Android外文文献翻译.docx
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Android外文文献翻译
AndroidApplicationFundamentals
AndroidapplicationsarewrittenintheJavaprogramminglanguage.TheAndroidSDKtoolscompilethecode—alongwithanydataandresourcefiles—intoan?
Androidpackage,anarchivefilewithan?
.apk?
suffix.Allthecodeinasingle?
.apk?
fileisconsideredtobeoneapplicationandisthefilethatAndroid-powereddevicesusetoinstalltheapplication.
Onceinstalledonadevice,eachAndroidapplicationlivesinitsownsecuritysandbox:
TheAndroidoperatingsystemisamulti-userLinuxsysteminwhicheachapplicationisadifferentuser.
Bydefault,thesystemassignseachapplicationauniqueLinuxuserID(theIDisusedonlybythesystemandisunknowntotheapplication).ThesystemsetspermissionsforallthefilesinanapplicationsothatonlytheuserIDassignedtothatapplicationcanaccessthem.
Eachprocesshasitsownvirtualmachine(VM),soanapplication'scoderunsinisolationfromotherapplications.
Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'scomponentsneedtobeexecuted,thenshutsdowntheprocesswhenit'snolongerneededorwhenthesystemmustrecovermemoryforotherapplications.
Inthisway,theAndroidsystemimplementsthe?
principleofleastprivilege.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
An?
activity?
representsasinglescreenwithauserinterface.Forexample,anemailapplicationmighthaveoneactivitythatshowsalistofnewemails,anotheractivitytocomposeanemail,andanotheractivityforreadingemails.Althoughtheactivitiesworktogethertoformacohesiveuserexperienceintheemailapplication,eachoneisindependentoftheothers.Assuch,adifferentapplicationcanstartanyoneoftheseactivities(iftheemailapplicationallowsit).Forexample,acameraapplicationcanstarttheactivityintheemailapplicationthatcomposesnewmail,inorderfortheusertoshareapicture.
Anactivityisimplementedasasubclassof?
Activity?
andyoucanlearnmoreaboutitinthe?
Activities?
developerguide.
Services
A?
service?
isacomponentthatrunsinthebackgroundtoperformlong-runningoperationsortoperformworkforremoteprocesses.Aservicedoesnotprovideauserinterface.Forexample,aservicemightplaymusicinthebackgroundwhiletheuserisinadifferentapplication,oritmightfetchdataoverthenetworkwithoutblockinguserinteractionwithanactivity.Anothercomponent,suchasanactivity,canstarttheserviceandletitrunorbindtoitinordertointeractwithit.
Aserviceisimplementedasasubclassof?
Service?
andyoucanlearnmoreaboutitinthe?
Services?
developerguide.
Contentproviders
A?
contentprovider?
managesasharedsetofapplicationdata.Youcanstorethedatainthefilesystem,anSQLitedatabase,ontheweb,oranyotherpersistentstoragelocationyourapplicationcanaccess.Throughthecontentprovider,otherapplicationscanqueryorevenmodifythedata(ifthecontentproviderallowsit).Forexample,theAndroidsystemprovidesacontentproviderthatmanagestheuser'scontactinformation.Assuch,anyapplicationwiththeproperpermissionscanquerypartofthecontentprovider(suchas?
ContactsContract.Data)toreadandwriteinformationaboutaparticularperson.
Contentprovidersarealsousefulforreadingandwritingdatathatisprivatetoyourapplicationandnotshared.Forexample,the?
NotePad?
sampleapplicationusesacontentprovidertosavenotes.
Acontentproviderisimplementedasasubclassof?
ContentProvider?
andmustimplementastandardsetofAPIsthatenableotherapplicationstoperformtransactions.Formoreinformation,seethe?
ContentProviders?
developerguide.
Broadcastreceivers
A?
broadcastreceiver?
isacomponentthatrespondstosystem-widebroadcastannouncements.Manybroadcastsoriginatefromthesystem—forexample,abroadcastannouncingthatthescreenhasturnedoff,thebatteryislow,orapicturewascaptured.Applicationscanalsoinitiatebroadcasts—forexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.Althoughbroadcastreceiversdon'tdisplayauserinterface,theymay?
createastatusbarnotification?
toalerttheuserwhenabroadcasteventoccurs.Morecommonly,though,abroadcastreceiverisjusta"gateway"toothercomponentsandisintendedtodoaveryminimalamountofwork.Forinstance,itmightinitiateaservicetoperformsomeworkbasedontheevent.
Abroadcastreceiverisimplementedasasubclassof?
BroadcastReceiver?
andeachbroadcastisdeliveredasan?
Intent?
object.Formoreinformation,seetheBroadcastReceiver?
class.
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'sno?
main()?
function,forexample).
Becausethesystemrunseachapplicationinaseparateprocesswithfilepermissionsthatrestrictaccesstootherapplications,yourapplicationcannotdirectlyactivateacomponentfromanotherapplication.TheAndroidsystem,however,can.So,toactivateacomponentinanotherapplication,youmustdeliveramessagetothesystemthatspecifiesyour?
intent?
tostartaparticularcomponent.Thesystemthenactivatesthecomponentforyou.
ActivatingComponents
Threeofthefourcomponenttypes—activities,services,andbroadcastreceivers—areactivatedbyanasynchronousmessagecalledan?
intent.Intentsbindindividualcomponentstoeachotheratruntime(youcanthinkofthemasthemessengersthatrequestanactionfromothercomponents),whetherthecomponentbelongstoyourapplicationoranother.
Anintentiscreatedwithan?
Intent?
object,whichdefinesamessagetoactivateeitheraspecificcomponentoraspecific?
type?
ofcomponent—anintentcanbeeitherexplicitorimplicit,respectively.
Foractivitiesandservices,anintentdefinestheactiontoperform(forexample,to"view"or"send"something)andmayspecifytheURIofthedatatoacton(amongotherthingsthatthecomponentbeingstartedmightneedtoknow).Forexample,anintentmightconveyarequestforanactivitytoshowanimageortoopenawebpage.Insomecases,youcanstartanactivitytoreceivearesult,inwhichcase,theactivityalsoreturnstheresultinan?
Intent?
(forexample,youcanissueanintenttolettheuserpickapersonalcontactandhaveitreturnedtoyou—thereturnintentincludesaURIpointingtothechosencontact).
Forbroadcastreceivers,theintentsimplydefinestheannouncementbeingbroadcast(forexample,abroadcasttoindicatethedevicebatteryislowincludesonlyaknownactionstringthatindicates"batteryislow").
Theothercomponenttype,contentprovid