QWidget top level widget

Alexey Serebryakov

Hi guys, how can I create a top level widgets?

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    
    QWidget w1; // is it top level widget?
    QWidget w2; // is it top level widget?

    w1.show();
    QString ptrStr1 = QString("0x%1").arg((quintptr)w1.windowHandle(), QT_POINTER_SIZE * 2, 16, QChar('0'));
    QMessageBox::information(&w1, "Info1", ptrStr1, QMessageBox::StandardButton::Ok); // ok, correct window handle

    w2.show();    
    QString ptrStr2 = QString("0x%1").arg((quintptr)w2.windowHandle(), QT_POINTER_SIZE * 2, 16, QChar('0'));
    QMessageBox::information(&w2, "Info2", ptrStr2, QMessageBox::StandardButton::Ok); // but 0x00000000

    return a.exec();
}

Whats wrong?
Thanks a lot.

mrjj

Hi
If you don't assign a parent to a QWidget it becomes top level. (a window)
You can also use setWiundowsFlags for the same effect
https://doc.qt.io/qt-6/qtwidgets-widgets-windowflags-example.html

Im surprised that w2 is zero. Does that happen every time?

what if you swap w1 and w2 processing ?

    w2.show();    
    QString ptrStr2 = QString("0x%1").arg((quintptr)w2.windowHandle(), QT_POINTER_SIZE * 2, 16, QChar('0'));
    QMessageBox::information(&w2, "Info1", ptrStr2, QMessageBox::StandardButton::Ok); 

   w1.show();
    QString ptrStr1 = QString("0x%1").arg((quintptr)w1.windowHandle(), QT_POINTER_SIZE * 2, 16, QChar('0'));
    QMessageBox::information(&w1, "Info", ptrStr1, QMessageBox::StandardButton::Ok);

Alexey Serebryakov

@mrjj yes every time.

Oh, but if I swap w1 and w2 processing there were correct handles. :-) Why that heppened?

mrjj

@Alexey-Serebryakov
ehh so swapping them, then both works? (o.O)

Well, I wonder if it's due to we do it before a.exec(); (before ever shown)

if you add a button and do it in its clicked slot, we still have one fail or what happens ?

JonB

@Alexey-Serebryakov said in QWidget top level widget:

// ok, correct window handle

How would you even know whether it's "correct"? :) Maybe all you mean is it's non-zero?

Since the code variants with the order swapped are functionally equivalent (the compiled code may well actually beidentical), I would guess no native window handle is actually assigned until the widgets are physically shown during the a.exec(), as @mrjj suggests.

Alexey Serebryakov

@mrjj is it correct code?

class Widget : public QWidget
{
    Q_OBJECT
public:
     Widget(QWidget *parent = nullptr) : QWidget(parent, Qt::Window) {}
    ~Widget();
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w1;
    Widget w2;

    w1.show();
    w1.windowHandle()->setScreen(QApplication::screens()[0]);

    w2.show();
    w2.windowHandle()->setScreen(QApplication::screens()[1]);

    return a.exec();
}

mrjj

Hi
Yes its correct
as such but windowHandle() can return null so could crash.

Also note with setScreen that on some systems with not work as one expect as
the monitors are group by the OS as one big screen.

Alexey Serebryakov

@mrjj For the top level widget windowHandle() must be valid always.

mrjj

@Alexey-Serebryakov

Yes they should and hopefully not null even the widget was not shown yet.

So you play around with a multi screen setup ?

Alexey Serebryakov

@mrjj yep.
So, I have Qt-widget based application. Also I have notification widget which must be pop up on the selected screen/monitor in right bottom corner. Like Windows notification center or how in MS Outlook.
User can select the screen which display that popup widget.
Idially I need something like this:

MyPopupWidget::show(QScreen *screen) {
  show();
  windowHandle()->setScreen(screen);
}

mrjj

@Alexey-Serebryakov
Ohh.
Like Win 10 Toast messages?

But you must run on other platforms and prefer your own ?

https://doc.qt.io/qt-5/qtwidgets-desktop-systray-example.html

also for full blown usage
https://github.com/mohabouje/WinToast

Alexey Serebryakov

@mrjj yes, like Win 10 Toast.
Application run on Windows 10 and Ubuntu. So I need show custom popup widget. :-(
Thank you but built-in systray not allowed.

mrjj

@Alexey-Serebryakov

Ok, it must be totally custom.
QSystray also had issues on ubuntu over the years.

So is the message also clickable like Toats are or is it more like a tooltip type ?

Alexey Serebryakov

@mrjj Yes there popup widget has title, text, image, close button, links and so on. User can click them and follow to the main window by context dependency. Also popup widget can contain other user-defined custom widget.

mrjj

@Alexey-Serebryakov

Oh so its a FancyToast widget ;)

Well it should not be that hard to place it. and the rich part sounds lovely.

Do notice on some virtual Desktops, its like one big screen and you will have to move your window
to right x pos as there will be only 1 screen.

Alexey Serebryakov

@mrjj what the FancyToast?

Alexey Serebryakov

@mrjj yeah virtual desktop is next my head ache... :-)

Axel Spoerl

Instantiating a QWidget and calling show()create a series of events that get processed asynchronously. It cannot be safely assumed that windowHandle()returns a valid pointer right after construction and/or calling show(). You might be more lucky on Win10, than on UBuntu/X11.

If you want to wait for the widget to be fully constructed, you have to wait for windowHandle() != nullptr, e.g. by

while (!w1.windowHandle())
    a.processEvents();

QWindow::isExposed()tells if the window has been rendered on a screen.

Alexey Serebryakov

@Axel-Spoerl thank you,
consider example code above,
how can I show w1 widget on first monitor and w2 on second?
Now both widgets showing on the same primary monitor.

Axel Spoerl

how can I show w1 widget on first monitor and w2 on second?

Suggest you take a look at the documentation and examine QWidget::setScreen(QScreen *screen).