Waiting for asynchronous calls in a swift script

Question

Im writing a swift script to be run in terminal that dispatches to the background thread a couple of operations. Without any extra effort, after all my dispatching is done, the code reaches the end of the file and quits, killing my background operations as well. What is the best way to keep the swift script alive until my background operations are finished?

The best I have come up with is the following, but I do not believe this is the best way, or even correct.

var semaphores = [dispatch_semaphore_t]()
while x {
  var semaphore = dispatch_semaphore_create(0)
  semaphores.append(semaphore)
  dispatch_background {
    //do lengthy operation
    dispatch_semaphore_signal(semaphore)
  }
}

for semaphore in semaphores {
  dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
}

possible duplicate of [Multiple workers in Swift Command Line Tool](http://stackoverflow.com/questions/28590701/multiple-workers-in-swift-command-line-tool) — Aaron Brager, Feb 27 '15 at 20:32

score 8 · Answer 1 · answered Aug 13 '19 at 14:58

8

In addition to using dispatch_groups, you can also do the following:

yourAsyncTask(completion: {
    exit(0)
})

RunLoop.main.run()

Some resources:

answered Aug 13 '19 at 14:58

jason z

1,377
13
19

score 2 · Accepted Answer · edited May 23 '17 at 12:32

2

Thanks to Aaron Brager, who linked to Multiple workers in Swift Command Line Tool ,

which is what I used to find my answer, using dispatch_groups to solve the problem.

edited May 23 '17 at 12:32

Community

1
1

answered Mar 04 '15 at 20:28

Gagan Singh

988
12
22

score 0 · Answer 3 · answered Feb 27 '15 at 23:49

0

How about something like this:

func runThingsInTheBackground() {
    var semaphores = [dispatch_semaphore_t]()

    for delay in [2, 3, 10, 7] {
        var semaphore = dispatch_semaphore_create(0)
        semaphores.append(semaphore)

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) {
            sleep(UInt32(delay))
            println("Task took \(delay) seconds")

            dispatch_semaphore_signal(semaphore)
        }
    }

    for semaphore in semaphores {
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
    }
}

This is very similar to what you have. My work 'queue' is an array of seconds to sleep, so that you can see that things are happening int he background.

Do note that this just runs all the tasks in the background. If you want to limit the number of active tasks to for example the number of CPU cores then you have to do a little more work.

Not sure if that is what you were looking for, let me know.

answered Feb 27 '15 at 23:49

Stefan Arentz

34,311
8
67
88

Maybe my question was unclear, but this is essentially exactly what I posted above. Thanks for your post though! – Gagan Singh Mar 04 '15 at 20:26
My example works. What is missing? I think it answers your question? – Stefan Arentz Mar 05 '15 at 00:13
Im sure your code works, but your code is almost the same as mine, not really adding anything. – Gagan Singh Mar 09 '15 at 16:50
It runs tasks in the background, waiting for them to finish. If that is not what you wanted then let me know and I'll change the code for a better answer. – Stefan Arentz Mar 09 '15 at 22:42

Waiting for asynchronous calls in a swift script

3 Answers3