Notify a method when binding changes

This is probably a very easy solution, but I am looking for a way to bind when a viewmodel's property changed and react to it within my own code.

For example

Activity.SetBinding (ActivityIndicator.IsRunningProperty, "IsLoading");

I want to be able to call a method whenever that IsLoading property changes, not just pass it into the Activity. How would I go about doing this?

Thanks!