title: Usernotification when the screen is locked
body: <p>I want set notifications to fire, even if the screen is locked. 
  The code: </p>
  
  <pre><code>let request = UNNotificationRequest(identifier: String(index), content: content, trigger: trigger)
  let center = UNUserNotificationCenter.current()
  center.add(request, withCompletionHandler: nil)
  </code></pre>
  
  <p>It works like a charm,while the app is in the background. But when the screen is locked there is no vibration and no sound. It seems like the notification is not being delivered. How can I make this work?</p>