Stop async dispatched queue on button clicked iphone

Question

Once I start a queue it takes almost 3-4 minutes, In case I want to stop this queue on a button(cancel button) click then can I do so?? If yes then how ?

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        @synchronized(self)
        {
            for (int i = 0; i < (4000); i++) {
                         (Some methods)
            }
        }
});

Can I stop or dismiss this thread?

score 2 · Answer 1 · edited May 23 '17 at 11:51

2

First don't synchronize in a gcd block. Use serial queues instead.

That being said, there is no way to stop a gcd block, but there is a way around it. In your queue you have a loop: Using a boolean flag you can simply exit your loop and basically terminate the gcd block.

See an example here

edited May 23 '17 at 11:51

Community

1
1

answered Jul 06 '13 at 07:17

Lefteris

14,550
2
56
95

The loop thing is not enough because the task are async. Tasks that are already in the queue when you flag for interruption will be executed anyway. The idea is being able to cancel future and present tasks. – Duck Jun 10 '15 at 22:36

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

1

Your first issue is that you block the main thread by doing a dispatch_sync

BUT as an answer to your question:

you should use a NSOperationQueue. that can be suspended between operations easily

off the top of my head:

NSOperationQueue *_myQueue; //instance var

_myQueue = [[NSOperationQueue alloc] init]; //init it

_myQueue.suspended = (buttonPressed) ? YES : NO; //toggle it like you need

for (int i = 0; i < (4000); i++) {
   [_queue addOperationWithInvocation:NSInvocation for method to call];  
}

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 06 '13 at 08:21

Daij-Djan

49,552
17
113
135

the import thing that a single block (GCD) or a single operation can't be stopped easily. but if you break it up, you can stop in between – Daij-Djan Jul 06 '13 at 08:29
instead of suspend there is also cancel ! – Daij-Djan Jul 06 '13 at 08:29
You cannot suspend a gcd block that started running, only those that haven't started yet. – Lefteris Jul 06 '13 at 22:23
that's exactly what I said Lefteris... and that is why I split up that block – Daij-Djan Jul 06 '13 at 22:36

Stop async dispatched queue on button clicked iphone

2 Answers2

you should use a NSOperationQueue. that can be suspended between operations easily