NetworkManager.Singlton.CustomMessagingManager is always null before IsListening

[UnityGeorgeLi]

NetworkManager.Singlton.CustomMessagingManager is not available before isListening is true. It means that the developer can't hookup listeners before the host, server, or client gets started.
There is no document that tells developers to wait till isListening to be true.

[LukeStampfli]

So the core issue is that many of our systems which live under NetworkManager are at null and then get created during the Initialize function when we start a client on a server. This is a problem because there is no point to interact with those systems.

We have hack fixes in our code base for PrefabHandler but that's not sustainable:
code from network manager for creating a prefab handler if none exists.

   public NetworkPrefabHandler PrefabHandler
        {
            get
            {
                if (m_PrefabHandler == null)
                {
                    m_PrefabHandler = new NetworkPrefabHandler();
                }

                return m_PrefabHandler;
            }
        }

I think there are a bunch of better approaches we can do here, for instance:

1. Create all sub systems in awake. During initialize call an initialize function on the already existing systems.

2. Expose an Initialized event on NetworkManager which should be used by users for setup/configuration. This might be not so great UX wise.

[luismasuelli]

I like the "Expose an Initialized event" approach to initialize the sub-systems (which IMHO should be already created in Awake, as you said). I would consider these events to be added (they would run in this order, if one runs Host mode, and some events would be excluded if running in dedicated server or client):

OnInitialized - Your suggestion I support.
OnHostStarted - IIRC it does not exist, yet.
OnServerStarted
OnClientConnected - In my opinion, also hosts' local connection should trigger this (this is not a bug but a design change).
OnClientDisconnect - In my opinion, also hosts' local connection should trigger this (this is not a bug but a design change).
OnServerStopped - I suggested this event in another issue.
OnHostStopped - IIRC it does not exist yet.
OnShutdown - A complement I add to your suggestion.

[LukeStampfli]

Unassigned myself since I'm not sure why I was assigned.

[NoelStephensUnity]

I am not quite sure why you would want to hookup listeners to NetworkManager.Singlton.CustomMessagingManager prior to establishing a connection because the server-host must be started in order to send or receive custom messages and clients need to be connected (and approved if applicable) before sending or receiving custom messages.

The current advised solution for registering NetworkManager.Singlton.CustomMessagingManager listeners would be to register for the OnServerStarted callback (server-host side) and the OnClientConnectedCallback (client-side) and both call the same method that registers with the CustomMessagingManager. While I can see where registering prior to connecting might simplify the listener registration process, there are many systems that are not available until a Server-Host or Client has been started.

Definitely this should be covered in the documentation, with the possibility that we could improve this user experience. The addressables pending PR-1758 has some updates in NetworkManager (i.e. adding states) that could be modified to invoke a callback when certain NetworkManager states are set and this could provide a more central/unified way for users to register for different sub-systems (i.e. CustomMessagingManager, NetworkSceneManager, etc).

[JesseOlmer]

Agreed this is likely a documentation thing for now since the "system not initialized yet" approach is very common in the codebase. I'd suggest we address this in the CustomMessagingManager xmldoc and maybe even a dedicated documentation page. This one's slightly tricky in some cases, but it seems to be doable at any point in the connection flow, even before connection approval:

    void ConnectionApproval(byte[] _, ulong connectionId, NetworkManager.ConnectionApprovedDelegate del)
    {
        // Send a message before connection approval is complete
        nm.CustomMessagingManager.SendNamedMessage("CustomTestMessage", connectionId, new FastBufferWriter(0, Allocator.Temp));
        del(false, null, true, null, null);
    }
    
    public void StartClient()
    {
        nm.StartClient();
        // Option 1: CMM is available at this point, and as long as you don't let another frame happen before you register you should be OK
        nm.CustomMessagingManager.RegisterNamedMessageHandler("CustomTestMessage", MessageHandler);
    }

    void OnClientConnected(ulong clientId)
    {
        if (clientId != nm.LocalClientId)
            return;

        // Option 2: This is too late to receive the message, and it will be dropped.
        nm.CustomMessagingManager.RegisterNamedMessageHandler("CustomTestMessage", MessageHandler);
    }

[ashwinimurt]

MTT-3459