外文翻译 - Android起航

外文资料原文BeginningAndroidMarkL.MurphyUsingXML-BasedLayoutsWhileitistechnicallypossibletocreateandattachwidgetstoouractivitypurelythroughJavacode,thewaywedidinChapter4,themorecommonapproachistouseanXML-basedlayoutfile.Dynamicinstantiationofwidgetsisreservedformorecomplicatedscenarios,wherethewidgetsarenotknownatcompile-time(eg.,populatingacolumnofradiobuttonsbasedondataretrievedofftheInternet).Withthatinmind,itstimetobreakouttheXMLandlearnhowtolayoutAndroidactivitiesthatway.WhatIsanXML-BasedLayout?Asthenamesuggests,anXML-basedlayoutisaspecificationofwidgetsrelationshipstoeachotherandtotheircontainers(moreonthisinChapter7)encodedinXMLformat.Specifically,AndroidconsidersXML-basedlayoutstoberesources,andassuchlayoutfilesarestoredintheres/layoutdirectoryinsideyourAndroidproject.EachXMLfilecontainsatreeofelementsspecifyingalayoutofwidgetsandtheircontainersthatmakeuponeviewhierarchy.TheattributesoftheXMLelementsareproperties,describinghowawidgetshouldlookorhowacontainershouldbehave.Forexample,ifaButtonelementhasanattributevalueofandroid:textStyle=bold,thatmeansthatthetextappearingonthefaceofthebuttonshouldberenderedinaboldfacefontstyle.AndroidsSDKshipswithatool(aapt)whichusesthelayouts.ThistoolshouldbeautomaticallyinvokedbyyourAndroidtoolchain(e.g.,Eclipse,Antsbuild.xml).OfparticularimportancetoyouasadeveloperisthataaptgeneratestheR.javasourcefilewithinyourproject,allowingyoutoaccesslayoutsandwidgetswithinthoselayoutsdirectlyfromyourJavacode.WhyUseXML-BasedLayouts?MosteverythingyoudousingXMLlayoutfilescanbeachievedthroughJavacode.Forexample,youcouldusesetTypeface()tohaveabuttonrenderitstextinbold,insteadofusingapropertyinanXMLlayout.SinceXMLlayoutsareyetanotherfileforyoutokeeptrackof,weneedgoodreasonsforusingsuchfiles.Perhapsthebiggestreasonistoassistinthecreationoftoolsforviewdefinition,suchasaGUIbuilderinanIDElikeEclipseoradedicatedAndroidGUIdesignerlikeDroidDraw1.SuchGUIbuilderscould,inprinciple,generateJavacodeinsteadofXML.Thechallengeisre-readingtheUIdefinitiontosupporteditsthatisfarsimplerifthedataisinastructuredformatlikeXMLthaninaprogramminglanguage.Moreover,keepinggeneratedXMLdefinitionsseparatedfromhand-writtenJavacodemakesitlesslikelythatsomebodyscustom-craftedsourcewillgetclobberedbyaccidentwhenthegeneratedbitsgetre-generated.XMLformsanicemiddlegroundbetweensomethingthatiseasyfortool-writerstouseandeasyforprogrammerstoworkwithbyhandasneeded.Also,XMLasaGUIdefinitionformatisbecomingmorecommonplace.MicrosoftsXAML2,AdobesFlex3,andMozillasXUL4alltakeasimilarapproachtothatofAndroid:putlayoutdetailsinanXMLfileandputprogrammingsmartsinsourcefiles(e.g.,JavaScriptforXUL).Manyless-well-knownGUIframeworks,suchasZK5,alsouseXMLforviewdefinition.While“followingtheherd”isnotnecessarilythebestpolicy,itdoeshavetheadvantageofhelpingtoeasethetransitionintoAndroidfromanyotherXML-centeredviewdescriptionlanguage.OK,SoWhatDoesItLookLike?HereistheButtonfromthepreviouschapterssampleapplication,convertedintoanXMLlayoutfile,foundintheLayouts/NowReduxsampleproject.ThiscodesamplealongwithallothersinthischaptercanbefoundintheSourceCodeareaof.TheclassnameofthewidgetButtonformsthenameoftheXMLelement.SinceButtonisanAndroid-suppliedwidget,wecanjustusethebareclassname.Ifyoucreateyourownwidgetsassubclassesofandroid.view.View,youwouldneedtoprovideafullpackagedeclarationaswell.TherootelementneedstodeclaretheAndroidXMLnamespace:xmlns:android=/apk/res/androidAllotherelementswillbechildrenoftherootandwillinheritthatnamespacedeclaration.BecausewewanttoreferencethisbuttonfromourJavacode,weneedtogiveitanidentifierviatheandroid:idattribute.Wewillcoverthisconceptingreaterdetaillaterinthischapter.TheremainingattributesarepropertiesofthisButtoninstance:android:textindicatestheinitialtexttobedisplayedonthebuttonface(inthiscase,anemptystring)android:layout_widthandandroid:layout_heighttellAndroidtohavethebuttonswidthandheightfillthe“parent”,inthiscasetheentirescreentheseattributeswillbecoveredingreaterdetailinChapter7.Sincethissinglewidgetistheonlycontentinouractivity,weonlyneedthissingleelement.ComplexUIswillrequireawholetreeofelements,representingthewidgetsandcontainersthatcontroltheirpositioning.AlltheremainingchaptersofthisbookwillusetheXMLlayoutformwheneverpractical,sotherearedozensofotherexamplesofmorecomplexlayoutsforyoutoperusefromChapter7onward.WhatswiththeSigns?ManywidgetsandcontainersonlyneedtoappearintheXMLlayoutfileanddonotneedtobereferencedinyourJavacode.Forexample,astaticlabel(TextView)frequentlyonlyneedstobeinthelayoutfiletoindicatewhereitshouldappear.ThesesortsofelementsintheXMLfiledonotneedtohavetheandroid:idattributetogivethemaname.AnythingyoudowanttouseinyourJavasource,though,needsanandroid:id.Theconventionistouse+id/.astheidvalue,wherethe.representsyourlocallyuniquenameforthewidgetinquestion.IntheXMLlayoutexampleintheprecedingsection,+id/buttonistheidentifierfortheButtonwidget.Androidprovidesafewspecialandroid:idvalues,oftheformandroid:id/.Wewillseesomeoftheseinvariouschaptersofthisbook,suchasChapters8and10.WeAttachThesetotheJavaHow?GiventhatyouhavepainstakinglysetupthewidgetsandcontainersinanXMLlayoutfilenamedmain.xmlstoredinres/layout,allyouneedisonestatementinyouractivitysonCreate()callbacktousethatlayout:setContentView(R.layout.main);ThisisthesamesetContentView()weusedearlier,passingitaninstanceofaViewsubclass(inthatcase,aButton).TheAndroid-builtview,constructedfromourlayout,isaccessedfromthatcode-generatedRclass.AllofthelayoutsareaccessibleunderR.layout,keyedbythebasenameofthelayoutfilemain.xmlresultsinR.layout.main.Toaccessouridentifiedwidgets,usefindViewById(),passinginthenumericidentifierofthewidgetinquestion.ThatnumericidentifierwasgeneratedbyAndroidintheRclassasR.id.something(wheresomethingisthespecificwidgetyouareseeking).ThosewidgetsaresimplysubclassesofView,justliketheButtoninstancewecreatedinChapter4.Marketisthemediumbetweendevelopersandusers,henceitsveryimportant.Somepredictedthattherewillbemoreandmoreapplicationmarketswhilesomedontthinkso.Incurrentmarkets,bothdoexist.Somespecifyonlyonemarketfortheirproducts,whileothersselltheirsoftwaresinvariousmarkets.SoftwaresfromNokia,MicrosoftandLinuxMobilearesoldineverymarket.Developersoftheseplatformscanreleasetheirownapplicationinwhatevermarkets,somarketshavetocompetewitheachotherforaliving.Thisisgoodforusers.However,thelackofuniversalmanagementmayleadtomessandchaos,softwaresthathavethesamefunctionalityexistindifferentmarkets,whichconfusesusersalot.Correspondingly,solemarketsclaimthatmostapplicationsshouldbesoldinthem.Thiskindofmonopolizationleadstonocompetitor.AppStoreandAndroidMarketaredeputyofsolemarkets.Normally,iPhoneapplicationscanonlybefoundinAppStore,andApplewillcheckeveryoneofthembyitself.GoodnewsisthateveryapplicationinAppStoreisofficiallytested,itssafe;Badnewsisthatalotofprettygoodsoftwaresarerejectedforvariousreasons.Andabigunofficialmechanismisbuiltbyhackers,thatisjailbreakandSIMunlock.JailbreakisaprocessthatallowsiPad,iPhoneandiPodTouchuserstogainrootaccessandunlocktheoperatingsystemthusremovinganylimitationsimposeduponthembyApple.Oncejailbroken,iPhoneusersareabletodownloadmanyextensionsandthemespreviouslyunavailablethroughtheAppStoreviainstallerssuchasCydia.AjailbrokeniPad,iPhoneoriPodTouchisstillabletousetheAppStoreandiTunes9.AndaSIMlockisacapabilitybuiltintoGSMphonesbymobilephonemanufacturers.Networkprovidersusethiscapabilitytorestricttheuseofthesephonestospecificcountriesandnetworkproviders.Generally,phonescanbelockedtoacceptonlySIMcardsbasedontheInternationalMobileSubscriberIdentity.SIMunlockmakeitpossibletouseamobilephonewithoutconsideringcountriesandnetworksspecifiedbymobilephonemanufacturers.HoweverinAndroid,Googledoesnttesteveryapplicationatall,soalthoughtheresanofficialmarketforAndroidapplications,youcanstillreleaseyourproductanywhereyouwant.Consideringsecurityproblems,Googlebannedtheuseofsomecomponents.LikejailbreakandSIMunlockiniPhone,rootinAndroidgivesusers100%controloftheirdevices,alongwithsomesecurityrisks.RootisaprocessthatallowsusersofcellphonesrunningtheAndroidoperatingsystemtoattainprivilegedcontrol(knownasrootaccess)withinAndroidsLinuxsubsystem,similartojailbreakingonAppledevicesrunningtheiOSoperatingsystem,overcominglimitationsthatthecarriersandmanufacturersputonsuchphones.RootingmakesitpossibletousecustomversionsoftheAndroidsystemsuchasCyanogenMod,supportingfeaturesunavailableinstockROMs.ItalsoallowsfornewerversionsofAndroidnotsuppliedbytheoriginaldevicemanufacturer.IncontrasttoiOSjailbreaking,rootingisnotneededtorunapplicationsnotdistributedbytheofficialAndroidMarket.Itisneededhowever,whentryingtoaccesspaidAndroidapplicationsfromcountrieswhicharenotpartofthepaidapplicationsmarket.