如何在Qt 4.4.1中检测应用程序级别聚焦？

#include <QApplication>
#include <QDebug>
#include <QEvent>
#include <QWidget>

class MyApplication : public QApplication {
public:
    MyApplication(int &argc, char **argv) : QApplication(argc, argv) {}

    bool event(QEvent *event) override {
        if (event->type() == QEvent::ApplicationActivate) {
            qDebug() << "应用程序获得焦点";
        } else if (event->type() == QEvent::ApplicationDeactivate) {
            qDebug() << "应用程序失去焦点";
        }
        return QApplication::event(event);
    }
};

int main(int argc, char *argv[]) {
    MyApplication app(argc, argv);

    QWidget window;
    window.show();

    return app.exec();
}