What is the API for class CRM_Civirules_TriggerData_TriggerData?
What you could do is use the getEntityData? That should give you all data that is avaiable for the entity in the triggerData. The contactId is special as we tend to want to do things with contacts :-)
Does that help?
So an example:

$contributionRecur = $triggerData-&gt;getEntityData('ContributionRecur');


which in your case is likely to become

$eventData = $triggerData-&gt;getEntityData('Event');