The question is puzzling to me, so perhaps you can explain more about what you need.
Here is some background:
Usually there's client-side validation or server-side validation.
The client-side validation is more efficient because your browser detects the issue, for example the message
"this field may not be blank"
In that example, when you click submit, the server doesn't even know you clicked submit because the client-side validation puts up that message and the submit never occurs, and hence the finish-assignment action doesn't occur on the server side either.
So the reason your question is puzzling is that you refer to doing something after the finish-assignment action which generally won't even occur if client-side validation puts up a message.