MFC使view(视图)透明

MFC SDI: How do I create transparent SDI views?

---

Q: How do I create transparent SDI views?

A: One can make a the view of an MFC application transparent by handling 'CView::OnEraseBkgnd()' function and returning 'FALSE' instead of the default 'CView::OnEraseBkgnd()' call.

Code:

BOOL CTransparentView::OnEraseBkgnd(CDC* pDC) 
{
  // TODO: Add your message handler code here and/or call default
  return FALSE;
//  return CView::OnEraseBkgnd(pDC);
}

But this will not give a real transparent window, since the original view background will be dragged along while moving the application window. To overcome this, I have handled 'OnMove()' and 'OnSize()' functions of 'CMainFrame' class to update the view to reflect the real background, by repainting the view each time the application window is moved / sized.

Code:

void CMainFrame::OnMove(int x, int y) 
{
  CFrameWnd::OnMove(x, y);
  
  // TODO: Add your message handler code here
  RepaintView();
}

void CMainFrame::OnSize(UINT nType, int cx, int cy) 
{
  CFrameWnd::OnSize(nType, cx, cy);
  
  // TODO: Add your message handler code here
  RepaintView();
}

I have defined a function 'RepaintWindow()' as part of the 'CMainFrame' class and implemented to repaint the view with correct background information.

Code:

void CMainFrame::RepaintView()
{
  // Gets current view
  CView* pView = GetActiveView();
  if (pView)    // Valid view
  {
    // Hides view to get background
    ShowWindow(SW_HIDE);
    // Gets desktop device context
    HDC hDC = ::GetDC(NULL);
    if (hDC)  // Valid device context handle
    {
      // Get clients rectangle in screen coordinates
      CRect Rect;
      pView->GetClientRect(Rect);
      pView->ClientToScreen(Rect);

      // Gets device context for current view
      CClientDC DC(pView);

      // Pastes background to view
      ::BitBlt(DC.m_hDC, 0, 0, Rect.Width(), Rect.Height(),
        hDC, Rect.left, Rect.top, SRCCOPY);

      // Releases desktop device context
      ::ReleaseDC(NULL, hDC);
    }

    // Shows view with right background info
    ShowWindow(SW_SHOW);
  }
}

At present, this is implemented for SDI application. With little modification (for updating all views), this methodology can be extended for MDI applications as well.