Implementing a Drop Target

This article is part of the drag and drop interface samples.

1. Serializing ASCII Data
2. Modeless child dialog
3. Modeless sibling dialog
4. The drag source
5. The MFC drop target
6. The TBTextTarget class

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Data Drag is Coming to Town... The Drop Target

You imagine? Right, there's more than one possibility, this time I'll give you three. One for your private data, one for the common data in order to receive data from other applications like Excel, WinWord, etc. and at last, I'll give you a handy-dandy class you can derive ANY MFC object from, to make it a drop target.

Getting the Data from the Dropped Object, the MFC Way

I assume you want to drag the contents of some rows of a CListControl of your CFormView into one of the modal dialogs mentioned earlier in this article series. In real-life, you can use this approach to implement the drop functions to any CWnd-derived object, whether in the modal dialog or not.

The first steps are the same as done when implementing the drop source:

1. To your drop target class, e.g., my CDialog based class CDropDialog, add a member variable of type UINT:

   UINT m_DragDropFormat;

2. In the constructor of the view, initialize it for private data exchange with:

   m_DragDropFormat = 
       ::RegisterClipboardFormat("YourInterfaceClipboardFormat");

   and for the common format, just use:

   m_DragDorpFormat = CF_TEXT;

3. Add the following lines to stdafx.h:

      #include <afxdisp.h>
      #include <afxole.h>

4. In your application's InitInstance function, make a call to AfxOleInit() before creating the document template.
5. If the window used as drop target is CView-derived, you are now in luck, most of it is at the hand. In the OnInitialUpdate function, call the Register() function and overwriting the functions OnDragOver and OnDrop (implementation same as CInterfaceDropTarget in step 9) should be enough in "normal" cases - READY!
6. If you have another kind of target, there's some more work.
7. Use the ClassWizard to create a new generic(!) class derived from the MFC(!) class COleDropTarget. I call it CInterfaceDropTarget.
8. Create and initialize a UINT m_DragDropFormat as in step 2.
9. Create - at least - the functions OnDragOver and OnDrop.

   DROPEFFECT CInterfaceDropTarget::OnDragOver (CWnd *pDrop, 
       COleDataObject* pDataObject, DWORD dwKeyState, CPoint point )
   {
       CFile *pFile = pDataObject->GetFileData(m_DragDropFormat);
       if (pFile != NULL)
           // perhaps some point checking here?
           return DROPEFFECT_COPY; // data fits
       else
           return DROPEFFECT_NONE; // data won't fit
   }
   
   BOOL CInterfaceDropTarget::OnDrop(CWnd *pDrop, 
       COleDataObject* pDataObject, DROPEFFECT dropEffect, CPoint point )
   {
       CFile *pFile = pDataObject->GetFileData(m_DragDropFormat);
       if (pFile != NULL)
       {
           int number, colCou;
           CString Data;
           // perhaps some point checking first?
           TRY
           {
               CArchive ar(&file, CArchive::load);
               TRY
               {
                   if (DragDropFormat != CF_TEXT)
                   {
                       // "Serialize" your data object from the archive
                       // (yes, you may use YourObject.Serialize(ar) here!)
                       ar >> number;
                       ar >> colCou;
   
                       for (int i=0; i<colCou; i++)
                       {
                           ar >> Data ;
   #pragma message(__FILE__ " dont't forget to process Data here.....")
                       }
                   }
                   else
                   {
                       CString Data, t;
   
                       TRY
                       {
                           while(ar.ReadString(t))
                               Data += t + "\n";
                       }
   #pragma message(__FILE__ " ... and here!")
                       CATCH_ALL(eInnerMost)
                       {
   #ifdef _DEBUG
                           TCHAR szCause[255];
                           CString strFormatted;
                           eInnerMost->GetErrorMessage(szCause, 255);
                           strFormatted = _T("Exception: ");
                           strFormatted += szCause;
                           AfxMessageBox(strFormatted); 
   #endif //_DEBUG
                           ASSERT(FALSE);
                       }
                       END_CATCH_ALL;
                   }
                   ar.Close();
               }
               CATCH_ALL(eInner)
               {
                   // exception while reading
                   // from or closing the archive
                   ASSERT(FALSE);
               }
               END_CATCH_ALL;
           }
           CATCH_ALL(eOuter)
           {
               // exception in the destructor of ar
               ASSERT(FALSE);
           }
           END_CATCH_ALL;
           return TRUE;
       }
       return COleDropTarget::OnDrop(pDrop, 
               pDataObject, dropEffect, point);
   }

10. Don't forget to process the data in the OnDrop function. You should send the just-dropped-object to your CWnd-class and "do something" with it there. The example does this with the functions ProcessData(CString data) and ProcessData(int rowcount, int linecount, CString Data).
11. In the CWnd-derived class used as drop target, add a member variable of type CInterfaceDropTarget, for example's sake, name it m_DT.
12. At any point where this window exists (for example: OnInitDialog is great), call the Register function of m_DT:

    BOOL CDropDialog::OnInitDialog() 
    {
        CDialog::OnInitDialog();
       m_DT.Register(this);
       // the other stuff follows ....
    }

Done!

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