func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                 fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {

  // Print message ID.
  if let messageID = userInfo[gcmMessageIDKey] {
    print("Message ID: (messageID)")
  }

  // Print full message.
  print(userInfo)

  completionHandler(UIBackgroundFetchResult.newData)
}

-- cordova 9.0.0
-- swift 5