Android外文文献翻译.docx

Android外文文献翻译.docx

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Android外文文献翻译

ThefinalrevisionwasonNovember23,2020

Android外文文献翻译

AndroidApplicationFundamentals

AndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecode—alongwithanydataandresourcefiles—intoanAndroidpackage,anarchivefilewith.AllthecodeinaisconsideredtobeoneapplicationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.

Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:

TheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.

Bydefault,thesystemassignseachapplicationauniqueLinuxuserID（theIDisusedonlybythesystemandisunknowntotheapplication）.ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.

Eachprocesshasitsownvirtualmachine（VM）,soanapplication'scoderunsinisolationfromotherapplications.

Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'snolongerneededorwhenthesystemmustrecovermemoryforotherapplications.

Inthisway,theAndroidsystemimplementstheprincipleofleastprivilege.Thatis,eachapplication,bydefault,hasaccessonlytothecomponentsthatitrequirestodoitsworkandnomore.Thiscreatesaverysecureenvironmentinwhichanapplicationcannotaccesspartsofthesystemforwhichitisnotgivenpermission.

However,therearewaysforanapplicationtosharedatawithotherapplicationsandforanapplicationtoaccesssystemservices:

It'spossibletoarrangefortwoapplicationstosharethesameLinuxuserID,inwhichcasetheyareabletoaccesseachother'sfiles.Toconservesystemresources,applicationswiththesameuserIDcanalsoarrangetoruninthesameLinuxprocessandsharethesameVM（theapplicationsmustalsobesignedwiththesamecertificate）.

Anapplicationcanrequestpermissiontoaccessdevicedatasuchastheuser'scontacts,SMSmessages,themountablestorage（SDcard）,camera,Bluetooth,andmore.Allapplicationpermissionsmustbegrantedbytheuseratinstalltime.

ThatcoversthebasicsregardinghowanAndroidapplicationexistswithinthesystem.Therestofthisdocumentintroducesyouto:

1、Thecoreframeworkcomponentsthatdefineyourapplication.

2、Themanifestfileinwhichyoudeclarecomponentsandrequireddevicefeaturesforyourapplication.

3、Resourcesthatareseparatefromtheapplicationcodeandallowyourapplicationtogracefullyoptimizeitsbehaviorforavarietyofdeviceconfigurations.

ApplicationComponents

ApplicationcomponentsaretheessentialbuildingblocksofanAndroidapplication.Eachcomponentisadifferentpointthroughwhichthesystemcanenteryourapplication.Notallcomponentsareactualentrypointsfortheuserandsomedependoneachother,buteachoneexistsasitsownentityandplaysaspecificrole—eachoneisauniquebuildingblockthathelpsdefineyourapplication'soverallbehavior.

Therearefourdifferenttypesofapplicationcomponents.Eachtypeservesadistinctpurposeandhasadistinctlifecyclethatdefineshowthecomponentiscreatedanddestroyed.

Herearethefourtypesofapplicationcomponents:

Activities

Anactivityrepresentsasinglescreenwithauserinterface.Forexample,anemailapplicationmighthaveoneactivitythatshowsalistofnewemails,anotheractivitytocomposeanemail,andanotheractivityforreadingemails.Althoughtheactivitiesworktogethertoformacohesiveuserexperienceintheemailapplication,eachoneisindependentoftheothers.Assuch,adifferentapplicationcanstartanyoneoftheseactivities（iftheemailapplicationallowsit）.Forexample,acameraapplicationcanstarttheactivityintheemailapplicationthatcomposesnewmail,inorderfortheusertoshareapicture.

Anactivityisimplementedasasubclassofandyoucanlearnmoreaboutitinthedeveloperguide.

