Dispatch queues: How to tell if they're running and how to stop them

This is a semi-common question when programming with GCD.

The short answer is that GCD does not have a cancelation API for queues. The rationale:

1. memory management would become vastly more complicated, because a given block might be responsible for free()ing a given allocation of memory. By always running the block, GCD ensures that memory management remains easy.
2. It is practically impossible to halt a running block without corrupting state.
3. Most code that needs cancellation logic is already tracking that state in private data structures.

Given all of these cases, it is far more efficient and powerful to write code like this:

dispatch_async(my_obj->queue, ^{    bool done = false;    // do_full_update() takes too long, therefore:    while ( !my_obj->cancelled && !done ) {        done = do_partial_update(my_obj);    }});

Oh, and to know if a queue has finished running all of the enqueued blocks, your code can simply execute an empty block with the synchronous API:

dispatch_sync(my_obj->queue, ^{});

As mentioned in the comments, a better way of knowing when your work is done is to use dispatch groups. Dispatch all your blocks to the group and then you can add a completion handler to the group. Once the work is complete, the completion block will run.

dispatch_group_t myGroup = dispatch_group_create();dispatch_group_async(myGroup, my_obj->queue, ^{    bool done = false;    while ( !my_obj->cancelled && !done ) {        done = do_partial_update(my_obj);    }});dispatch_group_notify(myGroup, my_obj->queue, ^{    NSLog(@"Work is done!");    dispatch_release(myGroup);});

Once all of your blocks have completed, the group will be empty and trigger the notification block. From there, you can update UI, etc.

Good luck and have fun!

How to tell if is running

BOOL dispatch_queue_is_empty(dispatch_queue_t queue){    dispatch_group_t group = dispatch_group_create();    dispatch_group_enter(group);    dispatch_async(queue, ^{        dispatch_group_leave(group);    });    int64_t maxWaitTime = 0.00000005 * NSEC_PER_SEC;    BOOL isReady = dispatch_group_wait(group, maxWaitTime) == 0;    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);        dispatch_release(group);    });    return isReady;}

To test in app

dispatch_queue_t queue = dispatch_queue_create("test", 0);NSLog(@"Is empty %@", dispatch_queue_is_empty(queue) ? @"YES" : @"NO");dispatch_async(queue, ^{    for(int i = 0; i < 100; i++)    {        NSLog(@"... %i", i);    }});NSLog(@"Is empty %@", dispatch_queue_is_empty(queue) ? @"YES" : @"NO");

Result

Is empty YESIs empty NO... 0... 1... 2... 3... 4... 5... 6... 7... 8... 9

The default value for the variable maxWaitTime can be tweaked to wanted result.

If you have a serial dispatch queue OR a concurrent dispatch queue, here is a code that can do the same thing.

BOOL __block queueIsEmpty = false;dispatch_barrier_async (_dispatchQueue, ^{    queueIsEmpty = true;});while (!queueIsEmpty) {    int i = 0;  // NOOP instruction}// At this point your queue should be empty.