Client-side NetworkList has duplicated values from server-side list after a specific scene flow

[elton-okawa]

Describe the bug
Client-side NetworkList has duplicated values from server-side list after a specific scene flow

Initially, everything syncs correctly but after leaving the ListScene and going back, it seems that client-side NetworkList is synched with values from server and also receive add NetworkListEvents which causes the observed duplication

To Reproduce

1. Clone this repository: https://github.com/elton-okawa/issue-unity-netcode-network-list-duplication
2. Start an editor and a clone using ParrelSync menu
3. Start as host in the editor
4. Start as client in the clone
5. Notice that both values is correctly synched and there are logs showing current NetworkList values
6. In host window, change the scene by clicking on DummyScene button
7. In host window, go back to the first scene by clicking on ListScene button
8. Notice that client window has duplicated values and its logs shows the same state

Actual outcome
Client-side NetworkList has 0, 1, 0, 1 values while server-side has 0, 1

Expected outcome
Client-side and server-side NetworkList with same values 0, 1

Environment:

• OS: Windows 10
• Unity Version: 2020.3.12f1
• Netcode Version: 1.0.0-pre.4
• Unity Transport for Netcode: 1.0.0-pre.4

[ashwinimurt]

Tracked in our backlog MTT-3021

[jeffreyrainy]

Hi @elton-okawa ,

Thanks for the report, that's a good one. I'll be working on a fix. For now, all I can offer is a workaround, that is, not updating the NetworkList during OnNetworkSpawn.

Please see diff:

--- a/Assets/Scripts/NetworkListController.cs
+++ b/Assets/Scripts/NetworkListController.cs
@@ -32,6 +32,7 @@ public class NetworkListController : NetworkBehaviour {
         lobbyList = new NetworkList<LobbyData>();
     }

+    private bool UpdateList = false;
     public override void OnNetworkSpawn() {
         base.OnNetworkSpawn();

@@ -39,14 +40,23 @@ public class NetworkListController : NetworkBehaviour {
             lobbyList.OnListChanged += HandleListChanged;
         }

-        if (IsServer) {
-            PopulateWithClientsConnected();
-
+        if (IsServer)
+        {
+            UpdateList = true;
             NetworkManager.Singleton.OnClientConnectedCallback += HandleClientConnected;
             NetworkManager.Singleton.OnClientDisconnectCallback += HandleClientDisconnected;
         }
     }

+    void Update()
+    {
+        if (UpdateList)
+        {
+            PopulateWithClientsConnected();
+            UpdateList = false;
+        }
+    }
+
     public override void OnDestroy() {
         base.OnDestroy();

You can go this way until we put in the proper fix.