Re: Why is OnPaint getting called while the redraw flag of the window is FALSE?

From:

"Mark Salsbery [MVP]" <MarkSalsbery[MVP]@newsgroup.nospam>

Newsgroups:

microsoft.public.vc.mfc

Date:

Thu, 31 Jan 2008 10:01:23 -0800

Message-ID:

<701B0ABA-113C-42DC-85A1-7018DA5C8ADA@microsoft.com>

I'm not sure how WM_SETREDRAW is handled by the system. It may not prevent
WM_PAINT messages from being sent, but instead, window classes that support
the message may just set an internal flag and ignore WM_PAINT messages when
the flag is set.

You may want to do something similar since you're doing your own painting -
add a redraw flag and only do painting in response to WM_PAINT if the flag
is not set.

Mark

--
Mark Salsbery
Microsoft MVP - Visual C++

"Timo Partanen" <timo.partanen@motivesys.com> wrote in message
news:ODKdcB$YIHA.984@TK2MSFTNGP06.phx.gbl...

Hi,

I have a list control that derives from CListCtrl. I noticed that my
OnPaint (i.e. the system sends a WM_PAINT message) is getting called while
the redraw flag of the window is FALSE. This temporarily results in a
black display for the window. When the system sends another WM_PAINT for
the window, the display starts looking OK again as the redraw flag of the
window is TRUE. This sequence results in a flickering effect that I would
definitely want to avoid. Is it okay that OnPaint is getting called while
the redraw flag of the window is FALSE? If yes, what should WM_PAINT
processing do in that case?

My OnPaint handler:
---
CPaintDC dc( this ); // Device context for painting.

// Get clip and client rectangle.
CRect rectClient;
CRect rectClip;
dc.GetClipBox( &rectClip );
GetClientRect( &rectClient );

// Create a compatible memory DC.
dcMem.CreateCompatibleDC( &dc );
CBitmap bitmap;
bitmap.CreateCompatibleBitmap( &dc,
ectClient.Width(), rectClient.Height() );
CBitmap* pbitmapOld = dcMem.SelectObject( &bitmap );

// Let the control do its default drawing.
CWnd::DefWindowProc( WM_PAINT, ( WPARAM )dcMem.m_hDC, 0 );

// Here I do my own drawing.
// .
// .
// .
// My own drawing ends.

// Bit-block-transfer the contents of the memory DC to the original DC.
dc.BitBlt( rectClip.left, rectClip.top,
rectClip.Width(), rectClip.Height(),
&dcMem, rectClip.left, rectClip.top, SRCCOPY );
---

If I change my code to be as below, the problem disappears. Can someone
say or guess why?

---
CPaintDC dc( this ); // Device context for painting.

// Let the control do its default drawing.
CWnd::DefWindowProc( WM_PAINT, ( WPARAM )dcMem.m_hDC, 0 );

// Here I do my own drawing.
// .
// .
// .
// My own drawing ends.
---

One thing that might affect is that the list-view control has the
LVS_EX_DOUBLEBUFFER extended style.

Thanks in advance.

Timo Partanen