How can I code "Execute during preview" capability in custom snap? (i.e. testing validation mode vs. run-time mode)

Thank you very much for your lighting-speed responses @rohithmadhavan and @robin.

I started on @rohithmadhavan’s suggestion first and the path was very educational for me as I started to implement it. However, my question was vague in detail, and a little misdirecting, so @robin’s response was the simpler solution I was looking for. I’ve developed 4 simple snaps, but none were write snaps, so that answer was not yet obvious to me.

It’s now working like a charm, so thank you both very much!

– Del