个人想法：因为不太清楚你要求，比如你在空白处绘制了东西上去，其他浮动窗口移动的时候呢？是否需要重新获取有效空白重新绘制呢?个人想法，未经过实践。
1.GetClientRect()获取整个窗口的客户区。然后获取其他窗口和BAR的区域，把窗口和BAR的区域去除，剩下的就是你所需要的空白处。
2.OnPaint()
void CMainFrame::OnPaint()
{
//CPaintDC dc(this); // device context for painting
// TODO: Add your message handler code here
// Do not call CMDIFrameWndEx::OnPaint() for painting messages
CMDIFrameWnd::OnPaint();
CRect rc;
CDC dc;
dc.m_hDC = ::GetDC(this->m_hWndMDIClient);
CBrush br(RGB(100, 0, 100) );
dc.SelectObject(&br);
this->GetClientRect(&rc); //这里需要算你要显示的区域
dc.PatBlt(rc.left-50,rc.top-40,rc.Width()-200,rc.Height()-200,PATCOPY);
ReleaseDC(&dc);
}
3.在PreTranslateMessage中添加下列内容
BOOL CMainFrame::PreTranslateMessage(MSG* pMsg)
{
// TODO: Add your specialized code here and/or call the base class
if( (pMsg->hwnd == m_hWndMDIClient) && (pMsg->message == WM_PAINT) )
PostMessage(WM_PAINT);
return CMDIFrameWndEx::PreTranslateMessage(pMsg);
}
