INFO: Making Transparent Controls with MFC

• Microsoft Visual C++, 32-bit Editions, versions 4.2, 5.0

SUMMARY

The OLE Control '96 Specification defines transparent controls and Visual C++ 4.2 will create controls that conform to this new specification. However, most available containers, including Internet Explorer 3.0, do not support this feature. This article demonstrates the process required to convert a standard Visual C++ ControlWizard control to be transparent in Internet Explorer 3.0.

MORE INFORMATION

Steps to Create a Transparent OLE Control

1. Create a default control using the ControlWizard.

2. Override PreCreateWindow for the control as shown below. You will need to add the prototype and handler manually because ClassWizard does not provide an override for PreCreateWindow in the case of controls:

         BOOL CTransCtrl::PreCreateWindow(CREATESTRUCT& cs)
         {
            // Add the Transparent style to the control
            cs.dwExStyle |= WS_EX_TRANSPARENT;
   
            return COleControl::PreCreateWindow(cs);
         }
   
   

3. To get the control to show what is behind it, use the “trick” shown below, which sends a WM_ERASEBKGND message to the parent with the handle to the control's device context. Use ClassWizard to add a handler for WM_ERASEBKGND and add the following code:

         BOOL CTransCtrl::OnEraseBkgnd(CDC* pDC)
         {
            // This is needed for transparency and the correct drawing...
            CWnd*  pWndParent;       // handle of our parent window
            POINT  pt;
   
            pWndParent = GetParent();
            pt.x       = 0;
            pt.y       = 0;
   
            MapWindowPoints(pWndParent, &pt, 1);
            OffsetWindowOrgEx(pDC->m_hDC, pt.x, pt.y, &pt);
            ::SendMessage(pWndParent->m_hWnd, WM_ERASEBKGND,
                         (WPARAM)pDC->m_hDC, 0);
            SetWindowOrgEx(pDC->m_hDC, pt.x, pt.y, NULL);
   
            return 1;
         }
   
   

4. Modify the OnDraw() function to display the desired image for the control. The example below draws an ellipse. If you do not select the NULL_BRUSH before drawing, the inside of the ellipse is filled in using the current brush.

   NOTE: If you do not select your own pens and brushes, the resulting behavior is unpredictable and depends on the GDI objects passed by the control container.

         void CTransCtrl::OnDraw(CDC* pdc, const CRect& rcBounds, const
                                 CRect& rcInvalid)
         {
            CPen* oldpen;  CBrush* oldbrush;
   
            oldpen = (CPen*) pdc->SelectStockObject(BLACK_PEN);
            oldbrush = (CBrush*) pdc->SelectStockObject(NULL_BRUSH);
   
            pdc->Ellipse(rcBounds);
   
            pdc->SelectObject(oldpen);
            pdc->SelectObject(oldbrush);
         }
   
   

5. Override the OnSetClientSite() function to set the m_bAutoClip member variable to TRUE. The COleControl::m_bAutoClip variable is an undocumented variable that is used to maintain a window that clips the contents of the control to its design-time size:

         void CTransCtrl::OnSetClientSite()
         {
            m_bAutoClip = TRUE;
   
            COleControl::OnSetClientSite();
         }
   

Keywords          : MfcOLE kbgraphic kbtool kbinf
Technology        : kbole kbMfc
Version           : Winnet:4.2,5.0
Platform          : NT WINDOWS