外文翻译-创建图形 视图窗口

附录一：英文原文CreatingGraphics/ViewWindowsByreimplementingacustomQWidgetsubclassspaintEvent()andusingaQPainter,wecandrawanythingwewant.Thisisidealforcustomwidgets,butisnotatallconvenientifwewanttodrawlotsofindividualitems,especiallyifwewanttoprovidetheuserwiththeabilitytointeractwiththeitems.Forexample,inthepastsomeusershavecreatedgraphicalapplicationsusingliterallythousandsofcustomwidgetstostandasgraphicalitems,andalthoughwidgetpaintingisveryfast,handlingasinglemouseclickinsuchsituationscouldeasilyconsumealmostthewholeCPUsprocessingcapability.Fortunately,Qt4.2introducedthegraphics/viewarchitecturewhichperfectlyfulfillstheneedforhigh-performanceitem-baseddrawingandinteraction.AlthoughoriginallydesignedasasuperiorreplacementforQt3sQCanvasclass,Qt4sgraphics/viewarchitecturehasgonefarbeyondthecanvassfunctionality.Infact,someapplicationsnowuseaQGraphicsViewastheirmainwindowscentralwidgetandplaceallthewidgetsusedtoprovidetheuserinterfaceinsidetheviewasgraphicsitemsintheirownright.Inthischaptersfirstsectionwebeginwithabriefoverviewofthegraphics/viewarchitecture,includingasidebaronsomesignificantchangesintroducedinQt4.6.Then,inthesecondsection,wewillreviewanapplicationwhosemainwindowscentralwidgetisaQGraphicsViewandwhichhasbothwidgetitemsandconventionalgraphicsitems.Andfinally,inthechaptersthirdsection,wewilllookatasimpleQGraphicsItemsubclassandtheQGraphicsItemAPI.Inthenextchapterwewilllookatamoreconventionalgraphics/viewapplicationabasicdrawingprogramandtherewewilllookmorecloselyatmostofthegraphics/viewclasses,andwillpresentmoreexamplesofhowtocreatecustomgraphicsitems.Incidentally,wewillrevisittheexamplespresentedinthischapterandthenextinChapter13wherewecreatemodifiedversionsofthemthatmakeuseofsomeQt4.6-specificfeatures.RatherlikeQtsmodel/viewarchitecture,thegraphics/viewarchitecturehasanon-visualclassforholdingtheitemdataasamodel(QGraphicsScene),andaclassforvisualizingthedata(QGraphicsView).Wecanvisualizethesamesceneinmanydifferentviewsifthatisrequired.AgraphicsscenecontainsitemsthatarederivedfromtheabstractQGraphicsItemclass.Sinceitsoriginalintroduction,Qtsgraphics/viewarchitecturehasbenefitedfromagreatdealofdevelopmentefforttoimprovebothitsspeedanditscapabilities.Scenescanbescaled,rotated,andprinted,andrenderingcanbedoneusingQtsrenderingengineorusingOpenGL.Thearchitecturealsosupportsanimationanddraganddrop.Graphicsscenescanbeusedtopresentanythingfromjustafewitemsuptotensofthousandsofitemsormore.Qtprovidesmanypredefinedgraphicsitemtypesthatcanbeusedoutofthebox;theyareshowninFigure11.1.Mostoftheclassnamesareself-explanatory,butwewillmentionafewthatmaynotbesoobvious.AQGraphicsPathItemrepresentsaQPainterPathessentiallyanarbitraryshapethatiscomposedofallthefundamentalthingsthatQtcandraw,including,arcs,Bziercurves,chords,