g_object_notify meaning

[Philippe Ribet <p ribet worldonline fr>]

Hello,

I'd like to understand the following line which stand in
gtk_widget_show/hide:
      g_object_notify (G_OBJECT (widget), "visible");

Is 'visible' some signal (who is intrested in receiving it) ? Is that
for GTK_VISIBLE flag delayed update ? If so, when will the update be
done, why not changing the flag at this point ?

    Thanks,

        Philippe Ribet