How to reference the actual owner of the User Task in process variable after task is complete?

Issue

• The requirement is to insert the actual owner/actor (the actor id of the person who worked on the User Task) as a process variable after a User Task is completed/aborted/failed. How can this be achieved?
• How can i get the task user id who has completed the task as task output parameter.. so that it can be mapped to a process variable...