畢業(yè)設計外文文獻-Android起航

附錄I英文原文BeginningAndroid. UsingXML-BasedLayoutsWhileitistechnicallypossibletocreateandattachwidgetstoouractivitypurelythroughJavacode,thewaywedidinChapter4,themorecommonapproachistouseanXML-basedlayoutfile.Dynamicinstantiationofwidgetsisreservedformorecomplicatedscenarios,wherethewidgetsarenotknownatcompile-time(eg.,populatingacolumnofradiobuttonsbasedondataretrievedofftheInternet).Withthatinmind,itimetobreakouttheXMLandlearnhowtolayoutAndroidactivitiesthatway.1 WhatIsanXML-BasedLayout?Asthenamesuggests,anXML-basedlayoutisaspecificationofwidgetslationshipstoeachother—andtotheircontainers(moreonthisinChapter7)—encodedinXMLformat.Specifically,AndroidconsidersXML-basedlayoutstoberesources,andassuchlayoutfilesarestoredintheresyoutdirectoryinsideyourAndroidproject.EachXMLfilecontainsatreeofelementsspecifyingalayoutofwidgetsandtheircontainersthatmakeuponeviewhierarchy.TheattributesoftheXMLelementsareproperties,describinghowawidgetshouldlookorhowacontainershouldbehave.Forexample,ifaButtonelementhasanattributevalueofandroid:textStyle="bold",thatmeansthatthetextappearingonthefaceofthebuttonshouldberenderedinaboldfacefontstyle.ASDKshipswithatool(aapt)whichusesthelayouts.ThistoolshouldbeautomaticallyinvokedbyyourAndroidtoolchain(e.g.,Eclipse,Abuild.xml).OfparticularimportancetoyouasadeveloperisthataaptgeneratestheR.javasourcefilewithinyourproject,allowingyoutoaccesslayoutsandwidgetswithinthoselayoutsdirectlyfromyourJavacode.2 WhyUseXML-BasedLayouts?MosteverythingyoudousingXMLlayoutfilescanbeachievedthroughJavacode.Forexample,youcouldusesetTypeface()tohaveabuttonrenderitstextinbold,insteadofusingapropertyinanXMLlayout.SinceXMLlayoutsareyetanotherfileforyoutokeeptrackof,weneedgoodreasonsforusingsuchfiles.Perhapsthebiggestreasonistoassistinthecreationoftoolsforviewdefinition,suchasaGUIbuilderinanIDElikeEclipseoradedicatedAndroidGUIdesignerlikeDroidDraw1.SuchGUIbuilderscould,inprinciple,generateJavacodeinsteadofXML.Thechallengeisre-readingtheUIdefinitiontosupportedits—thatisfarsimplerifthedataisinastructuredformatlikeXMLthaninaprogramminglanguage.Moreover,keepinggeneratedXMLdefinitionsseparatedfromhand-writtenJavacodemakesitlesslikelythatsomebodycustom-craftedsourcewillgetclobberedbyaccidentwhenthegeneratedbitsgetre-generated.XMLformsanicemiddlegroundbetweensomethingthatiseasyfortool-writerstouseandeasyforprogrammerstoworkwithbyhandasneeded.Also,XMLasaGUIdefinitionformatisbecomingmorecommonplace.MXAML2,AdobeFlex3,andMozillaXUL4alltakeasimilarapproachtothatofAndroid:putlayoutdetailsinanXMLfileandputprogrammingsmartsinsourcefiles(e.g.,JavaScriptforXUL).Manyless-well-knownGUIframeworks,suchasZK5,alsouseXMLforviewdefinition.Whilelowingtheherd”isnotnecessarilythebestpolicy,itdoeshavetheadvantageofhelpingtoeasethetransitionintoAndroidfromanyotherXML-centeredviewdescriptionlanguage.layoutfile,foundintheLayouts/NowReduxsampleproject.ThiscodesamplealongwithallothersinthischaptercanbefoundintheSourceCodeareaofhttp://r/.<?xmlversion="1.0"encoding="utf-8"?