ellipses,lines,rectangles,andtext.AQGraphicsSimpleTextItemrepresentsapieceofplaintextandaQGraphicsTextItemrepresentsapieceofQtrichtext(whichcanbespecifiedusingHTML;wediscussedrichtextintheprevioustwochapters).TheQGraphicsWidgetclassisprovidedasabaseclassforthecreationofcustomwidgetsthataredesignedtoliveingraphicsscenes.ItisalsopossibletoembedstandardQWidget-derivedwidgetsinscenesthisisdonebyaddingthewidgettoaQGraphicsProxyWidgetandputtingtheproxywidgetintothescene.Usingproxywidgets(orQWidgetsdirectly)is“slow”butwhetherthisisnoticeablewilldependontheapplication.TheQGraphicsWebViewclasswasintroducedinQt4.6andprovidesagraphicsitemversionoftheQWebViewclasswediscussedinChapter1,forpresentingwebcontentinascene.ForscenesthathavesmallnumbersofitemswecanusetheQGraphicsObjectsintroducedinQt4.6orforQt4.5andearlier,wecanderivefrombothQObjectandQGraphicsItemasthebasisforourowncustomitems.Thisincreaseseachitemsoverhead(e.g.,itemsconsumemorememory),butprovidestheconvenienceofsupportforsignalsandslotsandforQtspropertysystem.ForscenesthathavelotsandlotsofitemsitisusuallybesttousethelightweightQGraphicsItemclassasthebasisforthecustomitemsthatwillappearinlargenumbers,andonlyuseQGraphicsObjectsforitemsthatwillappearinsmallnumbers.Thegraphicsviewclassesareessentiallytwo-dimensional,althougheveryitemhasazvalue,withhigherz-valueditemsbeingdrawninfrontofthosewithlowerzvalues.Collisiondetectionisbasedonitem(x,y)positions.Inadditiontoinformationaboutcollisions,thescenecantelluswhichitemscontainaparticularpointorareinaparticularregion,andwhichareselected.Scenesalsohaveaforegroundlayerwhichisuseful,forexample,todrawagridthatoverlaysalltheitemsinthescene;theyalsohaveabackgroundlayerthatisdrawnunderneathalltheitems,usefulforprovidingabackgroundimageorcolor.Itemsareeitherchildrenofthescene,orachildofanotheritem,ratherlikeQtsnormalparentchildwidgetrelationships.Whentransformationsareappliedtoanitem,theyareautomaticallyappliedtoalltheitemschildren,recursivelytothegreatestdescendant.Thismeansthatifanitemismovedforexample,draggedbytheuserallofitschildren(andtheirchildren,andsoon)willbedraggedwithit.ItispossibletomakeachilditemignoreitsparentstransformationsbycallingQGraphicsItem:setFlag(QGraphicsItem:ItemIgnoresTransformations).Other,morecommonlyusedflagsincludeonesformakinganitemmovable,selectable,andfocusablebytheuser.(AlltheflagsarelistedinTables11.6and11.7;.)ItemscanalsobegroupedbymakingthemchildrenofaQGraphicsItemGroup;thisisusefulforcreatingadhoccollectionsofitems.Thegraphicsviewclassesusethreedifferentcoordinatesystems,althoughinpracticeweusuallycareaboutonlytwoofthem.Viewsusethephysicalcoordinatesystem.ScenesusealogicalcoordinatesystemthatwenormallydefinebypassingaQRectFtotheirconstructor.Qtautomaticallymapsscenecoordinatestoviewcoordinates.Inessence