Using MFC OLE Drag & Drop to Drag Text Between Windows

ID: Q135299

The information in this article applies to:

The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.50, 1.51, 1.52, 1.52b
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 5.0, 6.0

SUMMARY

In a Microsoft Foundation Classes (MFC) application, you may want to enable the user to drag text between various windows, including those in CWnd-derived objects as well as standard control windows in CListBox and CEdit-derived objects.

A sample (LSTDRG) is available that demonstrates how to add OLE drag and drop functionality to a pre-existing MFC application. This sample demonstrates using OLE drag and drop functionality to drag text between two list boxes, an edit control, and a CWnd-derived object's window. You can also use any other application with OLE text drag and drop functionality enabled as a drag source or a drop target (Word for Windows 6.x is one such application).

The following files are available for download from the Microsoft Download Center. Click the file names below to download the files:

Lstdrg.exe

For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

http://www.microsoft.com/downloads/search.asp

and then click How to use the Microsoft Download Center.

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:

COleDropTarget::OnDragEnter()
COleDropTarget::OnDragLeave()
COleDropTarget::OnDragOver()
COleDropTarget::OnDrop()

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.

REFERENCES

"MFC 2.5 OLE 2 Classes" reference book, available in the Books Online of Visual C++ version 1.5. See sections on drag and drop beginning with the section titled "Drag and Drop." Also review the COleDataSource class and the COleDataTarget class in the same reference book.

For good background information on how OLE handles drag and drop, please see "Inside OLE 2" by Kraig Brockschmidt, published by Microsoft Press.

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Keywords : kbfile kbole kbprg kbsample kbMFC kbVC
Version : winnt:2.0,2.1,2.2,4.0,4.1,4.2,5.0,6.0
Platform : winnt
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