拖拉演示.docx
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拖拉演示.docx
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拖拉演示
Introduction
I'veseenseveralquestionsaroundtheCodeProjectboardslatelyaskingaboutdoingdraganddropbetweenaprogramandExplorerwindows.LikemanythingsinWindows,itseemseasyonceyouknowhowit'sdone,butfindingtheanswercanbequiteachore.Inthisarticle,IdemonstratehowtohookupdraganddropsoyourprogramcanacceptdropsfromanExplorerwindow,andbeadragsourcesoyouruserscandragfilesintoanExplorerwindow.
ThesampleprojectisanMFCapp,andthearticleassumesyou'refamiliarwithC++,MFC,andusingCOMobjectsandinterfaces.IfyouneedhelpwithCOMobjectsandinterfaces,checkoutmyIntrotoCOMarticle.TheprogramisMultiFiler,alittleutilitythatactslikeadraganddrop"stagingarea".YoucandraganynumberoffilesintoMultiFiler,anditshowsthemallinalist.YoucanthendragfilesbackouttoExplorerwindows,usingtheShiftorCtrlkeystotellExplorertomoveorcopytheoriginalfiles,respectively.
DragandDropwithExplorer
Asyouknow,ExplorerletsyoudragfilesamongExplorerwindowsandthedesktop.Whenyoubeginadragoperation,theExplorerwindowyoudragfrom(thedropsource),createsaCOMobjectimplementingtheIDataObjectinterface,andputssomedataintotheobject.Thewindowyoudraginto(thedroptarget),thenreadsthatdatausingIDataObjectmethods;that'showitknowswhatfilesarebeingdropped.
IfyoucheckoutthedatacontainedintheIDataObjectwithaviewerlikeClipSpy,you'llseethatExplorerputsseveraldataformatsinthedataobject:
TheimportantformatisCF_HDROP.TheotherformatsarecustomformatsregisteredbyExplorerforitsownuse.Ifwewriteanappthatregistersitswindowasadroptarget,andifweknowhowtoreadCF_HDROPdata,we'llbeabletoacceptdroppedfiles.Similarly,ifwecanfilladataobjectwithCF_HDROPdata,Explorerwillletourappbeadragsource.So,what'scontainedinthatCF_HDROPformat?
Readon...
TheDROPFILESdatastructure
SowhatexactlyistheCF_HDROPformat?
Itturnsoutthatit'sjustaDROPFILESstruct.There'salsotheHDROPtype,whichissimplyapointertoaDROPFILESstruct.
DROPFILESisn'taverycomplexstructure.Hereisitsdefinition:
structDROPFILES
{
DWORDpFiles;//offsetoffilelist
POINTpt;//droppoint(clientcoords)
BOOLfNC;//isitonNonClientareaandptisinscreencoords
BOOLfWide;//widecharacterflag
};
Theonethingthatisn'tlistedinthestructdefinitionisthelistoffilenames.Thelistisformattedasadouble-nullterminatedlistofstrings.Butwhereisitstored?
ItisactuallystoredrightafterthefWidemember,andpFilesholdstheoffset(relativetothebeginningofthestruct)wherethelistislocatedinmemory.Theonlyothermemberthat'susedindraganddropisfWide,whichindicateswhetherthefilenamesareinANSIorUnicodecharacters.
AcceptingadraganddropfromExplorer
Acceptingadraganddropismucheasierthaninitiatingone,soI'llcoveracceptingfirst.
Therearetwowaysforyourwindowtoacceptdraganddrop.ThefirstwayisaholdoverfromWindows3.1andusestheWM_DROPFILESmessage.TheotherwayistoregisteryourwindowasanOLEdroptarget.
Theoldway-WM_DROPFILES
Tousetheoldmethod,youfirstsetthe"acceptfiles"styleinyourwindow.Fordialogs,thisisonthe"ExtendedStyles"page,asshownhere:
Ifyouwanttosetthisstyleatruntime,calltheDragAcceptFiles()API,whichtakestwoparameters.Thefirstisyourmainwindowhandle,andthesecondisTRUEtoindicateyoucanacceptdraganddrop.IfyourmainwindowisaCViewinsteadofadialog,you'llneedtosetthisstyleatruntime.
Nomatterwhichofthetwomethodsyouuse,yourwindowbecomesadroptarget.WhenyoudragfilesorfoldersfromanExplorerwindowanddroptheminyourwindow,thewindowreceivesaWM_DROPFILESmessage.TheWPARAMofaWM_DROPFILESmessageisanHDROPthatlistswhatfilesarebeingdropped.TherearethreeAPIsyouusetogetthefilelistoutoftheHDROP:
DragQueryFile(),DragQueryPoint(),andDragFinish().
∙DragQueryFile()doestwothings:
returnsthenumberoffilesbeingdragged,andenumeratesthroughthelistoffiles.DragQueryPoint()returnstheptmemberoftheDROPFILESstruct.DragFinish()freesupmemoryallocatedduringthedraganddropprocess.
∙DragQueryFile()takesfourparameters:
TheHDROP,theindexofthefilenametoreturn,abufferallocatedbythecallertoholdthename,andthesizeofthebufferincharacters.Ifyoupass-1astheindex,DragQueryFile()returnsthenumberoffilesinthelist.Otherwise,itreturnsthenumberofcharactersinthefilename.Youcantestthisreturnagainst0totellifthecallsucceeded.
∙DragQueryPoint()takestwoparameters,theHDROPandapointertoaPOINTstructthatreceivesthevalueintheptmemberoftheDROPFILESstruct.DragFinish()justtakesoneparameter,theHDROP.
