class SwiftWebSocketServer {
 let port: NWEndpoint.Port
 var listener: NWListener?
 var listenerState: NWListener.State
 let eventHandler:()->Void
 var connection: ServerConnection?
init(port: UInt16, handler:@escaping ()->Void) {
 self.port = NWEndpoint.Port(rawValue: port)!
 listenerState = .cancelled
 self.eventHandler = handler
 let parameters = NWParameters(tls: nil)
 parameters.allowLocalEndpointReuse = true
 parameters.includePeerToPeer = true
 let wsOptions = NWProtocolWebSocket.Options()
 wsOptions.autoReplyPing = true
 parameters.defaultProtocolStack.applicationProtocols.insert(wsOptions, at: 0)
 do {
 listener = try NWListener(using: parameters, on: self.port)
 listener!.stateUpdateHandler = self.stateDidChange(to:)
 listener!.newConnectionHandler = self.didAccept(nwConnection:)
 } catch {
 print(#function, error)
 }
 }
func start() throws {
 print("Server starting...")
 listener!.stateUpdateHandler = self.stateDidChange(to:)
 listener!.newConnectionHandler = self.didAccept(nwConnection:)
 listener!.start(queue: .main)
 print("Server started.")
 eventHandler()
 }
func stop() {
 self.listener!.stateUpdateHandler = nil
 self.listener!.newConnectionHandler = nil
 self.listener!.cancel()
 print("Server cancelled")
 connection?.stop()
 connection?.didStopCallback = nil
 connection = nil
 eventHandler()
 }
 func stateDidChange(to newState: NWListener.State) {
 print(#function, newState)
 switch newState {
 case .ready:
 print("Server ready.")
 case .failed(let error):
 print("Server failure, error: \(error.localizedDescription)")
 exit(EXIT_FAILURE)
 default:
 break
 }
 listenerState = newState
 eventHandler()
 }
}

Server starting...
2020-08-12 16:29:42.476799-0400 rfa-ios-native[821:464825]  nw listener set queue Error in client: nw listener set queue called after nw listener start
2020-08-12 16:29:42.480218-0400 rfa-ios-native[821:464825]  nw listener set queue Error in client: nw listener set queue called after nw listener start, dumping backtrace:
[arm64] libnetcore-1880.100.30
0 libnetwork.dylib 0x000000019218b060 _ nw create backtrace string + 120
1 libnetwork.dylib 0x00000001920a68bc nw listener set queue + 228
2 libswiftNetwork.dylib 0x00000001c6556e74 $s7Network10NWListenerC5start5queueySo012OS dispatch D0C tF + 56
...
Server started.
2020-08-12 16:29:42.482909-0400 rfa-ios-native[821:464835]  nw listener start block invoke [L1] In wrong state for start
2020-08-12 16:29:42.483533-0400 rfa-ios-native[821:464835]  nw listener start block invoke [L1] In wrong state for start, dumping backtrace:
[arm64] libnetcore-1880.100.30
0 libnetwork.dylib 0x000000019218b060 _ nw create backtrace string + 120
1 libnetwork.dylib 0x00000001920a89e8 6B86071A-0DFC-3EFD-A798-615EC6A7D591 + 3578344
2 libdispatch.dylib 0x000000010068dd10 dispatch call block and_release + 32