Implementing Reusable Drag ‘& Drop Classes

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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