AtypicalWM_DROPFILEShandlerwouldlooklikethis:
voidCMyDlg:
:
OnDropFiles(HDROPhdrop)
{
UINTuNumFiles;
TCHARszNextFile[MAX_PATH];
//Getthe#offilesbeingdropped.
uNumFiles=DragQueryFile(hdrop,-1,NULL,0);
for(UINTuFile=0;uFile { //GetthenextfilenamefromtheHDROPinfo. if(DragQueryFile(hdrop,uFile,szNextFile,MAX_PATH)>0) { //*** //DowhateveryouwantwiththefilenameinszNextFile. //*** } } //Freeupmemory. DragFinish(hdrop); } DragQueryPoint()isn'tnecessaryifallyouwantisthelistoffiles.(Actually,I'veneverhadtouseitmyself.) Thenewway-usinganOLEdroptarget TheothermethodofacceptingdraganddropistoregisteryourwindowasanOLEdroptarget.Normally,doingsowouldrequirethatyouwriteaC++classthatimplementstheIDropTargetinterface.However,MFChasaCOleDropTargetclassthattakescareofthatforus.TheprocessisabitdifferentdependingonwhetheryourmainwindowisadialogoraCView,soI'llcoverbothbelow. MakingaCViewadroptarget CViewalreadyhassomedraganddropsupportbuilt-in,howeverit'snotnormallyactivated.Toactivateit,youaddaCOleDropTargetmembervariabletotheview,andthencallitsRegister()functioninyourview'sOnInitialUpdate()tomaketheviewadroptarget,asshownbelow: voidCMyView: : OnInitialUpdate() { CView: : OnInitialUpdate(); //Registerourviewasadroptarget. //m_droptargetisaCOleDropTargetmemberofCMyView. m_droptarget.Register(this); } Oncethat'sdone,youthenoverridefourvirtualfunctionsthatarecalledwhentheuserdragsoveryourview: ∙OnDragEnter(): Calledwhenthecursorentersyourwindow. ∙OnDragOver(): Calledwhenthecursormovesinsideyourwindow. ∙OnDragLeave(): Calledwhenthecursorleavesyourwindow. ∙OnDrop(): Calledwhentheuserdropsinyourwindow. OnDragEnter() OnDragEnter()isthefirstfunctioncalled.Itsprototypeis: DROPEFFECTCView: : OnDragEnter(COleDataObject*pDataObject, DWORDdwKeyState,CPointpoint); Theparametersare: ∙pDataObject: PointertoaCOleDataObjectthatcontainsthedatabeingdragged. ∙dwKeyState: Asetofflagsindicatingwhichmousebuttonisclickedandwhichshiftkeys(ifany)arepressed.TheflagsareMK_CONTROL,MK_SHIFT,MK_ALT,MK_LBUTTON,MK_MBUTTON,andMK_RBUTTON. ∙point: Thecursorposition,expressedintheview'sclientcoordinates. OnDragEnter()returnsaDROPEFFECTvalue,whichtellsOLEwhetherthedropwillbeaccepted,andifso,whatcursorshouldbedisplayed.Thevaluesandtheirmeaningsare: ∙DROPEFFECT_NONE: Thedropwillnotbeaccepted.Thecursorchangesto: . ∙DROPEFFECT_MOVE: Thedatawillbemovedbythedroptarget.Thecursorchangesto: . ∙DROPEFFECT_COPY: Thedatawillbecopiedbythedroptarget.Thecursorchangesto: ∙DROPEFFECT_LINK: Thedatawillbelinkedtobythedroptarget.Thecursorchangesto: . Normally,inOnDragEnter()youexaminethedatabeingdraggedandseeifitmeetsyourcriteria.Ifnot,youreturnDROPEFFECT_NONEtorejectthedraganddrop.Otherwise,youcanreturnoneoftheothervaluesdependingonwhatyouintendtodowiththedata. OnDragOver() IfyoureturnavalueotherthanDROPEFFECT_NONEfromOnDragEnter(),OnDragOver()iscalledwheneverthemousecursormoveswithinyourwindow.TheprototypeofOnDragOver()is: DROPEFFECTCView: : OnDragOver(COleDataObject*pDataObject, DWORDdwKeyState,CPointpoint); TheparametersandreturnvalueareidenticaltoOnDragEnter().OnDragOver()letsyoureturndifferentDROPEFFECTvaluesdependingonthecursorpositionandtheshiftkeystate.Forinstance,ifyourmainviewwindowhasseveralareas,displayingdifferentlistsofinformation,andyouwanttoallowdropsonlyinonepart,you'dcheckthecursorpositioninthepointparameter,andreturnDROPEFFECT_NONEifthecursorisnotinthatarea. Asfortheshiftkeys,younormallyreacttothemasdescribedbelow: ∙SHIFTpressed(MK_SHIFTindwKeyState): ReturnDROPEFFECT_MOVE. ∙CONTROLpressed(MK_CONTROL): ReturnDROPEFFECT_COPY. ∙Bothpressed(MK_SHIFT|MK_CONTROL): ReturnDROPEFFECT_LINK. Theseareonlyguidelines,althoughit'sbesttoadheretothem,sincetheyarewhatExploreruses.Butifsomeoftheactions(copy,move,orlink)doesn'tmakesenseforyourapp,youdon'thavetoreturnthecorrespondingDROPEFFECT.Forinstance,inMultiFiler(I'llgettoit,Ipromise! )OnDragOver()alwaysreturnsDROPEFFECT_COPY.Justbesuretoreturntherightvalue,sothatthecursoraccuratelyindicatestotheuser,whatwillhappenifhedropsinyourwindow. OnDragLeave() OnDragLeave()iscallediftheuserdragsoutofyourwindowwithoutdropping.Theprototypeis: voidCView: : OnDragLeave(); Ithasnoparametersor
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