How do I detect if a modeless CDialog has been closed?

Question

I have followed this question to make a non-modal/modeless dialog: How to display a non-modal CDialog?

I'm using MFC/C++ in VS2008. I'm more fluent with C# and .net than with MFC and C++.

I have a menu item in my form that launches the dialog. There can only be one instance of the dialog opened. The dialog displays fine. I can close it by clicking the X in the corner and it closes when I close the main form. The problem I am having is the dialog cannot be opened again after I click the X to close the dialog. I know it is because the pointer is never set back to NULL.

I have this in my form's header file:

CChildDialog *m_pDialog;

I have this part in my form's constructor:

m_pDialog = NULL;

When clicking on a menu item I have this code in the menu item's method (I modified it from the other SO answer because I only want one instance of the dialog opened):

if(m_pDialog == NULL)
{
    // Invoking the Dialog
    m_pDialog = new CChildDialog();
    BOOL ret = m_pDialog->Create(IDD_CHILDDIALOG, this);

    if (!ret)   //Create failed.
    {
        AfxMessageBox(_T("Error creating Dialog"));
    }    

    m_pDialog->ShowWindow(SW_SHOW);
}

Now I know I need to execute this part and set the pointer to NULL, but I don't know where to put this:

// Delete the dialog once done
delete m_pDialog;
m_pDialog = NULL;

Do I need to keep monitoring if the dialog has been disposed? Is there an event triggered to the parent form when the dialog is closed?

Answer 1

If you want to recycle the contents of the window after closing it with X, you have to handle the WM_CLOSE message in your dialog:

void CChildDialog::OnClose()
{
    ShowWindow(SW_HIDE);
}

Then in the code that opens the window:

if(m_pDialog == NULL)
{
    // Invoking the Dialog
    m_pDialog = new CChildDialog();
    BOOL ret = m_pDialog->Create(IDD_CHILDDIALOG, this);

    if (!ret)   //Create failed.
    {
        AfxMessageBox(_T("Error creating Dialog"));
    }    
}

m_pDialog->ShowWindow(SW_SHOW); //moved outside the if(m_pDialog == NULL)

Hope it can help

Answer 2

If you want to delete the modeless dialog, then just do so.

If you want to delete the dialog's object when the user closed the modeless dialog you might take a look at WM_PARENTNOTIFY. If a child window is destroyed and the child windows has not the extended window style WS_EX_NOPARENTNOTIFY set, then windows sends a WM_PARENTNOTIFY with wParam=WM_DESTROY to the parent window. You should implement a handler for that message in the parent window and check if it's the modeless dialog that is being destroyed.