Hook for buffer or mode change?

I had some students in our Software Engineering class here take on the job of providing my ViaVoice module with a reasonable user interface last semester. They did a pretty good job, but I still need to clean up the code a bit. One thing that is still extremely kludgy is an attempt to set the ViaVoice grammar by mode. What I need to make this work right is a way to run a hook whenever: (a) some buffer becomes the current buffer; or (b) the mode (either major or minor) of the current buffer changes. I don't see anything suitable. The menubar, toolbar, and gutter are all changed in those situations, but it looks like they are marked so that redisplay notices the change and updates accordingly. I don't see a way to hook into that process either. Is there a clean way to do this? My students tried redefining or advising all of the user-visible buffer changing commands (yikes!), which still leaves (b) without a solution. Thanks, -- Jerry James