidpackage,anarchivefilewithan.apksuffix.Allthecodeinasingle.apkfileisconsideredtobeoneapplicationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:TheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.Bydefault,thesystemassignseachapplicationauniqueLinuxuserID(theIDisusedonlybythesystemandisunknowntotheapplication).ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.Eachprocesshasitsownvirtualmachine(VM),soanapplicationscoderunsinisolationfromotherapplications.Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplicationscomponentsneedtobeexecuted,thenshutsdowntheprocesswhenitsnolongerneededorwhenthesystemmustrecovermemoryforotherapplications.Inthisway,theAndroidsystemimplementstheprincipleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenvironmentinwhichanapplicationcannotaccesspartsofthesystemforwhichitisnotgivenpermission.However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:ItspossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachothersfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocessandsharethesameVM(theapplicationsmustalsobesignedwiththesamecertificate).1Anapplicationcanrequestpermissiontoaccessdevicedatasuchastheuserscontacts,SMSmessages,themountablestorage(SDcard),camera,Bluetooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Therestofthisdocumentintroducesyouto:1、Thecoreframeworkcomponentsthatdefineyourapplication.2、Themanifestfileinwhichyoudeclarecomponentsandrequireddevicefeaturesforyourapplication.3、Resourcesthatareseparatefromtheapplicationcodeandallowyourapplicationtogracefullyoptimizeitsbehaviorforavarietyofdeviceconfigurations.ApplicationComponentsApplicationcomponentsaretheessentialbuildingblocksofanAndroidapplication.Eachcomponentisadifferentpointthroughwhichthesystemcanenteryourapplication.Notallcomponentsareactualentrypointsfortheuserandsomedependoneachother,buteachoneexistsasitsownentityandplaysaspecificroleeachoneisauniquebuildingblockthathelpsdefineyourapplicationsoverallbehavior.Therearefourdifferenttypesofapplicationcomponents.Eachtypeservesadistinctpurposeandhasadistinctlifecyclethatdefineshowthecomponentiscreatedanddestroyed.Herearethefourtypesofapplicationcomponents:ActivitiesAnactivityrepresentsasinglescreenwithauserinterface.Forexample,anemailapplicationmighthaveoneactivitythatshowsalistofnewemails,anotheractivitytocomposeanemail,andanotheractivityforreadingemails.Althoughtheactivitiesworktogethertoformacohesiveuserexperienceintheemailapplication,eachoneisindependentoftheothers.Assuch,adifferentapplicationcanstartanyoneoftheseactivities(iftheemailapplicationallowsit).Forexample,acameraapplicationcanstarttheactivityintheemailapplicationthatcomposesnewmail,inorderfortheusertoshareapicture.AnactivityisimplementedasasubclassofActivityandyoucanlearnmoreaboutitintheActivitiesdeveloperguide.Services2Aserviceisacomponentthatrunsinthebackgroundtoperformlong-runningoperationsortoperformworkforremoteprocesses.Aservicedoesnotprovideauserinterface.Forexample,aservicemightplaymusicinthebackgroundwhiletheuserisinadifferentapplication,oritmightfetchdataoverthenetworkwithoutblockinguserinteractionwithanactivity.Anothercomponent,suchasanactivity,canstarttheserviceandletitrunorbindtoitinordertointeractwithit.AserviceisimplementedasasubclassofServiceandyoucanlearnmoreaboutitintheServicesdeveloperguide.ContentprovidersAcontentprovidermanagesasharedsetofapplicationdata.Youcanstorethedatainthefilesystem,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata(ifthecontentproviderallowsit).Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuserscontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider(suchasContactsContract.Data)toreadandwriteinformationaboutaparticularperson.Contentprovidersarealsousefulforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,theNotePadsampleapplicationusesacontentprovidertosavenotes.AcontentproviderisimplementedasasubclassofContentProviderandmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,seetheContentProvidersdeveloperguide.BroadcastreceiversAbroadcastreceiverisacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystemforexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcastsforexