Performing a Completion Block When a Task Is Done (!!!)

void average_async(int *data, size_t len,
dispatch_queue_t queue, void (^block)(int))
{
// Retain the queue provided by the user to make
// sure it does not disappear before the completion
// block can be called.
dispatch_retain(queue);
// Do the work on the default concurrent queue and then
// call the user-provided block with the results.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
int avg = average(data, len);
dispatch_async(queue, ^{ block(avg);});
// Release the user-provided queue when done
dispatch_release(queue);
});
}