Services

Aserviceisacomponentthatrunsinthebackgroundtoperformlong-runningoperationsortoperformworkforremoteprocesses.Aservicedoesnotprovideauserinterface.Forexample,aservicemightplaymusicinthebackgroundwhiletheuserisinadifferentapplication,oritmightfetchdataoverthenetworkwithoutblockinguserinteractionwithanactivity.Anothercomponent,suchasanactivity,canstarttheserviceandletitrunorbindtoitinordertointeractwithit.

Aserviceisimplementedasasubclassofandyoucanlearnmoreaboutitinthedeveloperguide.

Contentproviders

Acontentprovidermanagesasharedsetofapplicationdata.Youcanstorethedatainthefilesystem,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata（ifthecontentproviderallowsit）.Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuser'scontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider（suchastoreadandwriteinformationaboutaparticularperson.

Contentprovidersarealsousefulforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,thesampleapplicationusesacontentprovidertosavenotes.

AcontentproviderisimplementedasasubclassofandmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,seethedeveloperguide.

Broadcastreceivers

Abroadcastreceiverisacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystem—forexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcasts—forexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdon'tdisplayauserinterface,theymaytoalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcastreceiverisjusta"gateway"toothercomponentsandisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.

Abroadcastreceiverisimplementedasasubclassofandeachbroadcastisdeliveredasanobject.Formoreinformation,seetheclass.

AuniqueaspectoftheAndroidsystemdesignisthatanyapplicationcanstartanotherapplication’scomponent.Forexample,ifyouwanttheusertocaptureaphotowiththedevicecamera,there'sprobablyanotherapplicationthatdoesthatandyourapplicationcanuseit,insteadofdevelopinganactivitytocaptureaphotoyourself.Youdon'tneedtoincorporateorevenlinktothecodefromthecameraapplication.Instead,youcansimplystarttheactivityinthecameraapplicationthatcapturesaphoto.Whencomplete,thephotoisevenreturnedtoyourapplicationsoyoucanuseit.Totheuser,itseemsasifthecameraisactuallyapartofyourapplication.

Whenthesystemstartsacomponent,itstartstheprocessforthatapplication（ifit'snotalreadyrunning）andinstantiatestheclassesneededforthecomponent.Forexample,ifyourapplicationstartstheactivityinthecameraapplicationthatcapturesaphoto,thatactivityrunsintheprocessthatbelongstothecameraapplication,notinyourapplication'sprocess.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdon'thaveasingleentrypoint（there'snomain（）function,forexample）.

Becausethesystemrunseachapplicationinaseparateprocesswithfilepermissionsthatrestrictaccesstootherapplications,yourapplicationcannotdirectlyactivateacomponentfromanotherapplication.TheAndroidsystem,however,can.So,toactivateacomponentinanotherapplication,youmustdeliveramessagetothesystemthatspecifiesyourintenttostartaparticularcomponent.Thesystemthenactivatesthecomponentforyou.

ActivatingComponents

Threeofthefourcomponenttypes—activities,services,andbroadcastreceivers—areactivatedbyanasynchronousmessagecalledanintent.Intentsbindindividualcomponentstoeachotheratruntime（youcanthinkofthemasthemessengersthatrequestanactionfromothercomponents）,whetherthecomponentbelongstoyourapplicationoranother.

Anintentiscreatedwithanobject,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecifictypeofcomponent—anintentcanbeeitherexplicitorimplicit,respectively.

Foractivitiesandservices,anintentdefinestheactiontoperform（forexample,to"view"or"send"something）andmayspecifytheURIofthedatatoacton（amongotherthingsthatthecomponentbeingstartedmightneedtoknow）.Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivitytoreceivearesult,inwhichcase,theactivityalsoreturnstheresultinan（forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyou—thereturnintentincludesaURIpointingtothechosencontact）.

Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast（forexample,abroadcasttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicates"batteryislow"）.

Theothercomponenttype,contentprovider,isnotactivatedbyintents.Rather,itisactivatedwhentargetedbyarequestfroma.Thecontentresolverhandlesalldirecttransactionswiththecontentprovidersothatthecompo