HOWTO: Draw ActiveX Controls with Child Controls in Design Time

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SUMMARY

This article applies to ActiveX controls created with the MFC AppWizard, and describes how to draw an ActiveX control, that contains child controls, in design time.

MORE INFORMATION

For child windows of an ActiveX control to be drawn, the HWND of the ActiveX control must be created. During design time, it is not necessary to fully instantiate an ActiveX control in order to draw it. As a result, the HWND of the ActiveX control isn't created, and therefore the child windows aren't created. Without the child windows actually being instantiated, there isn't a way to get a design-time representation for them.

The way a container draws its controls in this case is to QueryInterface() the ActiveX control for its IViewObject interface, and call the IViewObject::Draw() method, which in turn calls the control's OnDraw() member function passing in a metafile HDC. Thus, even if the child controls had been created, they still would not be drawn because the framework does not pass a WM_PAINT message to the child controls.

To work around this problem, two things must be done:

1. Call RecreateControlWindow() in the OnDraw() member function of the ActiveX control when m_hWnd is NULL. If m_hWnd is not NULL, then the control already has a window object associated with it and RecreateControlWindow will not be called.
   
   RecreateControlWindow() creates the ActiveX controls window as a child of the m_pWndPark window. Note that m_pWndPark is a member variable of the _AFX_THREAD_STATE structure. If m_pWndPark is NULL then the desktop becomes the parent of the control. This is not what we want, therefore before RecreateControlWindow() is called, it is necessary to set the m_pWndPark variable to the window associated with the device context passed to OnDraw().



2. Call the helper function PaintChildWindows() from OnDraw(). This function draws the individual child controls of the ActiveX control to the device context.
   
   NOTE: The paint code below uses the BackColor and ForeColor stock properties for erasing the background. Alternatively, you could use the ambient properties. If you use this code and your control does not implement the BackColor or ForeColor stock properties, the control will not paint properly.

AmbientUserMode

// Override of AmbientUserMode, which will tell us whether if the UserMode
// ambient property is even implemented in the container. 
bool CFormControl::AmbientUserMode(bool *bUserMode, bool bInit)
{
   ASSERT (bUserMode);
   if (!GetAmbientProperty(DISPID_AMBIENT_USERMODE, 
                           VT_BOOL,
                           bUserMode) && bInit)
   {
      *bUserMode = true;
      return false;
   }else{
      return true;
   }
} 

PaintChildWindows()

void CFormControl::PaintChildWindows(HWND hWndParent, CDC* pDC,
                                     CPoint ptOffset)
{
    for (HWND hWndChild = ::GetTopWindow(hWndParent);
         hWndChild != NULL;
         hWndChild = ::GetNextWindow(hWndChild, GW_HWNDNEXT))
    {
        CRect rect;
        ::GetWindowRect(hWndChild, rect);  // wnd rect in screen coords
        ScreenToClient(&rect);             // relative to this view

        HDC hdcOut = pDC->m_hDC;

#ifdef _DEBUG
        CPoint pt = pDC->GetWindowOrg();
        ASSERT(pt.x == 0 && pt.y == 0);
#endif

        DWORD dwStyle = ::GetWindowLong(hWndChild, GWL_STYLE);
        if (dwStyle & (WS_HSCROLL|WS_VSCROLL))
        {
            TRACE("Warning: printing control with scrollbars not
                   supported\n");
        }
        if (dwStyle & WS_BORDER)
        {
            // The only case we special case--manually drawn border.
            ::Rectangle(hdcOut, ptOffset.x + rect.left,
                ptOffset.y + rect.top, ptOffset.x + rect.right,
                ptOffset.y + rect.bottom);
            rect.InflateRect(-1,-1);        // One logical pixel.
        }

        pDC->SaveDC();
        {
            CPoint pt(ptOffset.x + rect.left, ptOffset.y + rect.top);
            pDC->LPtoDP(&pt);
            pDC->OffsetViewportOrg(pt.x, pt.y);
            // Set the viewport origin so that the window origin
            // can be changed by the control. Draw it using a 
            // non-virtual HDC.
            // Error dialog in Excel95 shows up after this call when
            // used on NT4SP3
            ::SendMessage(hWndChild, WM_PAINT, (WPARAM)hdcOut, 0L);
        }
        pDC->RestoreDC(-1);

        if (::GetTopWindow(hWndChild) != NULL)
            PaintChildWindows(hWndChild, pDC, ptOffset);
    }
} 

OnDraw()

void CFormControl::OnDraw(CDC* pdc, const CRect& rcBounds,
                          const CRect& rcInvalid)
{
    // If in design mode.
    bool bUserMode;
    if((AmbientUserMode(&bUserMode) && !bUserMode) || !m_hWnd) {
    {
        // This drawing code creates the window and has it draw its
        // child windows into it.
        _AFX_THREAD_STATE* pThreadState = AfxGetThreadState();
        ASSERT(pThreadState != NULL);
        pThreadState->m_pWndPark = new CWnd;
        pThreadState->m_pWndPark->m_hWnd = pdc->GetWindow()->GetSafeHwnd();
        RecreateControlWindow();
    }

    CBrush* pOldBrush;
    CBrush bkBrush;
    bkBrush.CreateSolidBrush(TranslateColor(GetBackColor()));

    // Set the ForeColor property color and transparent background mode
    // into the device context.
    pdc->SetTextColor(TranslateColor(GetForeColor()));
    pOldBrush = pdc->SelectObject(&bkBrush);
    pdc->SetBkMode(TRANSPARENT);

    // Paint the background using the BackColor property.
    pdc->FillRect(rcBounds, &bkBrush);

    CPen pen(PS_SOLID, 1, RGB(0,0,0));  // Solid black pen.
    CPen* pOldPen = pdc->SelectObject(&pen);

    PaintChildWindows(m_hWnd, pdc, CPoint(rcBounds.left, rcBounds.top));
    ASSERT(pdc->GetWindowOrg() == CPoint(0,0));
    pdc->SelectObject(pOldPen);
} 

REFERENCES

For additional information about this implementation, please see the following articles in the Microsoft Knowledge Base:

Q127192 HOWTO: Draw Controls in an OLE Metafile

Q143299 SAMPLE: CFormView-Based Server Shows Metafile When Not Active

Additional query words:

Keywords : kbole kbActiveX kbCtrl kbMFC kbVC500 kbVC600
Version : winnt:5.0,6.0
Platform : winnt
Issue type : kbhowto