def METHOD_NAME(self):
    self._system_dbus = Gio.bus_get_sync(Gio.BusType.SYSTEM, None)
    self._system_dbus.signal_subscribe('org.freedesktop.login1',
                                       'org.freedesktop.login1.Manager',
                                       'PrepareForSleep',
                                       '/org/freedesktop/login1',
                                       None,
                                       Gio.DBusSignalFlags.NONE,
                                       self.on_prepare_for_sleep,
                                       None)