Nothing prints out in the console in command line tool Xcode
<p>Nothing prints out in the console in command line tool Xcode when I run the following code:</p>

<pre><code>import Foundation

class A {
  var someValue = 0

  let concurrentQueue = dispatch_queue_create("queue_for_property", DISPATCH_QUEUE_CONCURRENT)

  func increaseValueBy1000() {
    dispatch_barrier_async(concurrentQueue) {
      for _ in 0 ..&lt; 1000 {
        let v = self.someValue + 1
        print(v)
        self.someValue = v
      }
    }
  }
}

let instance1 = A()

dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)) {
  instance1.increaseValueBy1000()
}

instance1.increaseValueBy1000()
</code></pre>

<p>I don't see any print statement in the console.
If I remove barrier line works pretty fine.
What I do wrong in this case why my barriers don't allow to print?</p>