Cannot load a scene from the OnUnloadEventCompleted

[tangx246]

Description

When trying to load a scene from OnUnloadEventCompleted, the scene does not load.

This is reported in a forum post with a workaround: https://forum.unity.com/threads/you-cannot-load-a-scene-from-the-onunloadeventcompleted-callback.1267208/

Reproduce Steps

private void SceneManager_OnUnloadEventCompleted(string sceneName, LoadSceneMode loadSceneMode, List<ulong> clientsCompleted, List<ulong> clientsTimedOut)
        {
            if (IsServer)
            {
               // this will NOT work
// NetworkManager.Singleton.SceneManager.LoadScene(m_ActiveEncounter, LoadSceneMode.Additive);
 
                // it will work if you wait a frame.
                StartCoroutine(OnUnloadEventCompleted());
            }
        }
 
private IEnumerator OnUnloadEventCompleted()
        {
            yield return 0;
            NetworkManager.Singleton.SceneManager.LoadScene(m_ActiveEncounter, LoadSceneMode.Additive);
}

Actual Outcome

The scene does not load

Expected Outcome

The scene should load without having to wait a frame for the LoadScene call

Screenshots

If applicable, add screenshots to help explain your problem.

Environment

• OS: Windows 10
• Unity Version: 2021.3.2
• Netcode Version: 1.0.0-pre.8

[will-mearns]

Hey @NoelStephensUnity , can you verify/triage this one?

[NoelStephensUnity]

Hi @tangx246,
I am sorry you are running into an issue with the NetworkSceneManager. Hopefully I can help you navigate around this issue.
If you check the status output of the LoadScene method like:
var status = NetworkManager.Singleton.SceneManager.LoadScene(m_ActiveEncounter, LoadSceneMode.Additive);
You will most likely get a SceneEventProgressStatus.SceneEventInProgress if you try to load a scene upon the unloading of another scene notifying it has completed.
The recommended action, if you want to load a scene upon the unloading of another, is to handle the loading of the scene in another coroutine or have some form of state change (server-side) that will then load the scene the next frame.
Your work around appears to follow the recommended usage pattern.

[tangx246]

Needing to load the scene in the next frame is quite unintuitive, and seems to be undocumented (at least in the API reference - https://docs-multiplayer.unity3d.com/netcode/current/api/Unity.Netcode.NetworkSceneManager#onunloadeventcompleted)

We've found the workaround so this is somewhat lower priority, and I think unintuitive is fine as long as it's documented

[NoelStephensUnity]

@tangx246
You make a good point and I will look into adjusting the xml documentation to make a note of this.

[NoelStephensUnity]

@tangx246
PR-1974 was merged and the xml documentation API should be updated hopefully sometime after v1.0.0-pre.10. If not, it should be updated by v1.0.0 release.

Thank you for your patience and feedback to help us improve Netcode for GameObjects!

Cheers,
Noel