Synchronization/wait design for cross-thread event signaling (Obj-C)?

Question

In a Cocoa app, I have a setup like this:

1. The main thread (M) can submit requests to a some background \"producer\" thread (B) to get some work done, say the r

Answer 1

An NSConditionLock could work nicely here. Perhaps a condition lock associated with each X. Initially condition "dormant" then set to "processing" by the background thread then set to "complete" when it is done. The calling thread could check for the "processing" condition and if it is set, wait until condition "complete" is reached.