译文Android起航MarkL.Murphy使用XML进行布局虽然纯粹通过Java代码在activity上创建和添加部件，在技术上是可行的，我们在第4章中做的一样，更常见的方法是使用一种基于XML的布局文件。动态的小部件实例保留更多，情况复杂，小工具在编译时不为人所知（例如，在数据检索了互联网基础上将单选按钮填充柱。考虑到这一点，现在是时候打破XML来学习如何用此种方式来布置Androidactivities。什么是基于XML的布局？正如其名称所示，一个基于XML的布局是一个关系到每个规格的小部件，和他们的容器（更多关于此内容的在第7章）编码的XML格式。具体来说，Android认为基于XML的布局是资源，因此布局文件存储在res/在你的Android项目布局目录中。每个XML文件包含一个指定的部件和容器布局元素树，一种意见认为构成层次。对XML元素的属性，描述一个部件应如何看或者一个容器应如何运转。例如，如果一个按钮元素。有一个Android的属性值：文字样式=“bold”，这意味着该文本出现在按钮的表面应该是呈现一个粗体字体样式.Android的SDK中附带一个使用的布局的工具（aapt）。这个工具应自动调用你的Android工具链（例如，Eclipse中，Antsbuild.xml）。作为一个开发人员，尤其重要的是，在您的项目中aapt生成R.java源文件，让您能在那些布局中直接从Java代码中获取布局和部件。为什么使用基于XML的布局？使用XML布局文件做的大部分都可以通过Java代码。例如，你可以使用setTypeface（）命令一个按钮使用粗体文本，而不是在一个XML布局中使用属性。由于XML布局是为你跟踪的另一个文件，所以我们需要好的理由来使用这样的文件。也许最大的原因是为了在视图定义中协助工具的建立，如IDE中一个GUI创建者像Eclipse或者一个像DroidDraw1设计GUI图形用户界面建设者。这样GUI建设者们，在原则上，生成Java代码而不是XML。目前的挑战是重新阅读用户界面的定义，以支持编辑，也就是说，如果是像XLM的结构公式数据比一个程序语言中的数据简单的多。此外，保持生成的XML定义从手写的Java代码中分离，使得某人定制的来源意外重新生成不太可能。XML形成一个良好的中间立场，使工具作家使用更简便，程序员需要时手工工作更简易。此外，XML作为一个GUI定义格式是越来越普遍。微软的XAML，Adobe的Flex，和Mozilla的XUL都采取Android类似的方法：把布局细节放在一个XML文件和把编程智慧资料放在源文件（例如，XUL中的JavaScript）。许多不太知名的图形用户界面框架，如ZK，还使用视图定义的XML。而“随大流”并不一定是最好的政策，但他们有优势帮助从任何其他XML为中心的观点描述语言轻松进入Android。好了，那么基于XML的布局是什么样子的？下面是以前的章节的示例应用程序按钮，转换成一个XML布局文件，布局/NowRedux示例项目，在这一章中可以找到源代码的领域。部件，按钮的类名称形成XML元素的名称。因为按钮是Android提供的部件，我们可以只使用裸类的名称。如果您创建自己的部件作为android.view.View子小部件，您也将需要提供一个完整的包声明（如monsware.android.MyWidget）。根元素需要Android的XML命名空间声明：xmlns:android=/apk/res/android所有其他要素将成为子根并继承该命名空间的声明。因为我们要引用这个来自Java代码的按钮，我们需要通过android给它一个标识符：id属性。我们将在本章后面更详细的介绍这个概念。其余的属性是此按钮实例属性：android：文字表示的初始文本将显示在按钮（这种情况显示空字符串）android：layout_width和Android：layout_height命令android有按钮的宽度和高度填写“parent”，这种情况下，整个屏幕。将这些属性将在第7章中详解。由于这个单一部件是activity的仅有内容，我们只需要这一个因素。复杂的用户界面将需要整个树的元素，代表工具和容器，控制自己的定位。所有的这本书余下的章节将使用XML布局，所以还有数十种更复杂的其他布局实例，请前进到第七章仔细阅读。符号有什么用途？许多部件和容器只需要出现在XML布局文件，不须引用在Java代码。例如，一个静态标签（TextView）只需要在布局文件中以表明它应该出现在那里。在XML文件中各种元素文件不需要有android：id属性给他们一个名称。任何你想要在Java资源中使用的东西，都需要一个android：id.该公约是使用+id.作为ID值，其中的.代表你locallyunique名称有问题的部件。在上一节的XML布局的例子中，+id是按钮控件的标识符。android提供了一些特殊的android：ID值，形式android：id/.我们将在这本书的不同章节中看到这些，例如第八章和第十章。我们将这些附加到Java如何？既然你有意建立一个XML配置文件的工具和容器，名为main.xml存储res/layout，所有你需要的是一个在您activity的OnCreate()回调以使用该版式：setContentView（R.layout.main）;这是相同的setContentView()，我们前面使用，通过它的一个视图子类的实例(在这种情况下，一个按钮)。该android制造的观点，来自我们的布局，是从访问该代码生成的R类。所有的布局都可以访问R.layout，由基地键控布局文件的名称-main