Why do wx.App::SetTopWindow?

Is there any thing beyond adding member for reference from original implementation of app? such as like this.

In another word: does widgets system any special thing to SetTopWindow'ed window?
The answer is Yes. It does.

src/common/appcmn.cpp

wxWindow* wxAppBase::GetTopWindow() const
{
    wxWindow* window = m_topWindow;
    if (window == NULL && wxTopLevelWindows.GetCount() > 0)
        window = wxTopLevelWindows.GetFirst()->GetData();
    return window;
}

Without setting, you gets nothing. :p


src/common/dlgcmn.cpp

wxWindow *wxDialogBase::GetParentForModalDialog(wxWindow *parent) const
{
    // creating a parent-less modal dialog will result (under e.g. wxGTK2)
    // in an unfocused dialog, so try to find a valid parent for it:
    if ( parent )
        parent = wxGetTopLevelParent(parent);

    if ( !parent || !CanBeUsedAsParent(parent) )
        parent = wxTheApp->GetTopWindow();

    if ( parent && !CanBeUsedAsParent(parent) )
    {
        // can't use this one, it's going to disappear
        parent = NULL;
    }

    return parent;
}

Setting TopWindow to app affects Modal state.


src/common/cshelp.pp

/ Begin 'context help mode'
bool wxContextHelp::BeginContextHelp(wxWindow* win)
{
    if (!win)
        win = wxTheApp->GetTopWindow();
    if (!win)
        return false;

    wxCursor cursor(wxCURSOR_QUESTION_ARROW);
    wxCursor oldCursor = win->GetCursor();
    win->SetCursor(cursor);

It affects context help too.

src/msw/msgdlg.cpp

int wxMessageDialog::ShowModal()
{
    if ( !wxTheApp->GetTopWindow() )
    {
        // when the message box is shown from wxApp::OnInit() (i.e. before the
        // message loop is entered), this must be done or the next message box
        // will never be shown - just try putting 2 calls to wxMessageBox() in
        // OnInit() to see it
        while ( wxTheApp->Pending() )
            wxTheApp->Dispatch();
    }