QGuiApplication::primaryScreen() not telling me what I need to know

[zsd-]

@Bonnie Here is the code I used to get canvas_pos. (And no, I'm not sure it is right. I'm certainly open to being corrected.)
QWidget * wind = ui->canvas->window();
QPoint canvas_pos = wind->pos();

(Is that enough info to answer your question?)
Thanks.

[Bonnie]

@zsd Your result may be not accurate, but should work if "the window is completely on the external monitor".
How about these approaches?

QPoint canvas_pos = ui->canvas->window()->mapToGlobal(QPoint(0, 0));
QScreen * canvas_screen = QGuiApplication::screenAt(canvas_pos);

QPoint canvas_pos = ui->canvas->mapToGlobal(QPoint(0, 0));
QScreen * canvas_screen = QGuiApplication::screenAt(canvas_pos);

if(QWindow * wind = ui->canvas->window()->windowHandle()) {
    QScreen * canvas_screen = wind->screen();
}

[zsd-]

@Bonnie ,
thanks very much. I now see that there is another issue involved.

I am looking for this information after the following code (in main()):
QApplication a(argc, argv);
MainWindow w;
w.show();
At that point (after calling w.show()) all three versions of your code are still returning the laptop screen information, even when the window opens completely on the external monitor.

On the good side, all three give the correct screen if I call a function to print things out when a menu item is chosen (nothing magic about a menu item, it just seemed a quick way to test things out).

So perhaps my question should be modified to the following:
How can I get the correct screen information after w.show() but before I call
a.exec()
In other words (possibly!), how can I get Qt to really look and see which monitor the window is on before calling a.exec() ?

Thanks to anyone who can provide enlightenment!

[SGaist]

Hi,

Here is the trick: show has no effect until exec runs. The show method schedules an event to bring the widget to screen. Hence you might want to move some logic in the showEvent method.

[zsd-]

@SGaist
Thanks for the information. I had previously tried a sleep(5) after show() before exec() and a window (albeit empty) showed up on the monitor before exec() was called, and so I thought (ha!) that Qt would know which monitor it was on.

I tried putting a showEvent() in my main window class and that was called when w.show() was called (and thus before a.exec()), so it still has the wrong answer.

I've hunted around on the net, but I haven't found any enlightenment suitable for my level of Qt n00bness. Are you able to either elaborate a bit further or point me in the direction of a "how to use showEvent() at just the right place and just the right time for n00bs" document?

Thanks.

[zsd-]

This post is deleted!

[zsd-]

@zsd In case anyone is still watching and interested, the problem persists in 5.15.0.
If anyone has any more thoughts to share, I'd be happy to hear them.

[Christian Ehrlicher]

@zsd said in QGuiApplication::primaryScreen() not telling me what I need to know:

In other words (possibly!), how can I get Qt to really look and see which monitor the window is on before calling a.exec() ?

You can't since even Qt does not know this.

[zsd-]

@Christian-Ehrlicher Thanks for the answer. I find this surprising, given that the w.show() causes an empty window frame to pop up on the monitor.

However, I still need the info. The program could ask for it each and every time a user does something which needs the information, or the program could wait for such an event and cache the information, but are you able to recommend a Qt-approved way of doing something once when a.exec() is called?

Thanks.

[jsulm]

@zsd said in QGuiApplication::primaryScreen() not telling me what I need to know:

Qt-approved way of doing something once when a.exec() is called?

Not sure what exactly your use case is, but what about QTimer?

[zsd-]

@jsulm Hmmm... that might work, although it strikes me as a bit kludgy (no offense intended). My "use case" is explained in my original message. Is that insufficient in some way? I can supply more information, but I'm not sure what would be relevant.

[zsd-]

@Christian-Ehrlicher By the way... given that a.show() causes the window manager to open a window on the display, can you explain why you say that Qt doesn't know whicih monitor its window in on before a.exec() is called?
Thanks.