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

gistfile1.m

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

__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.
        }
    }];
}