Re: Multithread and Dialog

From:
Scot T Brennecke <ScotB@Spamhater.MVPs.org>
Newsgroups:
microsoft.public.vc.mfc
Date:
Sun, 13 Sep 2009 13:58:01 -0500
Message-ID:
<O#kTfQKNKHA.3412@TK2MSFTNGP04.phx.gbl>
Kilo wrote:

When I press the OK button of the main dialog (CMainDialog), it will
create another dialog (CMyDialog) and a worker thread. Just before the
worker thread end, it will post WM_CLOSE message to CMyDialog. I have
added a breakpoint in both CMyDialog::OnClose and
CMyDialog::OnNcDestroy. I supposed it will stop just after the WM_CLOSE
is sent. However, it is sent when the Cancel button of CMainDialog is
pressed. Any reason to explain that?

Also, since CMyDialog does not belong to the worker thread, it is good
to call UI related method CMyDialog::step (which call
CProgressCtrl::StepIt) directly from the worker thread?

Thanks.

UINT MyThreadProc( LPVOID pParam ) {
   CMyDialog *pMyDialog = (CMyDialog*)pParam;

   if (pMyDialog == NULL) {
       return 1;
   }

   for (int i=0; i<5; ++i) {
       ::Sleep(1000);
       pMyDialog->step();
   }

   pMyDialog->PostMessage(WM_CLOSE, 0, 0);

   return 0;
}

void CMainDialog::OnBnClickedOk()
{
   pMyDialog = new CMyDialog(5);
   pMyDialog->Create(IDD_MY_DIALOG);
   pMyDialog->ShowWindow(SW_SHOW);

   AfxBeginThread(MyThreadProc, pMyDialog);
}

void CMyDialog::step() {
   _progressBar.StepIt();
}

void CMyDialog::OnClose() {
   DestroyWindow();
}

void CMyDialog::PostNcDestroy() {
   CDialog::PostNcDestroy();
   delete this;
}


The default behavior of CDialog is to close and destroy the dialog by calling EndDialog with either IDOK (from OnOK) or IDCANCEL
(from OnCancel) whenever the OK or Cancel button is pushed. You have overridden the behavior for the OK button, but I suspect you
did not do so for the Cancel button.

You are correct that it is NOT good to call UI-related methods from threads which did not create the windows being acted upon. To
influence UI from worker threads, use PostMessage or other forms of asynchronous intra-thread communication.

Perhaps you should tell us what you REALLY want.

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