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Hi

I’m trying to create a child window (custom drawn control) on a window (which happens to the child of another). The parent window is filled with a custom gardient, when I create the child it is always filling in the rectangle in white. I am more than likely be silly here. Both windows are derived from a class which extends CWnd. It handles the WM_ERASEBKGRND & WM_PAINT messages. The WM_PAINT function is defined as follows:

void CSEWnd::OnPaint() {

CRect clientRect;

CPaintDC dcPaint(this);

GetClientRect(clientRect);

CExtMemoryDC dc( &dcPaint, &clientRect );

// Exclude the child areas from repaints

CExtPaintManager::stat_ExcludeChildAreas(

dcPaint.GetSafeHdc(),

GetSafeHwnd()

);

// Emulate erase background

DefWindowProc( WM_ERASEBKGND, WPARAM( dc.GetSafeHdc() ), LPARAM(0) );

// Store original colour & modes

int nOldBkMode = dc.GetBkMode();

COLORREF crOldTxtColour = dc.GetTextColor();

CExtMemoryDC dcsrc;

dcsrc.CreateCompatibleDC( &dc );

// Call the derived classes OnDraw function

OnDraw( dc, clientRect );

// Restore old colours

dc.SetTextColor( crOldTxtColour );

dc.SetBkMode(nOldBkMode);

}

Any ideas what I am doing wrong here?

Cheers

James Hughes

Dear James,


We believe that the WM_PAINT handler is not the best place to process WM_ERASEBKGND messages. To resolve the problem with an invalid background drawing, please modify the WindowProc method of your control (or alternatively you could modify the handlers of the WM_ERASEBKGND and WM_PAINT messages):

virtual LRESULT WindowProc(    
            UINT uMsg,
            WPARAM wParam,
            LPARAM lParam
            )
      {
            if( uMsg == WM_ERASEBKGND )
                  return (!0);
            
            if( uMsg == WM_PAINT )
            {
                  CRect rcClient;
                  GetClientRect( &rcClient );
                  CPaintDC dcPaint( this );
                  CExtMemoryDC dc( &dcPaint, &rcClient );
                  bool bTransparentBk = false;
                  if( g_PaintManager->GetCb2DbTransparentMode(this) )
                  {
                        // paint gradient backgroung
                        CExtPaintManager::stat_ExcludeChildAreas(
                             dc,
                             GetSafeHwnd(),
                             CExtPaintManager::stat_DefExcludeChildAreaCallback
                             );
                        if( g_PaintManager->PaintDockerBkgnd( dc, this ) )
                             bTransparentBk = true;
                  } // if( g_PaintManager->GetCb2DbTransparentMode(this) )
                  if( !bTransparentBk )
                        // paint solid backgroung
                        dc.FillSolidRect( &rcClient, g_PaintManager->GetColor( CExtPaintManager::CLR_3DFACE_OUT) );
                  
                  // place your painting code here
                  // .........
 
                  g_PaintManager->OnPaintSessionComplete( this );
                  return 0;
            }
            return CWnd::WindowProc(uMsg, wParam, lParam);
      }

All should work OK after that.