[plt-scheme] DrScheme, detecting frame activation and deactivation, where should I look next?

Hi folks,

My goal is to detect DrScheme frame activation and deactivation.

Reading the PLT Tools manual I found that I can extend DrScheme
classes to add functionality to them. Additionally, I read about all
of the other features (skimmed Zodiac).

Also, I read the first section of the PLT MrEd manual, reading about
all of the available widgets.

Neither resource got me in the right direction when it comes to my goal.

Where is the next place that I should look?