public void SubscribeEvent(SystemListenerEventHandler func)
        {
            bool needToStart = (SystemListenerEvent == null);

            SystemListenerEvent += new SystemListenerEventHandler(func);
            if (needToStart)
            {
                StartListening();
            }
        }
 public void UnsubscribeEvent(SystemListenerEventHandler func)
 {
     SystemListenerEvent -= func;
 }