feat: initialized callback

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -139,6 +139,11 @@ public bool DAHost
         /// </summary>
         public event OnSessionOwnerPromotedDelegateHandler OnSessionOwnerPromoted;
 
+        /// <summary>
+        /// When a server or client is initializing, before the network connecting is established
+        /// </summary>
+        public event Action OnInitialized;
+
         internal void SetSessionOwner(ulong sessionOwner)
         {
             var previousSessionOwner = CurrentSessionOwner;
@@ -1166,6 +1171,17 @@ internal void Initialize(bool server)
 
             NetworkConfig.InitializePrefabs();
             PrefabHandler.RegisterPlayerPrefab();
+
+            // Invoke initialized callback
+            try
+            {
+                OnInitialized?.Invoke();
+            }
+            catch (Exception)
+            {
+                Shutdown();
+                throw;
+            }
         }
 
         private enum StartType

com.unity.netcode.gameobjects/Tests/Runtime/NetworkManagerInitializedTests.cs

@@ -0,0 +1,61 @@
+using System;
+using NUnit.Framework;
+using Unity.Netcode.Transports.UTP;
+using UnityEngine;
+using Object = UnityEngine.Object;
+
+namespace Unity.Netcode.RuntimeTests
+{
+    internal class NetworkManagerInitializedTests
+    {
+        [Test]
+        [TestCase(true)]
+        [TestCase(false)]
+        public void OnInitializedIsCalled(bool isServer)
+        {
+            var networkManagerObject = CreateNetworkManager(out var networkManager);
+
+            var callbackHit = false;
+
+            networkManager.OnInitialized += () => callbackHit = true;
+            networkManager.Initialize(isServer);
+
+            Assert.True(callbackHit, "OnInitialized callback was never triggered");
+
+            // Clean up
+            Object.Destroy(networkManagerObject);
+        }
+
+        [Test]
+        public void OnInitializedShutsDownOnException()
+        {
+            var networkManagerObject = CreateNetworkManager(out var networkManager);
+
+            networkManager.OnInitialized += () => throw new Exception();
+
+            try
+            {
+                networkManager.StartServer();
+            }
+            catch (Exception)
+            {
+                // do nothing
+            }
+
+            Assert.True(networkManager.ShutdownInProgress, "Manager did not shutdown");
+
+            // Clean up
+            Object.Destroy(networkManagerObject);
+        }
+
+        private GameObject CreateNetworkManager(out NetworkManager networkManager)
+        {
+            var networkManagerObject = new GameObject(nameof(OnInitializedIsCalled));
+
+            var unityTransport = networkManagerObject.AddComponent<UnityTransport>();
+            networkManager = networkManagerObject.AddComponent<NetworkManager>();
+            networkManager.NetworkConfig = new NetworkConfig() { NetworkTransport = unityTransport };
+            return networkManagerObject;
+        }
+    }
+}