OnNetworkDespawn is not called on server side when a client disconnects

[tynitoon]

Description

The function OnNetworkDespawn() on server side is not called when a client leave the game or call the NetworkManager.Singleton.Shutdown() function.

Normaly, OnNetworkDespawn() should be call on server and client :
https://docs-multiplayer.unity3d.com/netcode/current/api/Unity.Netcode.NetworkBehaviour/index.html
In the version 0.1.0 this bug wasn't existing.

Reproduce Steps

1. Create a new project (the bug has been tested on 2021.3.13.f1 and 2021.2.8f1)
2. Create a new gameobject "Network" and add a NetworkManager component and a Unity Transport Component (the bug has also been tested with UNet transport)
3. Create a script "BuggyPlayer" that inherit from NetworkBehaviour and write the function "public override void OnNetworkDespawn()", add a Debug.Log("Despawn"); in it
4. Create a Prefab "Player", and add Network Object Component and your BuggyPlayer component
5. Drag and drop your prefab in the NetworkManager component in the inspector
6. Create a script "DummyHUD" then create 3 functions "void HostServer()", "void JoinServer()", "void ShutdownNetwork()", and call in HostServer function "NetworkManager.Singleton.StartHost();" in JoinServer "NetworkManager.Singleton.StartClient();" and in ShutdownNetwork "NetworkManager.Singleton.Shutdown();".
7. Add your DummyHUD component to the gameobject "Network"
8. create 3 buttons and link them to each of the functions from DummyHUD
9. Make a build
10. Start server on the editor then click your button linked to the HostServer function
11. Launch a client (which you have just built) and click on your button linked to the JoinServer function
12. Check that the scene has 2 Player(clone) in the hierarchy (that means that the client is correctly connected)
13. Click on the client the button linked to "ShutdownNetwork" or close the client (both give the same result)

Actual Outcome

You will see that there is no output "Despawn" in the console of the server side.

Expected Outcome

The output "Despawn" should be visible in the consol on the server side (by the way it is visible on client side)

Environment

• OS: Windows 11
• Unity Version: 2021.3.13.f1 and 2021.2.8f1
• Netcode Version: 1.1.0

[NoelStephensUnity]

@tynitoon
Following your steps I have not been able to replicate this issue.
I used the following script that I attached to our test project's playercube prefab (created a variant):

using UnityEngine;
using Unity.Netcode;

public class LogDespawnAndDestroy : NetworkBehaviour
{
    private string m_PlayerIdentifier = "(Unknown)";

    public override void OnNetworkSpawn()
    {
        if (NetworkObject.IsPlayerObject)
        {
            if (IsServer)
            {
                m_PlayerIdentifier = $"[Server-Side] Player-{OwnerClientId}";
            }
            else
            {
                m_PlayerIdentifier = $"[Client-Side] Player-{OwnerClientId}";
            }
        }
        base.OnNetworkSpawn();
    }

    public override void OnNetworkDespawn()
    {
        Debug.Log($"{m_PlayerIdentifier} NetworkObject despawned!");
        base.OnNetworkDespawn();
    }

    public override void OnDestroy()
    {
        Debug.Log($"{m_PlayerIdentifier} NetworkObject destroyed!");
        base.OnDestroy();
    }
}

I updated the Telport Sample's NetworkManager to use the variant with the above script attached:

The prefab variant used (script is last component):

I then started the editor as a host and the client I ran as a stand alone build:

I then disconnected the client by clicking the "X" top right button which invokes this:

    public void OnExitScene()
    {
        if (NetworkManager.Singleton)
        {
            NetworkManager.Singleton.Shutdown();
            Destroy(NetworkManager.Singleton.gameObject);
        } 

Doing this yielded the following result on the host side:

Performing the same steps but running just as a server (non-host) in the editor:

Yielded the same result as well:

Perhaps you could share a project that replicates this issue?

[tynitoon]

Hello, I tried to copy past your script to have exactly the same thing. But it didn't change the result on my side, so here the project that I'm using to test this bug :
https://github.com/tynitoon/netcode-bug-ondespawn/tree/master/Bug_Networking

here a video where I open the project after a fresh clone and show you the bug :
https://www.youtube.com/watch?v=RDJO0RP2mKI

[NoelStephensUnity]

@tynitoon
Thank you for your project! It really helped me narrow down a rather "odd" bug it would seem.
For the time being, using your same project, move the NetworkObject component above the "Log Despawn And Destroy" script so it looks like this:

Then let me know if you start seeing the OnNetworkDespawn messages appear.

[tynitoon]

Ahah I found the exact same thing 2 minutes ago. Yes it's the component order that cause this bug. If the Network Object component is first, the OnNetworkDespawn() is correctly call. But if it's last, it doesn't.

[NoelStephensUnity]

Correct.
But it is only specific to a client disconnecting from a server and will only happen on the server side.
For all other cases the ordering of components shouldn't matter.
I already found the area in NetworkManager that needs to be adjusted within OnClientDisconnectFromServer

                                SpawnManager.DespawnObject(playerObject, true);
                                //Destroy(playerObject.gameObject);

This just makes sure the player is despawned and then destroyed on the server side if the client initiates the disconnect.

For now, you can just move the NetworkObject component before any NetworkBehaviour components for your player prefab as that is the only time that should occur.

Will have a PR for the fix and it should be included in the update after the coming v1.2 update. 👍