Added a new internal message that is sent out to all the clients when…

… the server recieves acknowledgement that all clients have successfully loaded the scene (or timed out)

com.unity.multiplayer.mlapi/Runtime/Configuration/NetworkConstants.cs

@@ -18,6 +18,7 @@ internal static class NetworkConstants
         internal const byte TIME_SYNC = 11;
         internal const byte NETWORK_VARIABLE_DELTA = 12;
         internal const byte NETWORK_VARIABLE_UPDATE = 13;
+        internal const byte ALL_CLIENTS_SWITCH_SCENE_COMPLETED = 14;
         internal const byte UNNAMED_MESSAGE = 20;
         internal const byte DESTROY_OBJECTS = 21;
         internal const byte NAMED_MESSAGE = 22;
@@ -42,7 +43,7 @@ internal static class NetworkConstants
             "TIME_SYNC",
             "NETWORK_VARIABLE_DELTA",
             "NETWORK_VARIABLE_UPDATE",
-            "",
+            "ALL_CLIENTS_SWITCH_SCENE_COMPLETED",
             "",
             "", // 16
             "",

com.unity.multiplayer.mlapi/Runtime/Core/NetworkManager.cs

@@ -1170,6 +1170,13 @@ internal void HandleIncomingData(ulong clientId, NetworkChannel networkChannel,
                             NetworkLog.LogWarning($"Server received {nameof(NetworkConstants.CLIENT_SWITCH_SCENE_COMPLETED)} from client id {clientId}");
                         }
 
+                        break;
+                    case NetworkConstants.ALL_CLIENTS_SWITCH_SCENE_COMPLETED:
+                        if (IsClient)
+                        {
+                            MessageHandler.HandleAllClientsSwitchSceneCompleted(clientId, messageStream);
+                        }
+
                         break;
                     case NetworkConstants.SERVER_LOG:
                         if (IsServer && NetworkConfig.EnableNetworkLogs)

com.unity.multiplayer.mlapi/Runtime/Messaging/IInternalMessageHandler.cs

@@ -23,5 +23,6 @@ internal interface IInternalMessageHandler
         void HandleUnnamedMessage(ulong clientId, Stream stream);
         void HandleNamedMessage(ulong clientId, Stream stream);
         void HandleNetworkLog(ulong clientId, Stream stream);
+        void HandleAllClientsSwitchSceneCompleted(ulong clientId, Stream stream);
     }
 }

com.unity.multiplayer.mlapi/Runtime/Messaging/InternalMessageHandler.cs

@@ -37,6 +37,7 @@ internal class InternalMessageHandler : IInternalMessageHandler
         private static ProfilerMarker s_HandleNetworkLog = new ProfilerMarker($"{nameof(InternalMessageHandler)}.{nameof(HandleNetworkLog)}");
         private static ProfilerMarker s_RpcReceiveQueueItemServerRpc = new ProfilerMarker($"{nameof(InternalMessageHandler)}.{nameof(RpcReceiveQueueItem)}.{nameof(RpcQueueContainer.QueueItemType.ServerRpc)}");
         private static ProfilerMarker s_RpcReceiveQueueItemClientRpc = new ProfilerMarker($"{nameof(InternalMessageHandler)}.{nameof(RpcReceiveQueueItem)}.{nameof(RpcQueueContainer.QueueItemType.ClientRpc)}");
+        private static ProfilerMarker s_HandleAllClientsSwitchSceneCompleted = new ProfilerMarker($"{nameof(InternalMessageHandler)}.{nameof(HandleAllClientsSwitchSceneCompleted)}");
 #endif
 
         public NetworkManager NetworkManager => m_NetworkManager;
@@ -606,6 +607,22 @@ public void HandleNetworkLog(ulong clientId, Stream stream)
             }
 #if DEVELOPMENT_BUILD || UNITY_EDITOR
             s_HandleNetworkLog.End();
+#endif
+        }
+
+        public void HandleAllClientsSwitchSceneCompleted(ulong clientId, Stream stream)
+        {
+#if DEVELOPMENT_BUILD || UNITY_EDITOR
+            s_HandleAllClientsSwitchSceneCompleted.Begin();
+#endif
+            using (var reader = PooledNetworkReader.Get(stream))
+            {
+                var clientIds = reader.ReadULongArray();
+
+                NetworkManager.SceneManager.AllClientsReady(clientIds);
+            }
+#if DEVELOPMENT_BUILD || UNITY_EDITOR
+            s_HandleAllClientsSwitchSceneCompleted.End();
 #endif
         }
     }

com.unity.multiplayer.mlapi/Runtime/SceneManagement/NetworkSceneManager.cs

@@ -33,6 +33,8 @@ public class NetworkSceneManager
 
         public delegate void OnAllClientsLoadedSceneDelegate(SceneSwitchProgress progress, bool timedOut);
 
