Cocoa thread synchronisation when using [ALAssetsLibrary enumerateGroupsWithTypes:]

Question

I have recently, like a few people, discovered that [ALAssetsLibrary enumerateGroupsWithTypes] likes to run its blocks on another thread. What a shame that Apple didn't document that :-)

In my current circumstance I need to wait for the enumeration to complete, before the main thread returns any results. I clearly need some sort of thread synchronisation.

I've read about NSLock & NSConditionLock, but nothing yet seems to fit the requirement of 'signal a blocked thread that this worker thread has completed'. It seems like a simple enough need - can anyone point me in the right direction?

Your clue & boos, are most welcome as always,

M.

Answer 1

The answer is to use the NSConditionLock class thusly ...

typedef enum {
    completed = 0,
    running = 1
} threadState;

...

NSConditionLock *lock = [[NSConditionLock alloc] initWithCondition:running];

Then spin off your thread, or in my case a call to [ALAssetsLibrary enumerateGroupsWithTypes:]. Then block the parent thread with this ...

// Await completion of the worker threads 
[lock lockWhenCondition:completed];
[lock unlockWithCondition:completed];

When all work is done in the child/worker thread, unblock the parent with this ...

// Signal the waiting thread
[lock lockWhenCondition:running];
[lock unlockWithCondition:completed];