Implementing Reusable Drag '& Drop Classes

Wednesday May 23rd 2001 by Leon Finker
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A good starting point if you're looking to add drag and drop support to your app. The demo project has sample code for various clipboard formats.

IDataObject,IEnumFORMATETC,IDropSource,IDropTarget,IDragSourceHelper,IDropTargetHelper

Tools used: Microsoft. Platform SDK Whistler Beta 1 and VC++ 6.0 (SP5), WTL
Platform used: Windows 2000

I tried to create as generic Drag and Drop classes as possible. Here is what I came up with. I think it's a good starting point if you're looking to add drag and drop support to your app. The demo project has sample code for various clipboard formats:
  • CF_TEXT
  • CF_HDROP
  • CF_BITMAP
  • CF_DIB
  • CF_ENHMETAFILE
and MEDIUMs:
  • TYMED_HGLOBAL
  • TYMED_ISTREAM
  • TYMED_ENHMF
  • TYMED_GDI

Some screenshots of image drag and drop from static window:

Fig. 1. Dragging from static window to WordPad

Fig. 2. Using Clipboard

Fig. 3. Pasting the above clipboard contents into WordPad


Usage:
To enable your window as a DropTarget.

  • Derive a class from CIDropTarget.
  • Override OnDrop. Return true or false from this method. If true, base class will free the medium. If false, it won't free the medium.
  • Call ::RegisterDragDrop for your window.
  • Add Supported formats by calling CIDropTarget::AddSuportedFormat.
  • Optionally override other methods such as DragOver and DragLeave. I used it for the tree to highlight the current item.

Example:

class CTreeDropTarget : public CIDropTarget { public: virtual bool OnDrop(FORMATETC* pFmtEtc, STGMEDIUM& medium, DWORD *pdwEffect) { if(pFmtEtc->cfFormat == CF_TEXT && medium.tymed == TYMED_HGLOBAL) { // Handle it } return true; } // etc...

};

In your Window derived class create a member of CTreeDropTarget. Then initialize it like this:

{
  // ...
  m_pDropTarget = new CTreeDropTarget(m_hWnd);
  RegisterDragDrop(m_hWnd,m_pDropTarget);
  // create the supported formats:
  FORMATETC ftetc = {0}; 
  ftetc.cfFormat  = CF_TEXT; 
  ftetc.dwAspect  = DVASPECT_CONTENT; 
  ftetc.lindex    = -1; 
  ftetc.tymed     = TYMED_HGLOBAL; 
  m_pDropTarget->AddSuportedFormat(ftetc); 
  ftetc.cfFormat  = CF_HDROP; 
  m_pDropTarget->AddSuportedFormat(ftetc);
  //...
}

That's all for drop target.

To enable your window as the Drag and Drop source:

  • Catch the Windows message that initiates the drag and drop such as TVN_BEGINDRAG.
  • In the message function handler create new CIDataObject and CIDropSource.
  • Create the clipboard formats and medium for those formats.
  • Call SetData to add the clipboard formats and medium to DataObject. Second parameter to SetData indicates if DataObject should take the ownership of medium or not. If set to TRUE, then DataObject takes the ownership of your medium, you don't need to free it. Otherwise, it will make a copy of your medium without releasing the one you gave it.

Example:

LRESULT OnBegindrag(...)
{
   CIDropSource* pdsrc = new CIDropSource;
   CIDataObject* pdobj = new CIDataObject(pdsrc);
   // Init the supported format
   FORMATETC fmtetc = {0}; 
   fmtetc.cfFormat  = CF_TEXT; 
   fmtetc.dwAspect  = DVASPECT_CONTENT; 
   fmtetc.lindex    = -1; 
   fmtetc.tymed     = TYMED_HGLOBAL;
   // Init the medium used
   STGMEDIUM medium = {0};
   medium.tymed     = TYMED_HGLOBAL;
 
   // medium.hGlobal = init to something
   // Add it to DataObject
   pdobj->SetData(&fmtetc,&medium,TRUE);   
// Release the medium for me   // add more formats and medium if needed // Initiate the Drag & Drop ::DoDragDrop(pdobj, pdsrc, DROPEFFECT_COPY, &dwEffect); }

To use the shell's drag image manager (comes with Windows 2000):

You don't need to add the support for it if you are acting as drop target. It is encapsulated in CIDropTarget class. If you're acting as data source:

  • Create an instance of CDragSourceHelper before calling ::DoDragDrop.
  • Call CDragSourceHelper::InitializeFromWindow or CDragSourceHelper::InitializeFromBitmap.

Adding the Copy/Paste through clipboard is not much work either.

Example:

LRESULT OnContextMenu(...)
{
   // ...
   CIDataObject* pdobj = new CIDataObject(NULL);  
   // Init FORMATETC and STGMEDIUM just like before
   // Add the format and medium to Dataobject>
   pdobj->SetData(&fmtetc,&medium,TRUE);
   // Add data to clipboard
   OleSetClipboard(pdobj);
 
   OleFlushClipboard();  // render the data on clipboard, 
                        // so it's available even if we 
                        // close the app
   // ...
}

References:

Downloads

Download demo project - 76 Kb
Download source - 6 Kb
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