How to add OLE drag-and-drop functionality to a pre-existing MFC application (135299)

MORE INFORMATION

To enable OLE for a pre-existing MFC application, you must initialize the OLE DLLs. This is done in the InitInstance() of your application object by calling AfxOleInit().

The LSTDRG sample was first created with no OLE support using AppWizard. The code that calls AfxOleInit() was then copied from InitInstance() of the OCLIENT MFC sample application that is supplied with the above mentioned products.

OLE drag-and-drop functionality was enabled for each window by following the procedure outlined in the MFC version 2.5 OLE 2 Classes documentation. Sections of interest are:

• "drag-and-drop: Implementing a Drop Source"
• "drag-and-drop: Implementing a Drop Target"
• "Data Objects and Data Sources: Creation and Destruction"
• "Data Objects and Data Sources: Manipulation"

You will also want to review the MFC Classes COleDataSource and COleDropTarget.

To turn a CWnd-derived object into a drop source, you must instantiate a COleDataSource object in your CWnd-derived object. You can then call COleDataSource::CacheGlobalData() to cache the data (text) that you are going to drag. Then call COleDataSource::DoDragDrop() to actually initiate the drag drop. The return value from DoDragDrop() gives you the result of the drag/drop operation. The DROPEFFECT return value from DoDragDrop() is a bit field, so you have to test specific bits of the return value to determine the drag/drop results.

Turning a CWnd-derived object into a drop target is a bit more complicated. You must derive your own class from COleDropTarget, and instantiate a member of your COleDropTarget-derived object in the CWnd-derived class. In your COleDropTarget object you have to, at a minimum, override these four member functions: You must also initialize the COleDropTarget derived object as a drop target with the OLE DLLs by calling COleDropTarget::Register(). This would normally be done in the OnCreate member function of a CWnd-derived object (see the CDDWnd::OnCreate method in LSTDRG). A problem in dialog template created classes (CListBox, CEdit, and so on created by AppStudio) is that OnCreate is not called. This is because control windows associated with the dialog template are created during the dialog creation process before their actual CWnd-derived C++ objects are created. These windows are then attached by using Attached() to the C++ CWnd-derived objects once they are created.

To work around this, an Initialize() method was created for the CListBox and CEdit-derived classes where COleDropTarget::Register() is called. This Initialize() method is then called for each template-created control in the CFormView-derived classes OnInitialUpdate() method (see CListdragView::OnInitialUpdate() and CDDListBox::Initialize()).

Another interesting aspect of the LSTDRG application is the use of a burning trashcan animation when text is dropped on a window. This animation uses a single bitmap composed of multiple 'cells' or images. The use of a single bitmap as opposed to multiple bitmaps conserves GDI resources and is more efficient to paint. It also turns off the mouse pointer (cursor) by loading an empty pointer during painting to eliminate pointer flicker on Windows version 3.x computers.