Re: Custom CStatusBar and double-buffering

Hi,

Wondering if anyone has any good ideas here...

I have written a custom CStatusBar control which basically updates
some panes in color as the mouse cursor is moved around. The panes
were flickering due to the amount of redrawing required so I added
some code that uses double-buffering to draw the panes off-screen
first, then blits to screen. I used a well-known piece of code from
Keith Rule on CodeProject in the CMemDC class for this (http://
www.codeproject.com/KB/GDI/flickerfree.aspx?msg=2531592#xx2531592xx).
Here's the code:

<code>
class CMemDC : public CDC {
private:
   CBitmap m_bitmap; // Offscreen bitmap
   CBitmap* m_oldBitmap; // bitmap originally found in CMemDC
   CDC* m_pDC; // Saves CDC passed in constructor
   CRect m_rect; // Rectangle of drawing area.
   BOOL m_bMemDC; // TRUE if CDC really is a Memory DC.
public:

   CMemDC(CDC* pDC, const CRect* pRect = NULL) : CDC()
   {
           ASSERT(pDC != NULL);

           // Some initialization
           m_pDC = pDC;
           m_oldBitmap = NULL;
           m_bMemDC = !pDC->IsPrinting();

           // Get the rectangle to draw
           if (pRect == NULL) {
                   pDC->GetClipBox(&m_rect);
           } else {
                   m_rect = *pRect;
           }

           if (m_bMemDC) {
                   // Create a Memory DC
                   CreateCompatibleDC(pDC);
                   pDC->LPtoDP(&m_rect);

                   m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(),
m_rect.Height());
                   m_oldBitmap = SelectObject(&m_bitmap);

                   SetMapMode(pDC->GetMapMode());

                   SetWindowExt(pDC->GetWindowExt());
                   SetViewportExt(pDC->GetViewportExt());

                   pDC->DPtoLP(&m_rect);
                   SetWindowOrg(m_rect.left, m_rect.top);
           } else {
                   // Make a copy of the relevent parts of the current DC for printing
                   m_bPrinting = pDC->m_bPrinting;
                   m_hDC = pDC->m_hDC;
                   m_hAttribDC = pDC->m_hAttribDC;
           }

           // Fill background
           FillSolidRect(m_rect, pDC->GetBkColor());
   }

   ~CMemDC()
   {
           if (m_bMemDC) {
                   // Copy the offscreen bitmap onto the screen.
                   m_pDC->BitBlt(m_rect.left, m_rect.top, m_rect.Width(),
m_rect.Height(),
                           this, m_rect.left, m_rect.top, SRCCOPY);

                   //Swap back the original bitmap.
                   SelectObject(m_oldBitmap);
           } else {
                   // All we need to do is replace the DC with an illegal value,
                   // this keeps us from accidently deleting the handles associated
with
                   // the CDC that was passed to the constructor.
                   m_hDC = m_hAttribDC = NULL;
           }
   }

   // Allow usage as a pointer
   CMemDC* operator->()
   {
           return this;
   }

   // Allow usage as a pointer
   operator CMemDC*()
   {
           return this;
   }
};
</code>

Applying this code requires overriding the OnPaint & OnEraseBkgnd
methods in the custom CStatusBar as follows:

<code>
void MyStatusBar::OnPaint()
{
   CPaintDC dc(this); // device context for painting
   // TODO: Add your message handler code here
   // Do not call CStatusBar::OnPaint() for painting messages
   CRect rect;
   GetClientRect(&rect);
   CMemDC memDC(&dc, &rect);

   DefWindowProc(WM_PAINT, (WPARAM)memDC->m_hDC, (LPARAM)0);

}

BOOL MyStatusBar::OnEraseBkgnd(CDC* pDC)
{
   // TODO: Add your message handler code here and/or call default
   //return CStatusBar::OnEraseBkgnd(pDC);
   return TRUE;

}
</code>

This reduces the flickering, but now the pane borders don't get drawn
at all - they just appear as blank white areas. I have tried
overriding the OnNcPaint function to avoid the call to
CControlBar::EraseNonClient(). No effect. I have also tried calling
DrawBorders explicitly with likewise no effect.

Does anyone have any idea what I'm missing here?

Any suggestions would be much appreciated.

Best,
Bruce