Ability to set conditions to post functions

As a JIRA Administrator, I would like to have a native feature to set conditions to every post functions I set in my transitions.

Use case: The JIRA Administrator has created a post function to successfully auto assign a user when they move an issue down the swim lane, however, when the issues already assigned, it will update the assignee as well, the JIRA Administrator would like to set a condition to only perform the post function when the issue is unassigned.