Event Fired when Calculation Mode Changed
 

I'm looking for an event that fires when the calculation mode is changed from
automation to manual or vice versa. (Not to be confused with when the
calculation state changes from being in a calc to having finished a calc).

I have a requirement to implement a drop down in the toolbar which shows the
current calculation mode selected. Getting the current state is simple, but
knowing that it got changed, either by a user or programatically is
presenting a challenge.

thanks in advance!