In /core/Tracker/Action.php there is this code:
Piwik_PostEvent('Tracker.Action.record', $this, $info);
It passes as notification the Action object, and some additional information in $info.
When I use it in the plugin, I have;
$this->action_info =& $notification->getNotificationObject();
This gives me the Action object. How do I access the $info too?