Message Map of Controls in CDialog (DoModal)

  CDialog::DoModal() ----> CWnd::RunModalLoop()---->CWinThread::PumpMessage()---->CWinThread::PreTranslateMessage()---->CWnd::WalkPreTranslateTree()

 

The Key is the red code

BOOL CWinThread::PreTranslateMessage(MSG* pMsg)
{
 ASSERT_VALID(this);

 // if this is a thread-message, short-circuit this function
 if (pMsg->hwnd == NULL && DispatchThreadMessageEx(pMsg))
  return TRUE;

 // walk from target to main window
 CWnd* pMainWnd = AfxGetMainWnd();
 if (CWnd::WalkPreTranslateTree(pMainWnd->GetSafeHwnd(), pMsg))
  return TRUE;

 // in case of modeless dialogs, last chance route through main
 //   window's accelerator table
 if (pMainWnd != NULL)
 {
   CWnd* pWnd = CWnd::FromHandle(pMsg->hwnd);
   if (pWnd->GetTopLevelParent() != pMainWnd)
   return pMainWnd->PreTranslateMessage(pMsg);
 }

 return FALSE;   // no special processing
}

BOOL PASCAL CWnd::WalkPreTranslateTree(HWND hWndStop, MSG* pMsg)
{
 ASSERT(hWndStop == NULL || ::IsWindow(hWndStop));
 ASSERT(pMsg != NULL);

 // walk from the target window up to the hWndStop window checking
 //  if any window wants to translate this message

 for (HWND hWnd = pMsg->hwnd; hWnd != NULL; hWnd = ::GetParent(hWnd))
 {
  CWnd* pWnd = CWnd::FromHandlePermanent(hWnd);
  if (pWnd != NULL)
  {
   // target window is a C++ window
   if (pWnd->PreTranslateMessage(pMsg))
    return TRUE; // trapped by target window (eg: accelerators)
  }

  // got to hWndStop window without interest
  if (hWnd == hWndStop)
   break;
 }
 return FALSE;       // no special processing
}