Respond to application-wide "hotkey" in Qt

I haven't tried, but here is what I would do :

Create a QShortcut and make sure its context (with setContext()) is Qt::ApplicationShortcut.

shortcut = new QShortcut(QKeySequence(Qt::Key_F12), parent);
shortcut->setContext(Qt::ApplicationShortcut);

Then you just need to connect a slot to the QShortcut::activated() signal.

Setting the shortcut context to Qt::ApplicationShortcut has a serious flaw. It will not work in modal dialogs. So if you want a trully real pan-application-wide shortcut, then you need to override application's notify() method. The alternative is to install event filter for the application object but that I suspect would be slower and requires slightly more code. With notify() it is very simple:

class MyApplication : public QApplication
{
    // TODO: constructor etc.

protected:

bool MyApplication::notify(QObject *receiver, QEvent *event) override
{
    if (event->type() == QEvent::KeyPress)
    {
        auto keyEvent = static_cast<QKeyEvent*>(event);
        if (keyEvent->key() == Qt::Key_F12 && keyEvent->modifiers() == Qt::NoModifiers)
        {
            // TODO: do what you need to do
            return true;
        }  
    }
    return QApplication::notify(receiver, event);
}

}

If you have a "central widget" which all of the other widgets are children of, then you can simply set that as the widget argument for QShortcut.

(Python, qt5)

self.centralwidget = QtWidgets.QWidget(MainWindow)

QtWidgets.QShortcut(QtGui.QKeySequence("F12"), self.centralwidget, self.goFullScreen)

I added this as an answer because the shortcut context flag: Qt.ApplicationShortcut did not work for me.