[Kde-bindings] Fw: [Qyoto] Casting QObjects

Dimitar Dobrev dpldobrev at yahoo.com
Thu May 24 22:03:26 UTC 2012

You're welcome. :) I hope you will join our community, we could definitely use any kind of help: development, testing, popularisation among developers, etc.

I've just checked "qt_rubberband" in a C++ Qt project. The debugger shows it is as a QObject in an empty QMainWindow, i.e. the same as in your C# code. This probably means that Qyoto is in no error in this case. About what "qt_rubberband" is, here is an explanation - http://www.qtcentre.org/threads/34998-qt_rubberband-blocking-events-of-pushbutton?p=161726#post161726.

Anyway, you may try casting it using the Qt.qobject_cast<> static method. Unfortunately, even if this works, I do not yet know a way to test if such an object is of a certain type (QWidget in this case).

 From: Mach <metator.pt at gmail.com>
To: Dimitar Dobrev <dpldobrev at yahoo.com>; KDE bindings for other programming languages <kde-bindings at kde.org> 
Sent: Friday, May 25, 2012 12:38 AM
Subject: Re: [Kde-bindings] [Qyoto] Casting QObjects

Hi Dimitar, 

Thank you for your quick reply. Well, i take back what i said about
    traditional casting in C#. Seems like this qt_rubberband object is
    somewhat special (i don't have much experience in Qt). For instance,
    following my example this line of code returns null: QObject
    rubberBand = win.FindChild<QObject>("qt_rubberband");

What i really find strange though, is that qt_rubberband is
    considered to be only a QObject. Shouldn't it be a QRubberBand
    (which is a QWidget...)?


PS: I take this opportunity to thank you (and also Steven Boswell
    regarding building Qyoto in Windows) for making Qt available to

Em 24-05-2012 21:42, Dimitar Dobrev escreveu: 
Hi, Mach,
>On my machine your sample behaves the same way with the addition that traditional casting (that is, replacing 
>QWidget widget = (QWidget) child.MetaObject().Cast(child);
>QWidget widget = (QWidget) child;
>work as well). Apparently There is a bug in the IsWidgetType function because it allows the "qt_rubberband" which the debugger shows as a QObject only, to pass. However, "if (child is QWidget)" works properly so you can use that.
> From: Mach <metator.pt at gmail.com>
>To: Kde-bindings at kde.org 
>Sent: Thursday, May 24, 2012 10:54 PM
>Subject: [Kde-bindings] [Qyoto] Casting QObjects
>Hi all,
>I've managed to compile qyoto on windows and now i'm doing
            some test applications to try it out. I'm having trouble
            trying to downcast QObjects. Traditional casting in C# does
            not work, but i was expecting that. So i started to look for
            something that could perform a cast and found the method
            Cast(QObject) in QMetaObject but it's not working always.
            Below is an example application that tries to cast all the
            children of the main window to widgets. Notice that i
            previously check if the object is a widget type:
>using System;
>using Qyoto;
>namespace QtTest {
>    class Program {
>        [STAThread]
>        static void Main(string[] args) {
>            QApplication app = new QApplication(args);
>            QMainWindow win = new QMainWindow();
>            QPushButton btn = new QPushButton(win);
>            win.WindowTitle = "Demo Application";
>            win.Show();
>            foreach (QObject child in win.Children()) {
>                if (child.IsWidgetType()) {
>                    try {
>                        QWidget widget = (QWidget)
>                        Console.WriteLine("Cast succeeded.
            It's now a " + widget.GetType().Name);
>                    } catch (InvalidCastException e) {
>                        Console.WriteLine(child.ObjectName +
            " of type " + child.MetaObject().ClassName() +
>                                          " cannot be cast
            to QWidget");
>                    }
>                }
>            }
>            QApplication.Exec();
>        }
>    }
>The output on the console is:
>qt_rubberband of type QRubberBand cannot be cast to QWidget
>Cast succeeded. It's now a QPushButton
>I suspect this has something to do with the fact that
            qt_rubberband was created on the unmanaged side, whereas
            QPushButton was created by the managed side. I also tried
            calling QWidget.StaticMetaObject.Cast(QObject) but it's
            always returning null.
>Can someone help me out solving this problem?
>Thanks in advance. Regards
>Kde-bindings mailing list
>Kde-bindings at kde.org
Kde-bindings mailing list Kde-bindings at kde.org https://mail.kde.org/mailman/listinfo/kde-bindings 
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