ADDIN: Help needed detecting changes in layers

Hi everyone. Hit a problem with an AddIn I've almost finished developing. Without going into detail, I need the AddIn to respond to any changes in datasources on layers and stand-alone tables. Basically, my AddIn needs at least one feature layer and one stand-alone table to work, so it's toolbar buttons are disabled until the criteria is met. So far I have an extension hooked into the NewDocument and OpenDocument to interrogate the documents layers and their validity, creating two arrays of valid feature layers and valid stand-alone tables. This used to be in the tool's update event but the checking process had a significant affect on performance.

I hope someone can advise?


Thank you.