Why doesn't QGuiApplication emit a signal?

Question

Qt

There is a code:

#include <qapplication.h>
#include <qguiapplication.h>
#include <qscreen.h>
#include <qlist.h>
#include <qdebug.h>

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

    for (;;) {
        auto countMonitor = QGuiApplication::screens();
        qDebug() << countMonitor.size();
    }

    return a.exec();
}

I connect 2 monitors, run the program, 2 is displayed, if I turn off 1 of the monitors (at runtime), 2 will be displayed as it was. If I run the program with 1 monitor turned on, 1 is displayed, if I connect the 2nd monitor (at runtime), 1 will be displayed as it was. Why is that?

Answer 1

I checked it on my computer. Windows 10, Qt 5.15.2. Indeed, when the monitor is connected/disconnected, the signals are not called. After a cursory review of the Qt sources, I came to the conclusion that this functionality is not implemented for Windows.

Answer 2

In the system, when a new monitor is added, a signal is sent, since you are counting the number of monitors in an infinite loop, the main event loop cannot go through the events until you exit your loop.
Try adding:
QGuiApplication::processEvents();