基于android的蔬菜市场信息系统设计与实现android技术英文文献翻译

ANDROIDAPPLICATIONFUNDAMENTALSANDROIDAPPLICATIONSAREWRITTENINTHEJAVAPROGRAMMINGLANGUAGETHEANDROIDSDKTOOLSCOMPILETHECODEALONGWITHANYDATAANDRESOURCEFILESINTOANANDROIDPACKAGE,ANARCHIVEFILEWITHANAPKSUFFIXALLTHECODEINASINGLEAPKFILEISCONSIDEREDTOBEONEAPPLICATIONANDISTHEFILETHATANDROIDPOWEREDDEVICESUSETOINSTALLTHEAPPLICATIONONCEINSTALLEDONADEVICE,EACHANDROIDAPPLICATIONLIVESINITSOWNSECURITYSANDBOXTHEANDROIDOPERATINGSYSTEMISAMULTIUSERLINUXSYSTEMINWHICHEACHAPPLICATIONISADIFFERENTUSERBYDEFAULT,THESYSTEMASSIGNSEACHAPPLICATIONAUNIQUELINUXUSERIDTHEIDISUSEDONLYBYTHESYSTEMANDISUNKNOWNTOTHEAPPLICATIONTHESYSTEMSETSPERMISSIONSFORALLTHEFILESINANAPPLICATIONSOTHATONLYTHEUSERIDASSIGNEDTOTHATAPPLICATIONCANACCESSTHEMEACHPROCESSHASITSOWNVIRTUALMACHINEVM,SOANAPPLICATIONSCODERUNSINISOLATIONFROMOTHERAPPLICATIONSBYDEFAULT,EVERYAPPLICATIONRUNSINITSOWNLINUXPROCESSANDROIDSTARTSTHEPROCESSWHENANYOFTHEAPPLICATIONSCOMPONENTSNEEDTOBEEXECUTED,THENSHUTSDOWNTHEPROCESSWHENITSNOLONGERNEEDEDORWHENTHESYSTEMMUSTRECOVERMEMORYFOROTHERAPPLICATIONSINTHISWAY,THEANDROIDSYSTEMIMPLEMENTSTHEPRINCIPLEOFLEASTPRIVILEGETHATIS,EACHAPPLICATION,BYDEFAULT,HASACCESSONLYTOTHECOMPONENTSTHATITREQUIRESTODOITSWORKANDNOMORETHISCREATESAVERYSECUREENVIRONMENTINWHICHANAPPLICATIONCANNOTACCESSPARTSOFTHESYSTEMFORWHICHITISNOTGIVENPERMISSIONHOWEVER,THEREAREWAYSFORANAPPLICATIONTOSHAREDATAWITHOTHERAPPLICATIONSANDFORANAPPLICATIONTOACCESSSYSTEMSERVICESITSPOSSIBLETOARRANGEFORTWOAPPLICATIONSTOSHARETHESAMELINUXUSERID,INWHICHCASETHEYAREABLETOACCESSEACHOTHERSFILESTOCONSERVESYSTEMRESOURCES,APPLICATIONSWITHTHESAMEUSERIDCANALSOARRANGETORUNINTHESAMELINUXPROCESSANDSHARETHESAMEVMTHEAPPLICATIONSMUSTALSOBESIGNEDWITHTHESAMECERTIFICATEANAPPLICATIONCANREQUESTPERMISSIONTOACCESSDEVICEDATASUCHASTHEUSERSCONTACTS,SMSMESSAGES,THEMOUNTABLESTORAGESDCARD,CAMERA,BLUETOOTH,ANDMOREALLAPPLICATIONPERMISSIONSMUSTBEGRANTEDBYTHEUSERATINSTALLTIMETHATCOVERSTHEBASICSREGARDINGHOWANANDROIDAPPLICATIONEXISTSWITHINTHESYSTEMTHERESTOFTHISDOCUMENTINTRODUCESYOUTOTHECOREFRAMEWORKCOMPONENTSTHATDEFINEYOURAPPLICATIONTHEMANIFESTFILEINWHICHYOUDECLARECOMPONENTSANDREQUIREDDEVICEFEATURESFORYOURAPPLICATIONRESOURCESTHATARESEPARATEFROMTHEAPPLICATIONCODEANDALLOWYOURAPPLICATIONTOGRACEFULLYOPTIMIZEITSBEHAVIORFORAVARIETYOFDEVICECONFIGURATIONSAPPLICATIONCOMPONENTSAPPLICATIONCOMPONENTSARETHEESSENTIALBUILDINGBLOCKSOFANANDROIDAPPLICATIONEACHCOMPONENTISADIFFERENTPOINTTHROUGHWHICHTHESYSTEMCANENTERYOURAPPLICATIONNOTALLCOMPONENTSAREACTUALENTRYPOINTSFORTHEUSERANDSOMEDEPENDONEACHOTHER,BUTEACHONEEXISTSASITSOWNENTITYANDPLAYSASPECIFICROLEEACHONEISAUNIQUEBUILDINGBLOCKTHATHELPSDEFINEYOURAPPLICATIONSOVERALLBEHAVIORTHEREAREFOURDIFFERENTTYPESOFAPPLICATIONCOMPONENTSEACHTYPESERVESADISTINCTPURPOSEANDHASADISTINCTLIFECYCLETHATDEFINESHOWTHECOMPONENTISCREATEDANDDESTROYEDHEREARETHEFOURTYPESOFAPPLICATIONCOMPONENTSACTIVITIESANACTIVITYREPRESENTSASINGLESCREENWITHAUSERINTERFACEFOREXAMPLE,ANEMAILAPPLICATIONMIGHTHAVEONEACTIVITYTHATSHOWSALISTOFNEWEMAILS,ANOTHERACTIVITYTOCOMPOSEANEMAIL,ANDANOTHERACTIVITYFORREADINGEMAILSALTHOUGHTHEACTIVITIESWORKTOGETHERTOFORMACOHESIVEUSEREXPERIENCEINTHEEMAILAPPLICATION,EACHONEISINDEPENDENTOFTHEOTHERSASSUCH,ADIFFERENTAPPLICATIONCANSTARTANYONEOFTHESEACTIVITIESIFTHEEMAILAPPLICATIONALLOWSITFOREXAMPLE,ACAMERAAPPLICATIONCANSTARTTHEACTIVITYINTHEEMAILAPPLICATIONTHATCOMPOSESNEWMAIL,INORDERFORTHEUSERTOSHAREAPICTUREANACTIVITYISIMPLEMENTEDASASUBCLASSOFACTIVITYANDYOUCANLEARNMOREABOUTITINTHEACTIVITIESDEVELOPERGUIDESERVICESASERVICEISACOMPONENTTHATRUNSINTHEBACKGROUNDTOPERFORMLONGRUNNINGOPERATIONSORTOPERFORMWORKFORREMOTEPROCESSESASERVICEDOESNOTPROVIDEAUSERINTERFACEFOREXAMPLE,ASERVICEMIGHTPLAYMUSICINTHEBACKGROUNDWHILETHEUSERISINADIFFERENTAPPLICATION,ORITMIGHTFETCHDATAOVERTHENETWORKWITHOUTBLOCKINGUSERINTERACTIONWITHANACTIVITYANOTHERCOMPONENT,SUCHASANACTIVITY,CANSTARTTHESERVICEANDLETITRUNORBINDTOITINORDERTOINTERACTWITHITASERVICEISIMPLEMENTEDASASUBCLASSOFSERVICEANDYOUCANLEARNMOREABOUTITINTHESERVICESDEVELOPERGUIDECONTENTPROVIDERSACONTENTPROVIDERMANAGESASHAREDSETOFAPPLICATIONDATAYOUCANSTORETHEDATAINTHEFILESYSTEM,ANSQLITEDATABASE,ONTHEWEB,ORANYOTHERPERSISTENTSTORAGELOCATIONYOURAPPLICATIONCANACCESSTHROUGHTHECONTENTPROVIDER,OTHERAPPLICATIONSCANQUERYOREVENMODIFYTHEDATAIFTHECONTENTPROVIDERALLOWSITFOREXAMPLE,THEANDROIDSYSTEMPROVIDESACONTENTPROVIDERTHATMANAGESTHEUSERSCONTACTINFORMATIONASSUCH,ANYAPPLICATIONWITHTHEPROPERPERMISSIONSCANQUERYPARTOFTHECONTENTPROVIDERSUCHASCONTACTSCONTRACTDATATOREADANDWRITEINFORMATIONABOUTAPARTICULARPERSONCONTENTPROVIDERSAREALSOUSEFULFORREADINGANDWRITINGDATATHATISPRIVATETOYOURAPPLICATIONANDNOTSHAREDFOREXAMPLE,THENOTEPADSAMPLEAPPLICATIONUSESACONTENTPROVIDERTOSAVENOTESACONTENTPROVIDERISIMPLEMENTEDASASUBCLASSOFCONTENTPROVIDERANDMUSTIMPLEMENTASTANDARDSETOFAPISTHATENABLEOTHERAPPLICATIONSTOPERFORMTRANSACTIONSFORMOREINFORMATION,SEETHECONTENTPROVIDERSDEVELOPERGUIDEBROADCASTRECEIVERSABROADCASTRECEIVERISACOMPONENTTHATRESPONDSTOSYSTEMWIDEBROADCASTANNOUNCEMENTSMANYBROADCASTSORIGINATEFROMTHESYSTEMFOREXAMPLE,ABROADCASTANNOUNCINGTHATTHESCREENHASTURNEDOFF,THEBATTERYISL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RECEIVERSCANBEEITHERDECLAREDINTHEMANIFESTORCREATEDDYNAMICALLYINCODEASBROADCASTRECEIVEROBJECTSANDREGISTEREDWITHTHESYSTEMBYCALLINGREGISTERRECEIVERDECLARINGCOMPONENTCAPABILITIESASDISCUSSEDABOVE,INACTIVATINGCOMPONENTS,YOUCANUSEANINTENTTOSTARTACTIVITIES,SERVICES,ANDBROADCASTRECEIVERSYOUCANDOSOBYEXPLICITLYNAMINGTHETARGETCOMPONENTUSINGTHECOMPONENTCLASSNAMEINTHEINTENTHOWEVER,THEREALPOWEROFINTENTSLIESINTHECONCEPTOFINTENTACTIONSWITHINTENTACTIONS,YOUSIMPLYDESCRIBETHETYPEOFACTIONYOUWANTTOPERFORMANDOPTIONALLY,THEDATAUPONWHICHYOUDLIKETOPERFORMTHEACTIONANDALLOWTHESYSTEMTOFINDACOMPONENTONTHEDEVICETHATCANPERFORMTHEACTIONANDSTARTITIFTHEREAREMULTIPLECOMPONENTSTHATCANPERFORMTHEACTIONDESCRIBEDBYTHEINTENT,THENTHEUSERSELECTSWHICHONETOUSETHEWAYTHESYSTEMIDENTIFIESTHECOMPONENTSTHATCANRESPONDTOANINTENTISBYCOMPARINGTHEINTENTRECEIVEDTOTHEINTENTFILTERSPROVIDEDINTHEMANIFESTFILEOFOTHERAPPLICATIONSONTHEDEVICEWHENYOUDECLAREACOMPONENTINYOURAPPLICATIONSMANIFEST,YOUCANOPTIONALLYINCLUDEINTENTFILTERSTHATDECLARETHECAPABILITIESOFTHECOMPONENTSOITCANRESPONDTOINTENTSFROMOTHERAPPLICATIONSYOUCANDECLAREANINTENTFILTERFORYOURCOMPONENTBYADDINGANELEMENTASACHILDOFTHECOMPONENTSDECLARATIONELEMENTFOREXAMPLE,ANEMAILAPPLICATIONWITHANACTIVITYFORCOMPOSINGANEWEMAILMIGHTDECLAREANINTENTFILTERINITSMANIFESTENTRYTORESPONDTO“SEND“INTENTSINORDERTOSENDEMAILANACTIVITYINYOURAPPLICATIONCANTHENCREATEANINTENTWITHTHE“SEND”ACTIONACTION_SEND,WHICHTHESYSTEMMATCHESTOTHEEMAILAPPLICATIONS“SEND”ACTIVITYANDLAUNCHESITWHENYOUINVOKETHEINTENTWITHSTARTACTIVITYFORMOREABOUTCREATINGINTENTFILTERS,SEETHEINTENTSANDINTENTFILTERSDOCUMENTDECLARINGAPPLICATIONREQUIREMENTSTHEREAREAVARIETYOFDEVICESPOWEREDBYANDROIDANDNOTALLOFTHEMPROVIDETHESAMEFEATURESANDCAPABILITIESINORDERTOPREVENTYOURAPPLICATIONFROMBEINGINSTALLEDONDEVICESTHATLACKFEATURESNEEDEDBYYOURAPPLICATION,ITSIMPORTANTTHATYOUCLEARLYDEFINEAPROFILEFORTHETYPESOFDEVICESYOURAPPLICATIONSUPPORTSBYDECLARINGDEVICEANDSOFTWAREREQUIREMENTSINYOURMANIFESTFILEMOSTOFTHESEDECLARATIONSAREINFORMATIONALONLYANDTHESYSTEMDOESNOTREADTHEM,BUTEXTERNALSERVICESSUCHASGOOGLEPLAYDOREADTHEMINORDERTOPROVIDEFILTERINGFORUSERSWHENTHEYSEARCHFORAPPLICATIONSFROMTHEIRDEVICEFOREXAMPLE,IFYOURAPPLICATIONREQUIRESACAMERAANDUSESAPISINTRODUCEDINANDROID21APILEVEL7,YOUSHOULDDECLARETHESEASREQUIREMENTSINYOURMANIFESTFILETHATWAY,DEVICESTHATDONOTHAVEACAMERAANDHAVEANANDROIDVERSIONLOWERTHAN21CANNOTINSTALLYOURAPPLICATIONFROMGOOGLEPLAYHOWEVER,YOUCANALSODECLARETHATYOURAPPLICATIONUSESTHECAMERA,BUTDOESNOTREQUIREITINTHATCASE,YOURAPPLICATIONMUSTPERFORMACHECKATRUNTIMETODETERMINEIFTHEDEVICEHASACAMERAANDDISABLEANYFEATURESTHATUSETHECAMERAIFONEISNOTAVAILABLEHEREARESOMEOFTHEIMPORTANTDEVICECHARACTERISTICSTHATYOUSHOULDCONSIDERASYOUDESIGNANDDEVELOPYOURAPPLICATIONSCREENSIZEANDDENSITYINORDERTOCATEGORIZEDEVICESBYTHEIRSCREENTYPE,ANDROIDDEFINESTWOCHARACTERISTICSFOREACHDEVICESCREENSIZETHEPHYSICALDIMENSIONSOFTHESCREENANDSCREENDENSITYTHEPHYSICALDENSITYOFTHEPIXELSONTHESCREEN,ORDPIDOTSPERINCHTOSIMPLIFYALLTHEDIFFERENTTYPESOFSCREENCONFIGURATIONS,THEANDROIDSYSTEMGENERALIZESTHEMINTOSELECTGROUPSTHATMAKETHEMEASIERTOTARGETTHESCREENSIZESARESMALL,NORMAL,LARGE,ANDEXTRALARGETHESCREENDENSITIESARELOWDENSITY,MEDIUMDENSITY,HIGHDENSITY,ANDEXTRAHIGHDENSITYBYDEFAULT,YOURAPPLICATIONISCOMPATIBLEWITHALLSCREENSIZESANDDENSITIES,BECAUSETHEANDROIDSYSTEMMAKESTHEAPPROPRIATEADJUSTMENTSTOYOURUILAYOUTANDIMAGERESOURCESHOWEVER,YOUSHOULDCREATESPECIALIZEDLAYOUTSFORCERTAINSCREENSIZESANDPROVIDESPECIALIZEDIMAGESFORCERTAINDENSITIES,USINGALTERNATIVELAYOUTRESOURCES,ANDBYDECLARINGINYOURMANIFESTEXACTLYWHICHSCREENSIZESYOURAPPLICATIONSUPPORTSWITHTHEELEMENTFORMOREINFORMATION,SEETHESUPPORTINGMULTIPLESCREENSDOCUMENTINPUTCONFIGURATIONSMANYDEVICESPROVIDEADIFFERENTTYPEOFUSERINPUTMECHANISM,SUCHASAHARDWAREKEYBOARD,ATRACKBALL,ORAFIVEWAYNAVIGATIONPADIFYOURAPPLICATIONREQUIRESAPARTICULARKINDOFINPUTHARDWARE,THENYOUSHOULDDECLAREITINYOURMANIFESTWITHTHEELEMENTHOWEVER,ITISRARETHATANAPPLICATIONSHOULDREQUIREACERTAININPUTCONFIGURATIONDEVICEFEATURESTHEREAREMANYHARDWAREANDSOFTWAREFEATURESTHATMAYORMAYNOTEXISTONAGIVENANDROIDPOWEREDDEVICE,SUCHASACAMERA,ALIGHTSENSOR,BLUETOOTH,ACERTAINVERSIONOFOPENGL,ORTHEFIDELITYOFTHETOUCHSCREENYOUSHOULDNEVERASSUMETHATACERTAINFEATUREISAVAILABLEONALLANDROIDPOWEREDDEVICESOTHERTHANTHEAVAILABILITYOFTHESTANDARDANDROIDLIBRARY,SOYOUSHOULDDECLAREANYFEATURESUSEDBYYOURAPPLICATIONWITHTHEELEMENTPLATFORMVERSIONDIFFERENTANDROIDPOWEREDDEVICESOFTENRUNDIFFERENTVERSIONSOFTHEANDROIDPLATFORM,SUCHASANDROID16ORANDROID23EACHSUCCESSIVEVERSIONOFTENINCLUDESADDITIONALAPISNOTAVAILABLEINTHEPREVIOUSVERSIONINORDERTOINDICATEWHICHSETOFAPISAREAVAILABLE,EACHPLATFORMVERSIONSPECIFIESANAPILEVELFOREXAMPLE,ANDROID10ISAPILEVEL1ANDANDROID23ISAPILEVEL9IFYOUUSEANYAPISTHATWEREADDEDTOTHEPLATFORMAFTERVERSION10,YOUSHOULDDECLARETHEMINIMUMAPILEVELINWHICHTHOSEAPISWEREINTRODUCEDUSINGTHEELEMENTITSIMPORTANTTHATYOUDECLAREALLSUCHREQUIREMENTSFORYOURAPPLICATION,BECAUSE,WHENYOUDISTRIBUTEYOURAPPLICATIONONGOOGLEPLAY,THESTOREUSESTHESEDECLARATIONSTOFILTERWHICHAPPLICATIONSAREAVAILABLEONEACHDEVICEASSUCH,YOURAPPLICATIONSHOULDBEAVAILABLEONLYTODEVICESTHATMEETALLYOURAPPLICATIONREQUIREMENTSFORMOREINFORMATIONABOUTHOWGOOGLEPLAYFILTERSAPPLICATIONSBASEDONTHESEANDOTHERREQUIREMENTS,SEETHEFILTERSONGOOGLEPLAYDOCUMENTAPPLICATIONRESOURCESANANDROIDAPPLICATIONISCOMPOSEDOFMORETHANJUSTCODEITREQUIRESRESOURCESTHATARESEPARATEFROMTHESOURCECODE,SUCHASIMAGES,AUDIOFILES,ANDANYTHINGRELATINGTOTHEVISUALPRESENTATIONOFTHEAPPLICATIONFOREXAMPLE,YOUSHOULDDEFINEANIMATIONS,MENUS,STYLES,COLORS,ANDTHELAYOUTOFACTIVITYUSERINTERFACESWITHXMLFILESUSINGAPPLICATIONRESOURCESMAKESITEASYTOUPDATEVARIOUSCHARACTERISTICSOFYOURAPPLICATIONWITHOUTMODIFYINGCODEANDBYPROVIDINGSETSOFALTERNATIVERESOURCESENABLESYOUTOOPTIMIZEYOURAPPLICATIONFORAVARIETYOFDEVICECONFIGURATIONSSUCHASDIFFERENTLANGUAGESANDSCREENSIZESFOREVER