Usage of QGuiApplication::focusWindowChanged()

[Please_Help_me_D]

Hi,

I have GUI app and in my mainwindow class and I need to know when a focused window of my app has changed.
For this I'm going to connect signal focusWindowChanged with slot. But this signal is from QGuiApplication and QApplication class and I have object of this clas only in my main() function:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

What is simplest way to connect signal focusWindowChanged in mainwindow class? Or maybe there are some other ways to solve my task?

[jsulm]

@Please_Help_me_D said in Usage of QGuiApplication::focusWindowChanged():

What is simplest way to connect signal focusWindowChanged in mainwindow class?

Do it in main? You have both, main window and QApplication instances there...

[Please_Help_me_D]

@jsulm thank you!
In main should fits me needs I think.
And if not in main then I guess I have to reimplement QMainWindow (inherit MyMainWindow class from it) class and pass a pointer of QApplication there?

[JonB]

@Please_Help_me_D
You can do it @jsulm's way.

However, so that you know you have an alternative and could do it in MainWindow if you prefer, there is always a globally accessible instance of your application available through static https://doc.qt.io/qt-5/qcoreapplication.html#instance. So in MainWindow you could do your connect via

QGuiApplication *app = qobject_cast<QGuiApplication *>(QCoreApplication::instance());
connect(app, &QGuiApplication::focusWindowChanged, this, &MainWindow::slot);

This would allow if, for example, you wish to keep your MainWindow::slot() method private.

[Please_Help_me_D]

@JonB thank you!
A half hour ago I was looking to some library's source code and there was such thing and I could understand how it connect this signal. The answer is that there is a global instance of application :)

[jsulm]

@Please_Help_me_D said in Usage of QGuiApplication::focusWindowChanged():

I have to reimplement QMainWindow

You already do, right?
There is no need to pass QApplication pointer to anywhere - you can get it everywhere in your app: https://doc.qt.io/qt-5/qcoreapplication.html#instance

[Please_Help_me_D]

@jsulm no I didn't do that yet
Now I just started to try QCoreApplication *QCoreApplication::instance()

[jsulm]

@Please_Help_me_D said in Usage of QGuiApplication::focusWindowChanged():

no I didn't do that yet

Then what is MainWindow? Isn't it a subclass of QMainWindow?

[Please_Help_me_D]

@jsulm MainWindow It is a clas inherited from QMainWindow object. It is just standard Qt mainwindow that appears in Qt GUI project

[jsulm]

@Please_Help_me_D So, "And if not in main then I guess I have to reimplement QMainWindow (inherit MyMainWindow class from it)" is already the case.

[Please_Help_me_D]

@jsulm Yes, haven't thought of that :)

[SimonSchroeder]

BTW, I think QCoreApplication::instance() is a mouthful. There are two shorter versions for that: qApp and qGuiApp. These will cast the instance to QApplication and QGuiApplication, respectively.