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1、外文原文BeginningAndroidMarkL.MurphywriteUsingXML-BasedLayoutsWhileitistechnicallypossibletocreateandattachwidgetstoouractivitypurelythroughJavacode,thewaywedidinChapter4,themorecommonapproachistouseanXML-basedlayoutfile.Dynamicinstantiationofwidgetsisreservedformorecomplicatedscenari
2、os,wherethewidgetsarenotknownatcompile-time(eg.,populatingacolumnofradiobuttonsbasedondataretrievedofftheInternet).Withthatinmind,it’stimetobreakouttheXMLandlearnhowtolayoutAndroidactivitiesthatway.WhatIsanXML-BasedLayout?Asthenamesuggests,anXML-basedlayoutisaspecificationofwidget
3、s’relationshipstoeachother—andtotheircontainers(moreonthisinChapter7)—encodedinXMLformat.Specifically,AndroidconsidersXML-basedlayoutstoberesources,andassuchlayoutfilesarestoredintheres/layoutdirectoryinsideyourAndroidproject.EachXMLfilecontainsatreeofelementsspecifyingalayoutofwi
4、dgetsandtheircontainersthatmakeuponeviewhierarchy.TheattributesoftheXMLelementsareproperties,describinghowawidgetshouldlookorhowacontainershouldbehave.Forexample,ifaButtonelementhasanattributevalueofandroid:textStyle="bold",thatmeansthatthetextappearingonthefaceofthebuttonshouldbe
5、renderedinaboldfacefontstyle.Android’sSDKshipswithatool(aapt)whichusesthelayouts.ThistoolshouldbeautomaticallyinvokedbyyourAndroidtoolchain(e.g.,Eclipse,Ant’sbuild.xml).OfparticularimportancetoyouasadeveloperisthataaptgeneratestheR.javasourcefilewithinyourproject,allowingyoutoacce
6、sslayoutsandwidgetswithinthoselayoutsdirectlyfromyourJavacode.WhyUseXML-BasedLayouts?MosteverythingyoudousingXMLlayoutfilescanbeachievedthroughJavacode.Forexample,youcouldusesetTypeface()tohaveabuttonrenderitstextinbold,insteadofusingapropertyinanXMLlayout.SinceXMLlayoutsareyetano
7、therfileforyoutokeeptrackof,weneedgoodreasonsforusingsuchfiles.Perhapsthebiggestreasonistoassistinthecreationoftoolsforviewdefinition,suchasaGUIbuilderinanIDElikeEclipseoradedicatedAndroidGUIdesignerlikeDroidDraw1.SuchGUIbuilderscould,inprinciple,generateJavacodeinsteadofXML.Thech
8、allengeisre-readingtheUIdefinitio