,scenesuse“window”(logical)coordinatesandviewsuse“viewport”(physical)coordinates.So,whenwearepositioningitemsweplacethemintermsofscenecoordinates.Thethirdcoordinatesystemistheoneusedbyitems.Thisisparticularlyconvenientbecauseitisalogicalcoordinatesystemcenteredonpoint(0,0).Eachitems(0,0)isactuallyattheitemscenterpositioninthescene(apartfromtextitemswhereitistheitemstop-leftcorner).Thismeansthatinpractice,wecanalwaysdrawitemsintermsoftheirowncenterpointandwedonothavetocareaboutanytransformationsthathavebeenappliedtothembyparentitems,sincethescenewillautomaticallytakecareoftheseforus.NotealsothatinQt,y-coordinatesincreasedownwardforexample,point(5,8)is6pixelsabovepoint(5,14).Figureillustratestherelationshipbetweensceneanditemcoordinates.CertainaspectsofQtsgraphics/viewarchitecturesbehaviorhavechangedbetweenQt4.5andQt4.6;thesearesummarizedinthe“Qt4.6Graphics/ViewBehaviorChanges”sidebar.InthissectionwewillreviewthePetriDishapplication(petridish1)showninFigureTheapplicationhasaMainWindowclassthatinheritsQMainWindowandusesaQGraphicsViewasitscentralwidget.PetriDishisadialog-stylesimulationapplicationthatsimulates“cells”thatgrowiftheyarenttoocrowded,butshrinkiftheyaretooisolated,toocrowded,ortoobig,andwheresmallcellsrandomly“die”.Wewontsaymuchmoreaboutthesimulationitself,orabouttheapplicationslogic,sincethefocusofthischapterisQtsgraphics/viewarchitecture.Herewewillreviewtherelevantmainwindowmethods(orextractsfromthesemethods)toshowhowtocreateamainwindowbasedonagraphicsscene.AndinthenextsectionwewilllookattheCellitems(derivedfromQGraphicsItem),focusingonthebasicsofcreatingacustomgraphicsitemandintroducingtheQGraphicsItemAPIwithoutlookingattheirrelevantsimulationlogic.(Thesourcecodeisinthepetridish1subdirectory.)Theapplicationhasstart,pause/resume,stop,andquitbuttonstocontrolthesimulation,andtheusercansetaninitialcellcountandwhethertoshowcellIDswhichisusefulforcellsthatareotherwisetoosmalltosee.(Theinitialcellcountisdisabledduringasimulationrun,asthescreenshotillustrates.)TheuserinterfaceusesacoupleofQLCDNumberstoshowhowmanycellsareleftandhowmanyiterationsthesimulationhasrunfor.Wewillbeginbylookingatthemainwindowsconstructor,andthenatsomeofthehelpermethodsthatarerelevanttothegraphics/viewprogrammingweareconcernedwithhere.MainWindow:MainWindow(QWidget*parent):QMainWindow(parent),simulationState(Stopped),iterations(0)scene=newQGraphicsScene(this);scene-setItemIndexMethod(QGraphicsScene:NoIndex);createWidgets();createProxyWidgets();createLayout();createCentralWidget();createConnections();startButton-setFocus();setWindowTitle(QApplication:applicationName();ThecreationoftheQGraphicsSceneisslightlyunusualsincewehavenotspecifiedthescenesdimensions.Althoughweknowtheheightweneed(highenoughforthepetridishplussomemargin),thewidthdependsonhowwidethewidgetsare,sowewillsetthedimensionsoncethewidgetshavebeencreatedandlaidout.Whenitemsareadded,moved,orremovedfromascene,locationcomputationsarerequired.Forexample,ifanitemisaddedinavisibleportionofthesceneitmustbedrawn,orifavisibleitemismovedorremoved,whateverithadcoveredandisnowrevealedmustbedrawn.ForsceneswithlotsofstaticitemsthesecomputationscanbeconsiderablyspeededupbyusinganindexmethodofQGraphicsScene:BspTreeIndex(BinarySpacePartitioning);butforhighlydynamicsceneswithlotsofitemsadded,moved,orremoved,itisbettertoswitchoffindexing(aswedohere),sincetheoverheadofusingitoutweighsthesavingsitdelivers.附录二：中文译文创建图形/视图窗口通过自定义QWidget源派生类，实现paintEvent()并使用QPainter，我们可以随心所欲地绘制任何需要的内容，这种方法对于自定义的窗口部件（widget）很理想，但如果要绘制大量独立的项（item）就不怎么方便了，尤其是当想要向用户提供与图形项的图像应用，虽然窗口部件的绘制非常快，单在这种情况下处理单个鼠标点击轻易就消耗了几乎所有的CPU处理能力。型号QT4引入了图形/视图架构，完美的满足了基于项的高性能绘制和交互的需求。QT4的图形视图架构原本的设计是取代并超越QT3的QCanvas类，但它已经远远炒股了画布的功能。实际应用中有些应用程序用QGraphicsView作为主窗口的中心窗体，把所有提供用户界面的窗口部件作为图形项放在视图中。在11.1节我们先从图形驶入架构的概览开始，涵盖一些QT4中新加入的关键变化。在11.2节将查看一个用QFraphicsView作为主窗口的中心窗体的应用程序，QGraphicsView中既包含窗口部件项又包含传统的图形项。最后在11.3节我们将介绍一个简单的QGriphicsItem的子类和QGriphicsItem的API。下一张将介绍一个更传统的图形视图应用程序（一个基本的绘图程序）更深入的了解大部分图形/视图类，还会介绍更多关于如何创建自定义图形项的例子。顺便提一句，这一章里出现的例子将在第13章中使用一些QT4中才有的特性创建一个修改版。与QT的墨香视图架构十分相似，图形视图架构有一个作为模型的不可见类（QGraphicsScene）容纳项的数据，同事用另一个（QGraphicsView）来可视化显示数据。如有需要，我们可以在多个不用的视图内可视化显示同一场景。图形场景包含了从抽象类QGraphicsItem派生类而来的项。从最初被引入到QT中至今，得益于大量的开发努力，图形视图架构在速度和功能上都有了很大的进步。场景可缩放、旋转并可打印，渲染则可以使用QT的渲染引擎或者使用OPENGL。该架构还支持动画和拖放操作。图形场景可以用来呈现任何内容，从几个项直至成千上万乃至更多的项。QT提供了可以直接使用的众多预定义图形项类型，如图11.1所示。多数类的名称可顾名思义，单我们会体积一些不那么显而易见的类。QGraphicsPathItem代表一个QPainterPath，本质上是一个任意的形状，由所有基本的QT可绘制的物体，包括户县、贝吉爱曲线、弦、椭圆形、线段、矩形和文本一一组合而成。QGraphicsWidget类是创建在图形场景中存在的自定义窗口部件的积累。通过把窗口部件添加到QGraphicsProxyWidget，再把代理窗口部件（或者直接使用窗口部件）比较慢，单至于会不会慢到被质疑道的成都还要看应用程序。Qt4引入了QGraphicsWebView类,提供QWebView类（我们在第一章中讨论了这个类）的图形项版本，用于在场景中呈现网络内容（Webcontent）.对于项不多的场景，可以使用在Qt4中引入的QGraphicsObjects类，对于Qt4及更早的版本，可以吧QObject和QGraphicsItem两个雷作为基础派生自定义的项。这会增加每个项的开销（如项会占用更多的内存），单可以提供对信号和槽以及Qt的属性系统的支持。对于有很多很多项的场景来说，通常最好使用轻量级的QGraphicsItem类作为数目众多的自定义项的基础，仅对数目较少的项使用QgraphicsObjects。虽然图形视图类的每个项都有一个z值，z值较大的项会被绘制在z值较小的项之上，但图形视图类本质上是二维的。冲突检测基于项的（x,y）坐标位置。除了冲突信息之外，场景还可以告诉我们哪些项包含一个特定的点，哪些项在某特定领域中以及哪些项被选中。场景还有一个有用的前景层，例如在场景中绘制一个覆盖所有项的网络：场景还有一个背景层在所有项之下绘制，可用于提供背景图或者背景色。项可以是场景或者另一个项的子对象，和Qt窗口部件的父子关系十分相似。当一个项应用了变换，该项的所有子对象自动应用该变换，地柜应用至最深层次的子对象。这意味着如果一个项被移动（如被永辉拖动），它的所有子项（及子项的子孙）将和它同事被拖动。通过调用QGraphicsItem:setFlag(QGraphicsItem:ItemIgnoresTransformations)可以让子项忽略父项的变换。除此以外更常用的标志包括设置项可移动、选中和获得焦点的标志。通过吧项设置为QGraphicsItemGroup的子项，可以吧它们分组；这对于创建激动的项集合很有用。图形视图类使用的三种不同的坐标系统，虽然在实际的应用中通常只关心其中的两种。视图使用坐标系统，场景使用一朝在构造函数中传入的QRectF定义的逻辑坐标系统。QT自动进行从场景坐标到视图坐标的映射。重点在于场景用窗口（window）的（逻辑）坐标而视图用视口的屋里坐标。所以我们放置项是用的是场景的坐标系统。第三个坐标系统是项使用的坐标系统，该系统非常使用，应为它是中心在（0,0）点的逻辑坐标系统。每个项的（0,0）点实际上在该项场景中所在位置的中心