pydevd - how to intercept mode switch - running, paused

[nathan-retrace]

I'm building a Python debugging extension and I'd like it to work with Pydevd. It'd be useful to be able to add a callback which is called just before the main program is paused and control handed to the debugger, and called just before resume when the program is resumed. Is there a callback interface to allow this, or what approach is suggested?

Looking for advice on approach