Concurrent insertion of blocks into concurrent queue

Question

I want to add blocks A, B, C into global concurrent queue. Immediately after that I want to add barrier to the same queue.

I expect that A, B, C all be called in some order but when all blocks terminate the barrier will be invoked.

How can I guarantee that NO additional block will be inserted in the middle of this sequence? For example, if in some other place I deploy block X on the same queue, I do not want it to be executed before the barrier.

Can the same solution be available both on MacOS and iOS?

Answer 1

Barriers don’t work on global queues. Create your own custom concurrent queue. As the dispatch_barrier_async documentation says:

The queue you specify should be a concurrent queue that you create yourself using the dispatch_queue_create function. If the queue you pass to this function is a serial queue or one of the global concurrent queues, this function behaves like the dispatch_async function.

Then anything dispatched to that queue after the barrier won’t run until after the barrier. So dispatch A, B, and C. Then dispatch D with a barrier. Then dispatch X. The A, B, and C will run concurrently with respect to each other, when they’re done then D will run, and when D is done, X will run. That’s how it works, on both macOS and iOS. Just create your own custom concurrent queue.

dispatch_queue_t queue = dispatch_queue_create("com.company.app.queue", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{ 
    NSLog(@"start - A!\n");  
    sleep(4);  
    NSLog(@"end - A!\n");
});

dispatch_async(queue, ^{  
    NSLog(@"start - B!\n");  
    sleep(2);  
    NSLog(@"end - B!\n"); 
});

dispatch_async(queue, ^{  
    NSLog(@"start - C!\n");  
    sleep(3);  
    NSLog(@"end - C!\n"); 
});

dispatch_barrier_async(queue, ^{  
    NSLog(@"Barrier - D\n");  
});

dispatch_async(queue, ^{  
    NSLog(@"start - X!\n");  
    sleep(3);  
    NSLog(@"end - X!\n"); 
});