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From: Nick Meyer on 9 May 2008 09:13
I'm working on a modeless dialog which needs to get some data each time it is
shown. If it fails to obtain the data, I want to show a message box and
prevent the dialog from being shown. I've tried handling this in
OnShowWindow:

void CMyDialog::OnShowWindow(BOOL bShow, UINT nStatus)
{
CDialog::OnShowWindow(bShow, nStatus);

if (bShow)
{
// get data here
AfxMessageBox(_T("Failed to get data!"), MB_ICONERROR | MB_OK);
ShowWindow(SW_HIDE);
}
}

Calling ShowWindow inside OnShowWindow doesn't seem to have an effect. The
dialog is shown anyway. Commenting out the call to the base class has no
effect, and neither does setting bShow to FALSE. The only thing I've found
that works is doing a SendMessage(WM_CLOSE). However, then the dialog
flickers on the screen, as it is clearly shown and then hidden very quickly.

What's the best way to do this? Can you "cancel the showing" from inside
OnShowWindow, or is that too late?

Thanks!

From: David Wilkinson on 9 May 2008 09:28
Nick Meyer wrote:
> I'm working on a modeless dialog which needs to get some data each time it is
> shown. If it fails to obtain the data, I want to show a message box and
> prevent the dialog from being shown. I've tried handling this in
> OnShowWindow:
>
> void CMyDialog::OnShowWindow(BOOL bShow, UINT nStatus)
> {
> CDialog::OnShowWindow(bShow, nStatus);
>
> if (bShow)
> {
> // get data here
> AfxMessageBox(_T("Failed to get data!"), MB_ICONERROR | MB_OK);
> ShowWindow(SW_HIDE);
> }
> }
>
> Calling ShowWindow inside OnShowWindow doesn't seem to have an effect. The
> dialog is shown anyway. Commenting out the call to the base class has no
> effect, and neither does setting bShow to FALSE. The only thing I've found
> that works is doing a SendMessage(WM_CLOSE). However, then the dialog
> flickers on the screen, as it is clearly shown and then hidden very quickly.
>
> What's the best way to do this? Can you "cancel the showing" from inside
> OnShowWindow, or is that too late?

Nick:

The life-cycle of this dialog is not clear to me. When is it showing, and when
not? What triggers the attempt to gather data?

--
David Wilkinson
Visual C++ MVP
From: Nick Meyer on 9 May 2008 09:41


"David Wilkinson" wrote:

> Nick Meyer wrote:
> > I'm working on a modeless dialog which needs to get some data each time it is
> > shown. If it fails to obtain the data, I want to show a message box and
> > prevent the dialog from being shown. I've tried handling this in
> > OnShowWindow:
> >
> > void CMyDialog::OnShowWindow(BOOL bShow, UINT nStatus)
> > {
> > CDialog::OnShowWindow(bShow, nStatus);
> >
> > if (bShow)
> > {
> > // get data here
> > AfxMessageBox(_T("Failed to get data!"), MB_ICONERROR | MB_OK);
> > ShowWindow(SW_HIDE);
> > }
> > }
> >
> > Calling ShowWindow inside OnShowWindow doesn't seem to have an effect. The
> > dialog is shown anyway. Commenting out the call to the base class has no
> > effect, and neither does setting bShow to FALSE. The only thing I've found
> > that works is doing a SendMessage(WM_CLOSE). However, then the dialog
> > flickers on the screen, as it is clearly shown and then hidden very quickly.
> >
> > What's the best way to do this? Can you "cancel the showing" from inside
> > OnShowWindow, or is that too late?
>
> Nick:
>
> The life-cycle of this dialog is not clear to me. When is it showing, and when
> not? What triggers the attempt to gather data?
>
> --
> David Wilkinson
> Visual C++ MVP
>

Hi David,

Thanks for the quick response, and you're right: I could have been much more
detailed.

Our app uses a large number of dialogs to configure an external piece of
hardware, which it communicates with via CORBA. The app has a DialogManager
class which takes responsibility for managing the lifetime of each of these
dialogs. When the user selects a menu item, the menu command handler calls
the DialogManager and asks it to display the appropriate dialog. The first
time, the DialogManager creates the modeless dialog. When the dialog is
closed by the user, it is not destroyed, but only hidden, so subsequent calls
simply cause DialogManager to call ShowWindow(SW_SHOW).

Each dialog needs to query the hardware for different aspects of its current
configuration so it can display the current values for the user to modify.
We've been having the dialogs do this in OnShowWindow (in response to
WM_SHOWWINDOW) because the current values may have been changed by a
different client while the dialog was not visible, so the dialog needs to
update itself each time it is shown. If, for example, the hardware returns
an error or the connection is lost and the dialog cannot retrieve the current
configuration from the hardware, then we want to show a message box and then
prevent the dialog from actually appearing. Since the data is updated in
OnShowWindow when bShow == TRUE, that's when we're responding to the error.

The closest we've come is by sending a WM_CLOSE message from inside
OnShowWindow, but that causes a visible flicker. We're hoping to find a
better solution that doesn't cause this.

Thanks again,
Nick
From: David Wilkinson on 9 May 2008 09:55
Nick Meyer wrote:
> Thanks for the quick response, and you're right: I could have been much more
> detailed.
>
> Our app uses a large number of dialogs to configure an external piece of
> hardware, which it communicates with via CORBA. The app has a DialogManager
> class which takes responsibility for managing the lifetime of each of these
> dialogs. When the user selects a menu item, the menu command handler calls
> the DialogManager and asks it to display the appropriate dialog. The first
> time, the DialogManager creates the modeless dialog. When the dialog is
> closed by the user, it is not destroyed, but only hidden, so subsequent calls
> simply cause DialogManager to call ShowWindow(SW_SHOW).
>
> Each dialog needs to query the hardware for different aspects of its current
> configuration so it can display the current values for the user to modify.
> We've been having the dialogs do this in OnShowWindow (in response to
> WM_SHOWWINDOW) because the current values may have been changed by a
> different client while the dialog was not visible, so the dialog needs to
> update itself each time it is shown. If, for example, the hardware returns
> an error or the connection is lost and the dialog cannot retrieve the current
> configuration from the hardware, then we want to show a message box and then
> prevent the dialog from actually appearing. Since the data is updated in
> OnShowWindow when bShow == TRUE, that's when we're responding to the error.
>
> The closest we've come is by sending a WM_CLOSE message from inside
> OnShowWindow, but that causes a visible flicker. We're hoping to find a
> better solution that doesn't cause this.
>
> Thanks again,
> Nick

Nick:

Why not just send a custom message to the dialog (or call a specific method)
when your user selects the menu item?

--
David Wilkinson
Visual C++ MVP
From: Nick Meyer on 9 May 2008 10:11
"David Wilkinson" wrote:
>
> Why not just send a custom message to the dialog (or call a specific method)
> when your user selects the menu item?
>
> --
> David Wilkinson
> Visual C++ MVP
>

Hi David,

That sounds like a pretty good bet. Thanks for the advice!

Nick
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