Android手机外文翻译应用程序基础Android Developers.docx

Android手机外文翻译应用程序基础Android Developers.docx

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Android手机外文翻译应用程序基础AndroidDevelopers

英文原文及译文

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.（SeetheseparateUserInterfacedocumentformoreinformationonviewsandthehierarchy.）

Services

Aservicedoesn'thaveavisualuserinterface,butratherrunsinthebackgroundforanindefiniteperiodoftime.Forexample,aservicemightplaybackgroundmusicastheuserattendstoothermatters,oritmightfetchdataoverthenetworkorcalculatesomethingandprovidetheresulttoactivitiesthatneedit.EachserviceextendstheServicebaseclass.

Aprimeexampleisamediaplayerplayingsongsfromaplaylist.Theplayerapplicationwouldprobablyhaveoneormoreactivitiesthatallowtheusertochoosesongsandstartplayingthem.However,themusicplaybackitselfwouldnotbehandledbyanactivitybecauseuserswillexpectthemusictokeepplayingevenaftertheyleavetheplayerandbeginsomethingdifferent.Tokeepthemusicgoing,themediaplayeractivitycouldstartaservicetoruninthebackground.Thesystemwouldthenkeepthemusicplaybackservicerunningevenaftertheactivitythatstarteditleavesthescreen.

It'spossibletoconnectto（bindto）anongoingservice（andstarttheserviceifit'snotalreadyrunning）.Whileconnected,youcancommunicatewiththeservicethroughaninterfacethattheserviceexposes.Forthemusicservice,thisinterfacemightallowuserstopause,rewind,stop,andrestarttheplayback.

Likeactivitiesandtheothercomponents,servicesruninthemainthreadoftheapplicationprocess.Sothattheywon'tblockothercomponentsortheuserinterface,theyoftenspawnanotherthreadfortime-consumingtasks（likemusicplayback）.SeeProcessesandThreads,later.

Broadcastreceivers

Abroadcastreceiverisacomponentthatdoesnothingbutreceiveandreacttobroadcastannouncements.Manybroadcastsoriginateinsystemcode—forexample,announcementsthatthetimezonehaschanged,thatthebatteryislow,thatapicturehasbeentaken,orthattheuserchangedalanguagepreference.Applicationscanalsoinitiatebroadcasts—forexample,toletotherapplicationsknowthatsomedatahasbeendownloadedtothedeviceandisavailableforthemtouse.

Anapplicationcanhaveanynumberofbroadcastreceiverstorespondtoanyannouncementsitconsidersimportant.AllreceiversextendtheBroadcastReceiverbaseclass.

Broadcastreceiversdonotdisplayauserinterface.However,theymaystartanactivityinresponsetotheinformationtheyreceive,ortheymayusetheNotificationManagertoalerttheuser.Notificationscangettheuser'sattentioninvariousways—flashingthebacklight,vibratingthedevice,playingasound,andsoon.Theytypicallyplaceapersistenticoninthestatusbar,whichuserscanopentogetthemessage.

Contentproviders

Acontentprovidermakesaspecificsetoftheapplication'sdataavailabletootherapplications.Thedatacanbestoredinthefilesystem,inanSQLitedatabase,orinanyothermannerthatmakessense.ThecontentproviderextendstheContentProviderbaseclasstoimplementastandardsetofmethodsthatenableotherapplicationstoretrieveandstoredataofthetypeitcontrols.However,applicationsdonotcallthesemethodsdirectly.RathertheyuseaContentResolverobjectandcallitsmethodsinstead.AContentResolvercantalktoanycontentprovider;itcooperateswiththeprovidertomanageanyinterprocesscommunicationthat'sinvolved.

SeetheseparateContentProvidersdocumentformoreinformationonusingcontentproviders.

Wheneverthere'sarequestthatshouldbehandledbyaparticularcomponent,Androidmakessurethattheapplicationprocessofthecomponentisrunning,startingitifnecessary,andthatanappropriateinstanceofthecomponentisavailable,creatingtheinstanceifnecessary.

Activatingcomponents:

intents

Contentprovidersareactivatedwhenthey'retargetedbyarequestfromaContentResolver.Theotherthreecomponents—activities,services,andbroadcastreceivers—areactivatedbyasynchronousmessagescalledintents.AnintentisanIntentobjectthatholdsthecontentofthemessage.Foractivitiesandservices,itnamestheactionbeingrequestedandspecifiestheURIofthedatatoacton,amongotherthings.Forexample,itmightconveyarequestforanactivitytopresentanimagetotheuserorlettheusereditsometext.Forbroadcastreceivers,the

Intentobjectnamestheactionbeingannounced.Forexample,itmightannouncetointerestedpartiesthatthecamerabuttonhasbeenpressed.

Thereareseparatemethodsforactivatingeachtypeofcomponent:

1.Anactivityislaunched（orgivensomethingnewtodo）bypassinganIntentobjectto

Context.startActivity（）orActivity.startActivityForResult（）.TherespondingactivitycanlookattheinitialintentthatcausedittobelaunchedbycallingitsgetIntent（）method.Androidcallstheactivity'sonNewIntent（）methodtopassitanysubsequentintents.Oneactivityoftenstartsthenextone.Ifitexpe