onMessage method
Set a handler
for messages to the socket.
Implementation
Future<void> onMessage(WebsocketHandler handler) async {
var registrationRequest = $wp.RegistrationRequest(
eventType: $wp.WebsocketEventType.Message, socketName: name);
var worker = WebsocketWorker(registrationRequest, handler,
client: _websocketHandlerClient);
await worker.start();
}