Thank you so much eskimo. Thanks to your answers, I will be able to do what I want to do!

Thanks for your reply eskimo. My target platform is iOS. And I am using network extension (packet tunnel provider). I would like to know how startTunnel() and stopTunnel(), which exist in the NEPacketTunnelProvider, use NotificationCenter to send messages to the ViewController. Kind regards

Thank you for your kind reply. When I made the changes as you suggested, I was able to use DispatchNotificationCenter. I had overlooked the Availability in the documentation. Thanks,