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdontdisplayauserinterface,theymaycreateastatusbarnotificationtoalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcast3receiverisjustagatewaytoothercomponentsandisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.AbroadcastreceiverisimplementedasasubclassofBroadcastReceiverandeachbroadcastisdeliveredasanIntentobject.Formoreinformation,seetheBroadcastReceiverclass.AuniqueaspectoftheAndroidsystemdesignisthatanyapplicationcanstartanotherapplicationscomponent.Forexample,ifyouwanttheusertocaptureaphotowiththedevicecamera,theresprobablyanotherapplicationthatdoesthatandyourapplicationcanuseit,insteadofdevelopinganactivitytocaptureaphotoyourself.Youdontneedtoincorporateorevenlinktothecodefromthecameraapplication.Instead,youcansimplystarttheactivityinthecameraapplicationthatcapturesaphoto.Whencomplete,thephotoisevenreturnedtoyourapplicationsoyoucanuseit.Totheuser,itseemsasifthecameraisactuallyapartofyourapplication.Whenthesystemstartsacomponent,itstartstheprocessforthatapplication(ifitsnotalreadyrunning)andinstantiatestheclassesneededforthecomponent.Forexample,ifyourapplicationstartstheactivityinthecameraapplicationthatcapturesaphoto,thatactivityrunsintheprocessthatbelongstothecameraapplication,notinyourapplicationsprocess.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdonthaveasingleentrypoint(theresnomain()function,forexample).Becausethesystemrunseachapplicationinaseparateprocesswithfilepermissionsthatrestrictaccesstootherapplications,yourapplicationcannotdirectlyactivateacomponentfromanotherapplication.TheAndroidsystem,however,can.So,toactivateacomponentinanotherapplication,youmustdeliveramessagetothesystemthatspecifiesyourintenttostartaparticularcomponent.Thesystemthenactivatesthecomponentforyou.ActivatingComponentsThreeofthefourcomponenttypesactivities,services,andbroadcastreceiversareactivatedbyanasynchronousmessagecalledanintent.Intentsbindindividualcomponentstoeachotheratruntime(youcanthinkofthemasthemessengersthatrequestanactionfromothercomponents),whetherthecomponentbelongstoyourapplicationoranother.4AnintentiscreatedwithanIntentobject,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecifictypeofcomponentanintentcanbeeitherexplicitorimplicit,respectively.Foractivitiesandservices,anintentdefinestheactiontoperform(forexample,tovieworsendsomething)andmayspecifytheURIofthedatatoacton(amongotherthingsthatthecomponentbeingstartedmightneedtoknow).Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivitytoreceivearesult,inwhichcase,theactivityalsoreturnstheresultinanIntent(forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyouthereturnintentincludesaURIpointingtothechosencontact).Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast(forexample,abroadcasttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicatesbatteryislow).Theothercomponenttype,contentprovider,isnotactivatedbyintents.Rather,itisactivatedwhentargetedbyarequestfromaContentResolver.ThecontentresolverhandlesalldirecttransactionswiththecontentprovidersothatthecomponentthatsperformingtransactionswiththeproviderdoesntneedtoandinsteadcallsmethodsontheContentResolverobject.Thisleavesalayerofabstractionbetweenthecontentproviderandthecomponentrequestinginformation(forsecurity).Thereareseparatemethodsforactivatingeachtypeofcomponent:Youcanstartanactivity(orgiveitsomethingnewtodo)bypassinganIntenttostartActivity()orstartActivityForResult()(whenyouwanttheactivitytoreturnaresult).Youcanstartaservice(orgivenewinstructionstoanongoingservice)bypassinganIntenttostartService().OryoucanbindtotheservicebypassinganIntenttobindService().YoucaninitiateabroadcastbypassinganIntenttomethodslikesendBroadcast(),sendOrderedBroadcast(),orsendStickyBroadcast().Youcanperformaquerytoacontentproviderbycallingquery()onaContentResolver.Formoreinformationaboutusingintents,seetheIntentsandIntentFiltersdocument.Moreinformationaboutactivatingspecificcomponentsisalsoprovidedinthefollowingdocuments:Activities,Services,BroadcastReceiverandContentProviders.Declaringcomponents5Theprimarytaskofthemanifestistoinformthesystemabouttheapplicationscomponents.Forexample,amanifestfilecandeclareanactivityasfollows:.Intheelement,theandroid:iconattributepointstoresourcesforaniconthatidentifiestheapplication.Intheelement,theandroid:nameattributespecifiesthefullyqualifiedclassnameoftheActivitysubclassandtheandroid:labelattributesspecifiesastringtouseastheuser-visiblelabelfortheactivity.Youmustdeclareallapplicationcomponentsthisway:1、elementsforactivities2、elementsforservices3、elementsforbroadcastreceivers4、elementsforcontentprovidersActivities,services,andcontentprovidersthatyouincludeinyoursourcebutdonotdeclareinthemanifestarenotvisibletothesystemand,consequently,canneverrun.However,broadcastreceiverscanbeeitherdeclaredinthemanifestorcreateddynamicallyincode(asBroadcastReceiverobjects)andregisteredwiththesystembycallingregisterReceiver().DeclaringcomponentcapabilitiesAsdiscussedabove,inActivatingComponents,youcanuseanIntenttostartactivities,services,andbroadcastreceivers.Youcandosobyexplicitlynamingthetargetcomponent(usingthecomponentclassname)intheintent.However,therealpowerofintentsliesintheconceptofintentactions.Withintentactions,yousimplydescribethetypeofactionyou6wanttoperform(andoptionally,thedatauponwhichyoudliketoperformtheaction)andallowthesystemtofindacomponentonthedevicethatcanperformtheactionandstartit.Iftherearemultiplecomponentsthatcanperformtheactiondescribedbytheintent,thentheuserselectswhichonetouse.Thewaythesystemidentifiesthecomponentsthatcanrespondtoanintentisbycomparingtheintentreceivedtotheintentfiltersprovidedinthemanifestfileofotherapplicationsonthedevice.Whenyoudeclareacomponentinyourapplicationsmanifest,youcanoptionallyincludeintentfiltersthatdeclarethecapabilitiesofthecomponentsoitcanrespondtointentsfromotherapplications.Youcandeclareanintentfilterforyourcomponentbyaddinganelementasachildofthecomponentsdeclarationelement.Forexample,anemailapplicationwithanactivityforcomposinganewemailmightdeclareanintentfilterinitsmanifestentrytorespondtosendintents(inordertosendemail).Anactivityinyourapplicationcanthencreateanintentwiththe“send”action(ACTION_SEND),whichthesystemmatchestotheemailapplications“send”activityandlaunchesitwhenyouinvoketheintentwithstartActivity().Formoreaboutcreatingintentfilters,seetheIntentsandIntentFiltersdocument.DeclaringapplicationrequirementsThereareavarietyofdevicespoweredbyAndroidandnotallofthemprovidethesamefeaturesandcapabilities.Inordertopreventyourapplicationfrombeinginstalledondevicesthatlackfeaturesneededbyyourapplication,itsimportantthatyouclearlydefineaprofileforthetypesofdevicesyourapplicationsupportsbydeclaringdeviceandsoftwarerequirementsinyourmanifestfile.Mostofthesedeclarationsareinformationalonlyandthesystemdoesnotreadthem,butexternalservicessuchasGooglePlaydoreadtheminordertoprovidefilteringforuserswhentheysearchforapplicationsfromtheirdevice.Forexample,ifyourapplicationrequiresacameraandusesAPIsintroducedinAndroid2.1(APILevel7),youshoulddeclaretheseasrequirementsinyourmanifestfile.Thatway,devicesthatdonothaveacameraandhaveanAndroidversionlowerthan2.1cannotinstallyourapplicationfromGooglePlay.However,youcanalsodeclarethatyourapplicationusesthecamera,butdoesnotrequireit.Inthatcase,yourapplicationmustperformacheckatruntimetodetermineifthedevicehasacameraanddisableanyfeaturesthatusethecameraifoneisnotavailable.7Herearesomeoftheimportantdevicecharacteristicsthatyoushouldconsiderasyoudesignanddevelopyourapplication:ScreensizeanddensityInordertocategorizedevicesbytheirscreentype,Androiddefinestwocharacteristicsforeachdevice:screensize(thephysicaldimensionsofthescreen)andscreendensity(thephysicaldensityofthepixelsonthescreen,ordpidotsperinch).Tosimplifyallthedifferenttypesofscreenconfigurations,theAndroidsystemgeneralizesthemintoselectgroupsthatmakethemeasiertotarget.Thescreensizesare:small,normal,large,andextralarge.Thescreendensitiesare:lowdensity,mediumdensity,highdensity,andextrahighdensity.Bydefault,yourapplicationiscompatiblewithallscreensizesanddensities,becausetheAndroidsystemmakestheappropriateadjustmentstoyourUIlayoutandimageresources.However,youshouldcreatespecializedlayoutsforcertainscreensizesandprovidespecializedimagesforcertaindensities,usingalternativelayoutresources,andbydeclaringinyourmanifestexactlywhichscreensizesyourapplicationsupportswiththeelement.Formoreinformation,seetheSupportingMultipleScreensdocument.InputconfigurationsManydevicesprovideadifferenttypeofuserinputmechanism,suchasahardwarekeyboard,atrackball,orafive-waynavigationpad.Ifyourapplicationrequiresaparticularkindofinputhardware,thenyoushoulddeclareitinyourmanifestwiththeelement.However,itisrarethatanapplicationshouldrequireacertaininputconfiguration.DevicefeaturesTherearemanyhardwareandsoftwarefeaturesthatmayormaynotexistonagivenAndroid-powereddevice,suchasacamera,alightsensor,bluetooth,acertainversionofOpenGL,orthefidelityofthetouchscreen.YoushouldneverassumethatacertainfeatureisavailableonallAndroid-powereddevices(otherthantheavailabilityofthestandardAndroidlibrary),soyoushoulddeclareanyfeaturesusedbyyourapplicationwiththeelement.PlatformVersion8DifferentAndroid-powereddevicesoftenrundifferentversionsoftheAndroidplatform,suchasAndroid1.6orAndroid2.3.EachsuccessiveversionoftenincludesadditionalAPIsnotavailableinthepreviousversion.InordertoindicatewhichsetofAPIsareavailable,eachplatformversionspecifiesanAPILevel(forexample,Android1.0isAPILevel1andAndroid2.3isAPILevel9).IfyouuseanyAPIsthatwereaddedtotheplatformafterversion1.0,youshoulddeclaretheminimumAPILevelinwhichthoseAPIswereintroducedusingtheelement.Itsimportantthatyoudeclareallsuchrequirementsforyourapplication,because,whenyoudistributeyourapplicationonGooglePlay,thestoreusesthesedeclarationstofilterwhichapplicationsareavailableoneachdevice.Assuch,yourapplicationshouldbeavailableonlytodevicesthatmeetallyourapplicationrequirements.FormoreinformationabouthowGooglePlayfiltersapplicationsbasedonthese(andother)requirements,seetheFiltersonGooglePlaydocument.ApplicationResourcesAnAndroidapplicationiscomposedofmorethanjustcodeitrequiresresourcesthatareseparatefromthesourcecode,suchasimages,audiofiles,andanythingrelatingtothevisualpresentationoftheapplication.Forexample,youshoulddefineanimations,menus,styles,colors,andthelayoutofactivityuserinterfaceswithXMLfiles.Usingapplicationresourcesmakesiteasytoupdatevariouscharacteristicsofyourapplicationwithoutmodifyingcodeandbyprovidingsetsofalternativeresourcesenablesyoutooptimizeyourapplicationforavarietyofdeviceconfigurations(suchasdifferentlanguagesandscreensizes).ForeveryresourcethatyouincludeinyourAndroidproject,theSDKbuildtoolsdefineauniqueintegerID,whichyoucanusetoreferencetheresourcefromyourapplicationcodeorfromotherresourcesdefinedinXML.Forexample,ifyourapplicationcontainsanimagefilenamedlogo.png(savedintheres/drawable/directory),theSDKtoolsgeneratearesourceIDnamedR.drawable.logo,whichyoucanusetoreferencetheimageandinsertitinyouruserinterface.Oneofthemostimportantaspectsofprovidingresourcesseparatefromyoursourcecodeistheabilityforyoutoprovidealternativeresourcesfordifferentdeviceconfigurations.Forexample,bydefiningUIstringsinXML,youcantranslatethestringsintootherlanguagesandsavethosestringsinseparatefiles.Then,basedonalanguagequalifierthat9youappendtotheresourcedirectorysname(suchasres/values-fr/forFrenchstringvalues)andtheuserslanguagesetting,theAndroidsystemappliestheappropriatelanguagestringstoyourUI.Androidsupportsmanydifferentqualifiersforyouralternativeresources.Thequalifierisashortstringthatyouincludeinthenameofyourresourcedirectoriesinordertodefinethedeviceconfigurationforwhichthoseresourcesshouldbeused.Asanotherexample,youshouldoftencreatedifferentlayoutsforyouractivities,dependingonthedevicesscreenorientationandsize.Forexample,whenthedevicescreenisinportraitorientation(tall),youmightwantalayoutwithbuttonstobevertical,butwhenthescreenisinlandscapeorientation(wide),thebuttonsshouldbealignedhorizontally.Tochangethelayoutdependingontheorientation,youcandefinetwodifferentlayoutsandapplytheappropriatequalifiertoeachlayoutsdirectoryname.Then,thesystemautomaticallyappliestheappropriatelayoutdependingonthecurrentdeviceorientation.Formoreaboutthedifferentkindsofresourcesyoucanincludeinyourapplicationandhowtocreatealternativeresourcesforvariousdeviceconfigurations,seetheApplicationResourcesdeveloperguide.10中文翻译安卓应用基础在Java编程语言编写的Android应用程序的Android的SDK工具编译代码以及与任何数据和到一个Android的包，一个归档文件档案资源的.apk后缀，所有的在一个单一的代码.apk文件被认为是一个应用程序，是