What is the best way to get notified when all callbacks have returned?

gistfile1.m

for (myObject in objects) {
	[myObject doWithCallback: ^(BOOL success){
		//
	}];
}

Solution:
GCD Dispatch Queues
http://mikeash.com/pyblog/friday-qa-2009-09-04-intro-to-grand-central-dispatch-part-ii-multi-core-performance.html

can you elaborate a bit more on the problem? I guess the callbacks spin of a new thread? You could consider a NSOperationQueue and listen to the case where it's empty again.

How do you listen to the "queue empty" event on a gdc queue?

__block NSUInteger counter = 0;
NSUInteger max = [objects count];

dispatch_queue_t q = dispatch_queue_create("com.mindnode.fafnerqueue", DISPATCH_QUEUE_SERIAL);

for (myObject in objects) {
    [myObject doWithCallback: ^(BOOL success){
        dispatch_async(q, ^{
            if(++counter >= max) {
                // Done!
                dispatch_release(q);
            }
    }];
}

Alternative (probably more efficient):

__block volatile int64_t counter = 0;
int64_t max = (int64_t)[objects count];

for (myObject in objects) {
    [myObject doWithCallback: ^(BOOL success){
        if(OSAtomicIncrement64Barrier(&counter) >= max) {
            // Done! Will only be executed once.
        }
    }];
}