Receiving system D-Bus events (signals) and responding to them

[swirl]

Title. I specifically need to respond to org.freedesktop.login1.Manager.PrepareForSleep, by closing out the application and deallocating all memory in order to prevent memory dumps and the such. However, after scouring the docs for a few hours--I've got nothing. I can't seem to find a way to respond to system events--Qt only provides an example for sending and receiving stuff at a custom destination.

Here's a snippet of code I tried:

class Test : public QDBusAbstractAdaptor
{
    Q_OBJECT
    Q_CLASSINFO("D-Bus Interface", "org.freedesktop.login1.Manager")
public:
    Test(QObject *obj) : QDBusAbstractAdaptor(obj)
    { }
signals:
    void PrepareForSleep();
};

And in the main function:

QObject obj;
Test *test = new Test(&obj);
QObject::connect(pong, &Test::PrepareForSleep, test, [] {
    qDebug() << "Preparing for sleep";
});

QDBusConnection::sessionBus().registerObject("/", &obj);

However, when this event occurs (like when I suspend my computer), nothing shows up and the app just continues.

Like I said I can't make anything of the docs. Maybe I'm doing something wrong, but I tried a bunch of different things and it just didn't work. I know it's possible because QDBusViewer can connect to signals and show their info, but I once again can't figure out how it does it.

[swirl]

I've found I can do something like

QDBusConnection::systemBus().connect("org.freedesktop.login1", "/org/freedesktop/login1", "org.freedesktop.login1.Manager", "PrepareForSleep")

however, I don't know what I'm supposed to do with this. It says I need to provide a slot with the correct arguments, but I don't know what arguments.