title: Why userNotificationCenter didReceive is not fired?
body: <p>This is a part of my code. I want to use <code>UNUserNotificationCenter</code> and <code>UNUserNotificationCenterDelegate</code> to handle notification events.</p>
  
  <p>This code catches the notification event when the app is in a foreground state. But <code>"didReceive"</code> is not fired for a background state.</p>
  
  <pre><code>func application(_ application: UIApplication,
                           didFinishLaunchingWithOptions launchOptions:
              [UIApplication.LaunchOptionsKey : Any]? = nil) -&gt; Bool {
              UNUserNotificationCenter.current().delegate = self
      application.registerForRemoteNotifications()
      return true
      }    
          func userNotificationCenter(_ center: UNUserNotificationCenter,
                                      willPresent notification: UNNotification,
                                      withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -&gt; Void)
          {
              print("willPresent") /// This works
              completionHandler([.alert, .badge, .sound])
          }
          func userNotificationCenter(_ center: UNUserNotificationCenter,
                                      didReceive response: UNNotificationResponse,
                                      withCompletionHandler completionHandler: @escaping () -&gt; Void) {
              print("didReceive") /// This doesn't work
              completionHandler()
          }
  </code></pre>
  
  <p>But if I don't use <code>UNUserNotificationCenter.current().delegate = self</code> the delegate method is correctly fired in the background.</p>
  
  <pre><code>func application(_ application: UIApplication,
                       didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                       fetchCompletionHandler completionHandler: @escaping FetchCompletionHandler) {
          print("didReceiveRemoteNotification... \(userInfo)")
  }
  </code></pre>
  
  <p>How can I use "didReceive"? I want to handle the notification in the background.</p>