[SHVDN3] NullReferenceException when calling Notification.Show from websocket eventhandler

Slluxx

Hey, i want to run a websocket server and recieve commands/messages made by some other client. The websockt server runs fine and i can connect to it but it seems like i can not execute anything GTA related inside the eventhandler and i dont know why or how to fix it. Help is apprechiated!

Error:

[01:32:26] [ERROR] Caught fatal unhandled exception:
System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei SHVDN.ScriptDomain.ExecuteTask(IScriptTask task)
   bei SHVDN.NativeFunc.Invoke(UInt64 hash, UInt64* argPtr, Int32 argCount)
   bei GTA.Native.Function.Call(Hash hash, InputArgument argument0)
   bei GTA.UI.Notification.Show(String message, Boolean blinking)
   bei Cmod.WebsocketServer.OnWebsocketEvent(Object sender, WebSocketEventArg args)
   bei SimpleWebSocketServerLibrary.SimpleWebSocketHandler.WebSocketHandler.OnWebsocketEvent(Object sender, WebSocketEventArg arg)
   bei SimpleWebSocketServerLibrary.WSocketServer.WebSocketServer.<StartServerAsync>d__10.MoveNext()
--- Ende der Stapelüberwachung vom vorhergehenden Ort, an dem die Ausnahme ausgelöst wurde ---
   bei System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   bei System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   bei SimpleWebSocketServerLibrary.SimpleWebSocketHandler.WebSocketHandler.<StartConnection>d__6.MoveNext()
--- Ende der Stapelüberwachung vom vorhergehenden Ort, an dem die Ausnahme ausgelöst wurde ---
   bei System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   bei System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   bei System.Threading.ThreadPoolWorkQueue.Dispatch()

Code:


using System;
using GTA;
using GTA.Native;
using System.Windows.Forms;
using SimpleWebSocketServerLibrary;
using System.Text;

namespace Cmod {


// Other code.


    public class WebsocketServer : Script
    {
        public SimpleWebSocketServer websocketServer;

        public WebsocketServer()
        {
            // this.KeyUp += onKeyUp;

            SimpleWebSocketServerSettings settings = new SimpleWebSocketServerSettings();
            settings.port = 9091;

            websocketServer = new SimpleWebSocketServer(settings);
            websocketServer.WebsocketServerEvent += OnWebsocketEvent;
            websocketServer.StartServer();
            GTA.UI.Notification.Show("WS STARTED");

        }

        public static void OnWebsocketEvent(object sender, WebSocketEventArg args)
        {
            try
            {
                if (args.data != null && args.isText)
                {
                    string received = Encoding.UTF8.GetString(args.data);
                    // _WebsocketServer.SendTextMessage("Client: " + args.clientId + " on url: " + args.clientBaseUrl + ", says: " + received);
                    GTA.UI.Notification.Show("MSG: " + received);
                }
            }
            catch (Exception)
            {
                throw;
            }
        }
    }
}

JustDancePC

@Slluxx Because the code executed on the socket event is not running on the main thread, and you can only call GTA functions from the main thread

Slluxx

@JustDancePC Ah i see, thanks! Is there a good/efficient way around that (eg can i
somehow execute code back on the main thread from within the socket thread)? Or is there nothing i can do about that?

JustDancePC

@Slluxx Probably not the best solution, but what I do in these cases is use a static variable as a flag inside a function that runs on tick, that way I can check every frame if my GTA-related code should run:

private static bool _flag;

private void MyEvent() {
  //Event triggered
  _flag = true;
}

private void OnTick() {
  if (_flag) {
    //Call GTA functions
    _flag = false;
  }
}

Slluxx

@JustDancePC Yeah i guess that would work but as you said, its kind of jank. With websockets i would like to get 2 variables (socket & whatever has been transmitted). storing those inside classes to get them from a different thread by looking at it every tick is probably not smart. I have found synchronization context but didnt manage to make that work either.

Slluxx

I finally did it myself with the help of this SO thread.

Here is an example snippe, but PLEASE read the SO thread. This example always assumes to be in a different thread, which might or might not (always) be the case for you.

    public class MyClass : Script
    {

        private Dispatcher _uiDispatcher = Dispatcher.CurrentDispatcher;

        public MyClass ()
        {
            var thing = new SomeThing();
            thing.runInDifferentThread(socket =>
            {
                socket.OnMessage = message =>
                {
                    _uiDispatcher.Invoke(delegate ()
                    {
                        OnMessage_Worker(socket, message);
                    });
                };
            });
        }

        public void OnMessage_Worker(IWebSocketConnection socket, string message){
            GTA.UI.Notification.Show(message);
            socket.Send("OK");
        }
    }

JustDancePC

@Slluxx Oh, awesome 👌