Using GCD in Swift CLI

Question

So I'm trying to use GCD in the CLI. To test it out i wrote some code like this:

import Foundation

var i = 0

print("a: ",i)

dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)) {

    for n in 1..<10{
        i++
    }

    print("c: ",i)

    dispatch_async(dispatch_get_main_queue()){
        print("d: ",i)
    }
}

print("b: ",i)

sleep(5)
print("e: ",i)

the output of this is: a: 0 b: 0 c: 9 e: 9

with the last line printing a few seconds later. What I'm trying to find out is what happened at d? Nothing I put in that block seems to execute. This works fine when I use it in iOS, just not in the CLI.

the execution terminated before print("d: ",i) has a chance to execute. add to the end your code dispatch_main() to interrupt execution use cmd-c — user3441734
– user3441734, Commented Jan 16, 2016 at 18:41

matt · Accepted Answer · 2016-01-16 21:19:56Z

1

The CLI lacks the persistence of an app. It came to an end (terminated) before d had a chance to be printed.

As @user3441734 rightly points out, you can work around this in the CLI by calling dispatch_main() as the last thing before exit. This call effectively forces us to dip into the main queue right now and pull out the main-queued block and execute it, before exiting.

edited Jan 16, 2016 at 21:19

answered Jan 16, 2016 at 16:01

matt

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4 Comments

user3441734 Over a year ago

dispatch_main() is the answer ( this executes blocks submitted to the main queue)

matt Over a year ago

Right you are. This call effectively forces main() to dip into the main queue right now and pull out the queued block and execute it, before exiting. You (or @user3441734) should give this as an answer (and, if it's you, in a couple of days you can accept it): I'll certainly upvote it!

user3441734 Over a year ago

update your answer, please. i am in the middle of nothing (no electricity and my battery will died in minutes ..) if it can helps somebody, that's fine ...

matt Over a year ago

@user3441734 OK, I did that. But the real answer is still yours.

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