The quick and easy way to achieve this IMO would be to implement a
lightweight ThreadManager, which every thread registers with upon
instantiation (run), and when it completes, it sends a message
(Callback) to this ThreadManager. That way, at any given point in time
the ThreadManager knows how many threads were spawned, completed, and
are under progress. The progressbar can listen to the ThreadManager via
publish/subscribe and render itself accordingly.

-cheers,
Manish