Scenes loaded on the client before connecting are unloaded if the server does not have them loaded

[JayPeet]

Description

We load a scene called Client which sticks around for the entire game. It contains client specific stuff (such as the player).
We has overwrote the VerifySceneBeforeLoading on SceneManager to just allow any scene to be loaded, as a quick hack to allow us to have the Client scene loaded while having the server synchronize the loaded scenes.

As of commit e0bd7eb (PR: #2383) that functionality has been broken, since it now unloads every scene not on the server.

a TODO exists in the new DefaultSceneManagerHandler which says that we need a way to validate if an unload should happen. On my fork (https://github.com/JayPeet/com.unity.netcode.gameobjects/tree/develop) I added a VerifySceneBeforeUnloading and hook it into the DefaultSceneManagerHandlers UnloadUnassignedScenes so we can bypass it for certain scenes.

Not sure if this is how the NGO team would prefer to fix this, so no PR has been opened.

[NoelStephensUnity]

@JayPeet
Ahh... yeah I haven't finished the documentation for this yet and it is not slated to be officially released until after the up-coming v1.3.0 release (i.e. most likely in v1.4.0)

What you are most likely wanting is to change the client synchronization mode on the server side.

                networkManager.StartHost();
                networkManager.SceneManager.SetClientSynchronizationMode(LoadSceneMode.Additive);

What this does is makes sure that all clients, when they first synchronize, will use any existing scenes and will not try to load a scene in LoadSceneMode.Single when synchronizing a client (i.e. that is most likely what is unloading your client scene).

Here is a project that demonstrates all of those features:
AdditiveClientSynchronization.zip

I also started looking at one area of the code and I am going to assume that you are using LoadSceneMode.Additive for client synchronization?
This is what I ran across (again develop branch... so apologies):

        /// <summary>
        /// Unloads any scenes that have not been assigned.
        /// TODO: There needs to be a way to validate if a scene should be unloaded
        /// or not (i.e. local client-side UI loaded additively)
        /// </summary>
        /// <param name="networkManager"></param>
        public void UnloadUnassignedScenes(NetworkManager networkManager = null)
        {

It would seem that this is most likely what is unloading your scene and so I will add a VerifySceneBeforeUnloading callback that will get invoked during that time to provide you with this capability.

[NoelStephensUnity]

@JayPeet
Just following up with the fix for the issue you were experiencing. The above draft PR will default to not automatically unloading scenes, but provides an additional flag property to NetworkSceneManager that enables this mechanism as well as a VerifySceneBeforeUnloading callback that is only invoked when both the flag and client synchronization mode is set to additive (the only real use case for it).
👍

[JayPeet]

Awesome! Thanks for the update. :)

All seems to do what I need it too, going to close this issue.