+        public delegate void AllClientsReadyDelegate(ulong[] clientIds);
+
         /// <summary>
         /// Event that is invoked when the scene is switched
         /// </summary>
@@ -43,10 +45,21 @@ public class NetworkSceneManager
         /// </summary>
         public event SceneSwitchStartedDelegate OnSceneSwitchStarted;
 
+        /// <summary>
+        /// Event that is invoked on the server when a client completes scene transition
+        /// </summary>
         public event OnClientLoadedSceneDelegate OnClientLoadedScene;
 
+        /// <summary>
+        /// Event that is invoked on the server when all clients have reported that they have completed scene transition
+        /// </summary>
         public event OnAllClientsLoadedSceneDelegate OnAllClientsLoadedScene;
 
+        /// <summary>
+        /// Event that is invoked on the client after all clients have successfully completed scene transition
+        /// </summary>
+        public event AllClientsReadyDelegate OnAllClientsReady; 
+
         internal readonly HashSet<string> RegisteredSceneNames = new HashSet<string>();
         internal readonly Dictionary<string, uint> SceneNameToIndex = new Dictionary<string, uint>();
         internal readonly Dictionary<uint, string> SceneIndexToString = new Dictionary<uint, string>();
@@ -149,7 +162,17 @@ public SceneSwitchProgress SwitchScene(string sceneName)
             CurrentSceneSwitchProgressGuid = switchSceneProgress.Guid;
 
             switchSceneProgress.OnClientLoadedScene += clientId => { OnClientLoadedScene?.Invoke(switchSceneProgress, clientId); };
-            switchSceneProgress.OnComplete += timedOut => { OnAllClientsLoadedScene?.Invoke(switchSceneProgress, timedOut); };
+            switchSceneProgress.OnComplete += timedOut =>
+            {
+                OnAllClientsLoadedScene?.Invoke(switchSceneProgress, timedOut);
+
+                using (var buffer = PooledNetworkBuffer.Get())
+                using (var writer = PooledNetworkWriter.Get(buffer))
+                {
+                    writer.WriteULongArray(switchSceneProgress.DoneClients.ToArray(), switchSceneProgress.DoneClients.Count);
+                    m_NetworkManager.MessageSender.Send(NetworkManager.Singleton.ServerClientId, NetworkConstants.ALL_CLIENTS_SWITCH_SCENE_COMPLETED, NetworkChannel.Internal, buffer);
+                }
+            };
 
             // Move ALL NetworkObjects to the temp scene
             MoveObjectsToDontDestroyOnLoad();
@@ -490,5 +513,10 @@ private void MoveObjectsToScene(Scene scene)
                 SceneManager.MoveGameObjectToScene(sobj.gameObject, scene);
             }
         }
+
+        internal void AllClientsReady(ulong[] clientIds)
+        {
+            OnAllClientsReady?.Invoke(clientIds);
+        }
     }
 }

com.unity.multiplayer.mlapi/Tests/Editor/NetworkManagerMessageHandlerTests.cs

@@ -333,6 +333,8 @@ internal class DummyMessageHandler : IInternalMessageHandler
         public void HandleNamedMessage(ulong clientId, Stream stream) => VerifyCalled(nameof(HandleNamedMessage));
 
         public void HandleNetworkLog(ulong clientId, Stream stream) => VerifyCalled(nameof(HandleNetworkLog));
+
+        public void HandleAllClientsSwitchSceneCompleted(ulong clientId, Stream stream) => VerifyCalled(nameof(HandleAllClientsSwitchSceneCompleted));
 
         private void VerifyCalled(string method)
         {