Add in Active Document Change

First off thanks for your time!

Could anyone let me know as to what event I need to tap if I have to catch ActiveDocument changes.
I have two files a.cs and b.cs in my solution and I would like to know when the user switches from one file  to another. I know the activedocument changes and I could use the activedocuments name but is there an event that is fired when this happens?

Thanks!
Sasha