Is there a way to know if this-command altered the buffer?

[Marcin Borkowski]

Hi all,

I want to run some function in a certain hook.  This function will
update some info displayed using an overlay.  The info depends on the
buffer contents and is pretty expensive to compute, so I'd like to only
display it if the buffer really changed.

Is there a way to know if `this-command' actually changed the buffer (as
opposed to e.g. point motion)?

TIA,

-- 
Marcin Borkowski
http://mbork.pl