fix: NetworkObject component should always precede NetworkBehaviour components [MTT-7216] (#2685)

* fix

MTT-7216
This resolves the issue where an in-scene placed NetworkObject can have NetworkBehaviour components not invoke the OnNetworkDespawn method when doing a full scene transition (LoadSceneMode.Single) due to the way GameObjects destroy the attached components.

* test

This is a unit test to validate the re-ordering of the NetworkObject component.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue where a `NetworkBehaviour` component's `OnNetworkDespawn` was not being invoked on the host-server side for an in-scene placed `NetworkObject` when a scene was unloaded (during a scene transition) and the `NetworkBehaviour` component was positioned/ordered before the `NetworkObject` component. (#2685)
 - Fixed issue where `SpawnWithObservers` was not being honored when `NetworkConfig.EnableSceneManagement` was disabled. (#2682)
 - Fixed issue where `NetworkAnimator` was not internally tracking changes to layer weights which prevented proper layer weight synchronization back to the original layer weight value. (#2674)
 - Fixed "writing past the end of the buffer" error when calling ResetDirty() on managed network variables that are larger than 256 bytes when serialized. (#2670)

com.unity.netcode.gameobjects/Editor/NetworkBehaviourEditor.cs

@@ -417,6 +417,48 @@ public static void CheckForNetworkObject(GameObject gameObject, bool networkObje
                     }
                 }
             }
+
+            if (networkObject != null)
+            {
+                OrderNetworkObject(networkObject);
+            }
+        }
+
+        // Assures the NetworkObject precedes any NetworkBehaviour on the same GameObject as the NetworkObject
+        private static void OrderNetworkObject(NetworkObject networkObject)
+        {
+            var monoBehaviours = networkObject.gameObject.GetComponents<MonoBehaviour>();
+            var networkObjectIndex = 0;
+            var firstNetworkBehaviourIndex = -1;
+            for (int i = 0; i < monoBehaviours.Length; i++)
+            {
+                if (monoBehaviours[i] == networkObject)
+                {
+                    networkObjectIndex = i;
+                    break;
+                }
+
+                var networkBehaviour = monoBehaviours[i] as NetworkBehaviour;
+                if (networkBehaviour != null)
+                {
+                    // Get the index of the first NetworkBehaviour Component
+                    if (firstNetworkBehaviourIndex == -1)
+                    {
+                        firstNetworkBehaviourIndex = i;
+                    }
+                }
+            }
+
+            if (firstNetworkBehaviourIndex != -1 && networkObjectIndex > firstNetworkBehaviourIndex)
+            {
+                var positionsToMove = networkObjectIndex - firstNetworkBehaviourIndex;
+                for (int i = 0; i < positionsToMove; i++)
+                {
+                    UnityEditorInternal.ComponentUtility.MoveComponentUp(networkObject);
+                }
+
+                EditorUtility.SetDirty(networkObject.gameObject);
+            }
         }
     }
 }

com.unity.netcode.gameobjects/Tests/Editor/NetworkObjectTests.cs

@@ -1,5 +1,6 @@
 using System.Text.RegularExpressions;
 using NUnit.Framework;
+using Unity.Netcode.Editor;
 using UnityEngine;
 using UnityEngine.TestTools;
 
@@ -64,9 +65,95 @@ public void GetBehaviourIndexOne()
             Object.DestroyImmediate(gameObject);
         }
 
+        /// <summary>
+        /// Verifies that a NetworkObject component that is positioned after a NetworkBehaviour component will
+        /// be migrated to a component index value that is before the lowest NetworkBehaviour component index value.
+        /// (The lowest NetworkBehaviour component's index value will also change when this happens)
+        /// </summary>
+        [Test]
+        public void NetworkObjectComponentOrder()
+        {
+            var gameObject = new GameObject(nameof(GetBehaviourIndexOne));
+            // Add the Networkbehaviour first
+            var networkBehaviour = gameObject.AddComponent<EmptyNetworkBehaviour>();
+            // Add an empty MonoBehaviour inbetween the NetworkBehaviour and NetworkObject
+            gameObject.AddComponent<EmptyMonoBehaviour>();
+            // Add the NetworkObject
+            var networkObject = gameObject.AddComponent<NetworkObject>();
+            var componentIndices = GetIndices(gameObject);
+
+            // Verify the NetworkObject procedes the NetworkBehaviour
+            Assert.True(componentIndices.NetworkObjectIndex > componentIndices.NetworkBehaviourIndex, $"[Initial Setup] NetworkObject index ({componentIndices.NetworkObjectIndex}) is not greater than the NetworkBehaviour index ({componentIndices.NetworkBehaviourIndex})!");
+
+            // Force-Invoke the CheckForNetworkObject method in order to verify the NetworkObject is moved
+            NetworkBehaviourEditor.CheckForNetworkObject(gameObject);
+            var adjustedIndices = GetIndices(gameObject);
+
+            Assert.True(ValidateComponentIndices(componentIndices, GetIndices(gameObject)), "NetworkObject did not get migrated below the NetworkBehaviour!");
+
+            // Cleanup
+            Object.DestroyImmediate(gameObject);
+        }
+
+        private bool ValidateComponentIndices(ComponentIndices previous, ComponentIndices current)
+        {
+            if (previous.NetworkObjectIndex != current.NetworkObjectIndex && previous.NetworkBehaviourIndex != current.NetworkBehaviourIndex)
+            {
+                if (current.NetworkObjectIndex < previous.NetworkObjectIndex && current.NetworkObjectIndex < current.NetworkBehaviourIndex)
+                {
+                    return true;
+                }
+            }
+            return false;
+        }
+
+        private ComponentIndices GetIndices(GameObject gameObject)
+        {
+            // Get the index/order values for the added NetworkBehaviour and NetworkObject
+            var components = gameObject.GetComponents<MonoBehaviour>();
+            var componentIndices = new ComponentIndices()
+            {
+                NetworkObjectIndex = -1,
+                NetworkBehaviourIndex = -1
+            };
+            for (int i = 0; i < components.Length; i++)
+            {
+                if (componentIndices.NetworkObjectIndex != -1 && componentIndices.NetworkBehaviourIndex != -1)
+                {
+                    break;
+                }
+                var component = components[i];
+                var networkObjectComponent = component as NetworkObject;
+                if (networkObjectComponent != null)
+                {
+                    componentIndices.NetworkObjectIndex = i;
+                    continue;
+                }
+                var networkBehaviourComponent = component as EmptyNetworkBehaviour;
+                if (networkBehaviourComponent != null)
+                {
+                    componentIndices.NetworkBehaviourIndex = i;
+                    continue;
+                }
+            }
+
+            return componentIndices;
+        }
+
+        private struct ComponentIndices
+        {
+            public int NetworkObjectIndex;
+            public int NetworkBehaviourIndex;
+        }
+
         public class EmptyNetworkBehaviour : NetworkBehaviour
         {
 
         }
+
+        public class EmptyMonoBehaviour : MonoBehaviour
+        {
+
+        }
     }
 }