外文翻译AndroidApplicationFundamentalsWord文档格式.docx

外文翻译AndroidApplicationFundamentalsWord文档格式.docx

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∙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:

∙Thecoreframeworkcomponentsthatdefineyourapplication.

∙Themanifestfileinwhichyoudeclarecomponentsandrequireddevicefeaturesforyourapplication.

∙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.

AnactivityisimplementedasasubclassofActivityandyoucanlearnmoreaboutitintheActivitiesdeveloperguide.

Services

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

AserviceisimplementedasasubclassofServiceandyoucanlearnmoreaboutitintheServicesdeveloperguide.

Contentproviders

Acontentprovidermanagesasharedsetofapplicationdata.Youcanstorethedatainthefilesystem,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata（ifthecontentproviderallowsit）.Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuser'

scontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider（suchasContactsContract.Data）toreadandwriteinformationaboutaparticularperson.

Contentprovidersarealsousefulforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,theNotePadsampleapplicationusesacontentprovidertosavenotes.

AcontentproviderisimplementedasasubclassofContentProviderandmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,seetheContentProvidersdeveloperguide.

Broadcastreceivers

Abroadcastreceiverisacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystem—forexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcasts—forexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdon'

tdisplayauserinterface,theymaycreateastatusbarnotificationtoalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcastreceiverisjusta"

gateway"

toothercomponentsandisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.

AbroadcastreceiverisimplementedasasubclassofBroadcastReceiverandeachbroadcastisdeliveredasanIntentobject.Formoreinformation,seetheBroadcastReceiverclass.

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.

AnintentiscreatedwithanIntentobject,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecifictypeofcomponent—anintentcanbeeitherexplicitorimplicit,respectively.

Foractivitiesandservices,anintentdefinestheactiontoperform（forexample,to"

view"

or"

send"

something）andmayspecifytheURIofthedatatoacton（amongotherthingsthatthecomponentbeingstartedmightneedtoknow）.Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivitytoreceivearesult,inwhichcase,theactivityalsoreturnstheresultinanIntent（forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyou—thereturnintentincludesaURIpointingtothechosencontact）.

Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast（forexample,abroadcasttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicates"

batteryislow"

）.

Theothercompon