><Buttonxmlns:android="http://r/pkndroid"android:id="@+id/button"android:text=""android:layout_width="fill_parent"android:layout_height="fill_parent"/>Theclassnameofthewidget—Button—formsthenameoftheXMLelement.SinceButtonisanAndroid-suppliedwidget,wecanjustusethebareclassname.Ifyoucreateyourownwidgetsassubclassesofandroid.view.View,youwouldneedtoprovideafullpackagedeclarationaswell.TherootelementneedstodeclaretheAndroidXMLnamespace:xmlns:android="http:/schem/r/pksndroid"Allotherelementswillbechildrenoftherootandwillinheritthatnamespacedeclaration.BecausewewanttoreferencethisbuttonfromourJavacode,weneedtogiveitanidentifierviatheandroid:idattribute.Wewillcoverthisconceptingreaterdetaillaterinthischapter.TheremainingattributesarepropertiesofthisButtoninstance:android:textindicatestheinitialtexttobedisplayedonthebuttonface(inthiscase,anemptystring)android:layout_widthandandroid:layout_heighttellAndroidtohavethewidthandheightfilltheinthiscasetheentirescreen—theseattributeswillbecoveredingreaterdetailinChapter7.Sincethissinglewidgetistheonlycontentinouractivity,weonlyneedthissingleelement.ComplexUIswillrequireawholetreeofelements,representingthewidgetsandcontainersthatcontroltheirpositioning.AlltheremainingchaptersofthisbookwillusetheXMLlayoutformwheneverpractical,sotherearedozensofotherexamplesofmorecomplexlayoutsforyoutoperusefromChapter7onward.3Whatswiththe@Signs?ManywidgetsandcontainersonlyneedtoappearintheXMLlayoutfileanddonotneedtobeeferencedinyourJavacode.Forexample,astaticlabel(TextView)frequentlyonlyneedstobeinthelayoutfiletoindicatewhereitshouldappear.ThesesortsofelementsintheXMLfiledonotneedtohavetheandroid:idattributetogivethemaname.AnythingyoudowanttouseinyourJavasource,though,needsanandroid:id.Theconventionistouse@+id/...astheidvalue,wherethe...representsyourlocallyuniquenameforthewidgetinquestion.IntheXMLlayoutexampleintheprecedingsection,@+id/buttonistheidentifierfortheButtonwidget.Androidprovidesafewspecialandroid:idvalues,oftheform@android:id/....Wewillseesomeoftheseinvariouschaptersofthisbook,suchasChapters8and10.WeAttachThesetotheJavaHow?GiventhatyouhavepainstakinglysetupthewidgetsandcontainersinanXMLlayoutfilenamedmain.xmlstoredinresyout,allyouneedisonestatementinyouractivityonCreate()callbacktousethatlayout:setContentView(R.layout.main);ThisisthesamesetContentView()weusedearlier,passingitaninstanceofaViewsubclass(inthatcase,aButton).TheAndroid-builtview,constructedfromourlayout,isaccessedfromthatcode-generatedRclass.AllofthelayoutsareaccessibleunderR.layout,keyedbythebasenameofthelayoutfile—main.xmlresultsinR.layout.main.Toaccessouridentifiedwidgets,usefindViewById(),passinginthenumericidentifierofthewidgetinquestion.ThatnumericidentifierwasgeneratedbyAndroidintheRclassasR.id.something(wheresomethingisthespecificwidgetyouareseeking).ThosewidgetsaresimplysubclassesofView,justliketheButtoninstancewecreatedinChapter4.TheRestoftheStoryIntheoriginalNowdemo,thefacewouldshowthecurrenttime,whichwouldreflectwhenthebuttonwaslastpushed(orwhentheactivitywasfirstshown,ifthebuttonhadnotyetbeenpushed).Mostofthatlogicstillworks,eveninthisreviseddemo(NowRedux).However,ratherthaninstantiatingtheButtoninouractivityonCreate()callback,wecanreferencetheonefromtheXMLlayout:package/r/monsware.android.layouts;importandroid.app.Activity;importandroid.os.Bundle;importandroid.view.View;importandroid.widget.Button;importjava.util.Date;publicclassNowReduxextendsActivityimplementsView.OnClickListener {Buttonbtn;@OverridepublicvoidonCreate(Bundleicicle) {super.onCreate(icicle);setContentView(R.layout.main);btn=(Button)findViewById(R.id.button);btn.setOnClickListener(this);updateTime();}publicvoidonClick(Viewview) {updateTime();}privatevoidupdateTime() {btn.setText(newDate().toString());}}ThefirstdifferenceisthatratherthansettingthecontentviewtobeaviewwecreatedinJavacode,wesetittoreferencetheXMLlayout(setContentView(R.layout.main)).TheR.javasourcefilewillbeupdatedwhenwerebuildthisprojecttoincludeareferencetoourlayoutfile(storedasmain.xmlinourresyoutdirectory).TheotherdifferenceisthatweneedtogetourhandsonourButtoninstance,forwhichweusethefindViewById()call.Sinceweidentifiedourbuttonas@+id/button,wecanreferencetheidentifierasR.id.button.Now,withtheButtoninstanceinhand,wecansetthecallbackandsetthelabelasneeded.. EmployingBasicWidgetsEveryGUItoolkithassomebasicwidgets:fields,labels,buttons,etc.Atoolkitisnodifferentinscope,andthebasicwidgetswillprovideagoodintroductionastohowwidgetsworkinAndroidactivities.Thesimplestwidgetisthelabel,referredtoinAndroidasaTextView.LikeinmostGUItoolkits,labelsarebitsoftextnoteditabledirectlybyusers.Typically,theyareusedtoidentifyadjacentwidgets(e.g.,ame:abelbeforeafieldwhereonefillsinaname).InJava,youcancreatealabelbycreatingaTextViewinstance.Morecommonly,though,youwillcreatelabelsinXMLlayoutfilesbyaddingaTextViewelementtothelayout,withanandroid:textpropertytosetthevalueofthelabelitself.Ifyouneedtoswaplabelsbasedoncertaincriteria,suchasinternationalization,youmaywishtousearesourcereferenceintheXMLinstead,aswillbedescribedinChapter9.TextViewhasnumerousotherpropertiesofrelevanceforlabels,Forexample,intheBasicabelproject,youwillfindthefollowinglayoutfile:<?xmlversion="1.0"encoding="utf-8"?><TextViewxmlns:android=http:/schem/r/pkndroidandroid:layout_width="fill_parent"android:layout_height="wrap_content"android:text="Youwereexpectingsomethingprofound?"2.1 Button,WhGottheButton?WeealreadyseentheuseoftheButtonwidgetinChapters4and5.Asitturnsout,ButtonisasubclassofTextView,soeverythingdiscussedintheprecedingsectionintermsofformattingthefaceofthebuttonstillholds.Androidhastwowidgetstohelpyouembedimagesinyouractivities:ImageViewandImageButton.Asthenamessuggest,theyareimage-basedanaloguestoTextViewandButton,respectively.Eachwidgettakesanandroid:srcattribute(inanXMLlayout)tospecifywhatpicturetouse.Theseusuallyreferenceadrawableresource,describedingreaterdetailinthechapteronresources.YoucanalsosettheimagecontentbasedonaUrifromacontentproviderviasetImageURI().ImageButton,asubclassofImageView,mixesinthestandardButtonbehaviors,forrespondingtoclicksandwhatnot.Forexample,takeapeekatthemain.xmllayoutfromtheBasicageViewsampleproject:<?xmlversion="1.0"encoding="utf-8"?><ImageViewxmlns:android=http://r/pksndroidandroid:id="@+id/icon"android:layout_width="fill_parent"android:layout_height="fill_parent"android:adjustViewBounds="true"android:src="@drawableolecule"2.2FieldsofGreen.OrOtherColors.Alongwithbuttonsandlabels,fieldsarethethirdnchor”ofmostGUItoolkits.InAndroid,theyareimplementedviatheEditTextwidget,whichisasubclassoftheTextViewusedforlabels.AlongwiththestandardTextViewproperties(e.g.,android:textStyle),EditTexthasmanyothersthatwillbeusefulforyouinconstructingfields,including:Beyondthose,youcanconfigurefieldstousespecializedinputmethods,suchasandroid:numericfornumeric-onlyinput,android:passwordforshroudedpasswordinput,andandroid:phoneNumberforenteringinphonenumbers.Ifyouwanttocreateyourowninputmethodscheme(e.g.,postalcodes,SocialSecuritynumbers),youneedtocreateyourownimplementationoftheInputMethodinterface,thenconfigurethefieldtouseitviaandroid:inputMethod.Forexample,fromtheBasic/Fieldproject,hereisanXMLlayout