应用程序基础Android Developers毕业设计外文翻译Word文档下载推荐.docx

应用程序基础Android Developers毕业设计外文翻译Word文档下载推荐.docx

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ApplicationFundamentals

AndroidapplicationsarewrittenintheJavaprogramminglanguage.ThecompiledJavacode—alongwithanydataandresourcefilesrequiredbytheapplication—isbundledbytheaapttoolintoanAndroidpackage,anarchivefilemarkedbyan.apksuffix.Thisfileisthevehiclefordistributingtheapplicationandinstallingitonmobiledevices;

it'

sthefileusersdownloadtotheirdevices.Allthecodeinasingle.apkfileisconsideredtobeoneapplication.

Inmanyways,eachAndroidapplicationlivesinitsownworld:

1.Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'

scodeneedstobeexecuted,andshutsdowntheprocesswhenit'

snolongerneededandsystemresourcesarerequiredbyotherapplications.

2.Eachprocesshasitsownvirtualmachine（VM）,soapplicationcoderunsinisolationfromthecodeofallotherapplications.

3.Bydefault,eachapplicationisassignedauniqueLinuxuserID.Permissionsaresetsothattheapplication'

sfilesarevisibleonlytothatuserandonlytotheapplicationitself—althoughtherearewaystoexportthemtootherapplicationsaswell.

It'

spossibletoarrangefortwoapplicationstosharethesameuserID,inwhichcasetheywillbeabletoseeeachother'

sfiles.Toconservesystemresources,applicationswiththesameIDcanalsoarrangetoruninthesameLinuxprocess,sharingthesameVM.

ApplicationComponents

AcentralfeatureofAndroidisthatoneapplicationcanmakeuseofelementsofotherapplications（providedthoseapplicationspermitit）.Forexample,ifyourapplicationneedstodisplayascrollinglistofimagesandanotherapplicationhasdevelopedasuitablescrollerandmadeitavailabletoothers,youcancalluponthatscrollertodothework,ratherthandevelopyourown.Yourapplicationdoesn'

tincorporatethecodeoftheotherapplicationorlinktoit.Rather,itsimplystartsupthatpieceoftheotherapplicationwhentheneedarises.

Forthistowork,thesystemmustbeabletostartanapplicationprocesswhenanypartofitisneeded,andinstantiatetheJavaobjectsforthatpart.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdon'

thaveasingleentrypointforeverythingintheapplication（nomain（）function,forexample）.Rather,theyhaveessentialcomponentsthatthesystemcaninstantiateandrunasneeded.Therearefourtypesofcomponents:

Activities

Anactivitypresentsavisualuserinterfaceforonefocusedendeavortheusercanundertake.Forexample,anactivitymightpresentalistofmenuitemsuserscanchoosefromoritmightdisplayphotographsalongwiththeircaptions.Atextmessagingapplicationmighthaveoneactivitythatshowsalistofcontactstosendmessagesto,asecondactivitytowritethemessagetothechosencontact,andotheractivitiestoreviewoldmessagesorchangesettings.Thoughtheyworktogethertoformacohesiveuserinterface,eachactivityisindependentoftheothers.EachoneisimplementedasasubclassoftheActivitybaseclass.

Anapplicationmightconsistofjustoneactivityor,likethetextmessagingapplicationjustmentioned,itmaycontainseveral.Whattheactivitiesare,andhowmanytherearedepends,ofcourse,ontheapplicationanditsdesign.Typically,oneoftheactivitiesismarkedasthefirstonethatshouldbepresentedtotheuserwhentheapplicationislaunched.Movingfromoneactivitytoanotherisaccomplishedbyhavingthecurrentactivitystartthenextone.

Eachactivityisgivenadefaultwindowtodrawin.Typically,thewindowfillsthescreen,butitmightbesmallerthanthescreenandfloatontopofotherwindows.Anactivitycanalsomakeuseofadditionalwindows—forexample,apop-updialogthatcallsforauserresponseinthemidstoftheactivity,orawindowthatpresentsuserswithvitalinformationwhentheyselectaparticularitemon-screen.

Thevisualcontentofthewindowisprovidedbyahierarchyofviews—objectsderivedfromthebaseViewclass.Eachviewcontrolsaparticularrectangularspacewithinthewindow.Parentviewscontainandorganizethelayoutoftheirchildren.Leafviews（thoseatthebottomofthehierarchy）drawintherectanglestheycontrolandrespondtouseractionsdirectedatthatspace.Thus,viewsarewheretheactivity'

sinteractionwiththeusertakesplace.

Forexample,aviewmightdisplayasmallimageandinitiateanactionwhentheusertapsthatimage.Androidhasanumberofready-madeviewsthatyoucanuse—includingbuttons,textfields,scrollbars,menuitems,checkboxes,andmore.

Aviewhierarchyisplacedwithinanactivity'

swindowbytheActivity.setContentView（）method.ThecontentviewistheViewobjectattherootofthehierarchy.（SeetheseparateUserInterfacedocument