Custom CStatusBar and double-buffering

[Bruce L]

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

[Joseph M. Newcomer]

See below...

****
Just note that calling DefWindowProc like this is essentially the same as calling
CStatusBar::OnPaint, if you follow the logic.

I would be inclined to use the virtual DrawItem method; see my essay on www.codeguru.com
(I posted it there a few months before they were bought out and lost their glitter);
there's also a link to this from my MVP Tips site (unless they moved the article)
joe
****

>}
>
>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

Joseph M. Newcomer [MVP]
email: newc...@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm

[Bruce L]

> email: newco...@flounder.com

> Web:http://www.flounder.com
> MVP Tips:http://www.flounder.com/mvp_tips.htm

Thanks for the reply Joe - and the pointer about my weird call to
OnPaint!

I posted this on CodeProject and got an alternative reply:

In order to use CMemDC.h in your controls you need to change the call
to 'FillSolidRectangle(...)' in CMemDC.h:

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

to this:

HBRUSH hbrBackGrnd = (HBRUSH)GetClassLong(pDC->GetWindow()-
>GetSafeHwnd(), GCL_HBRBACKGROUND);
::FillRect(GetSafeHdc(), &m_rect, hbrBackGrnd);

Hey Presto! the control borders now get repainted.