fix: correctly setting PreviousValue when changing the element of NetworkList (#2069)

Author: jeffreyrainy

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 - Fixed NetworkLists not populating on client. NetworkList now uses the most recent list as opposed to the list at the end of previous frame, when sending full updates to dynamically spawned NetworkObject. The difference in behaviour is required as scene management spawns those objects at a different time in the frame, relative to updates. (#2062)
 
+- Fixed NetworkList Value event on the server. PreviousValue is now set correctly when a new value is set through property setter. (#2067)
+
 ## [1.0.0] - 2022-06-27
 
 ### Changed

com.unity.netcode.gameobjects/Runtime/NetworkVariable/Collections/NetworkList.cs

@@ -452,13 +452,15 @@ public T this[int index]
             get => m_List[index];
             set
             {
+                var previousValue = m_List[index];
                 m_List[index] = value;
 
                 var listEvent = new NetworkListEvent<T>()
                 {
                     Type = NetworkListEvent<T>.EventType.Value,
                     Index = index,
-                    Value = value
+                    Value = value,
+                    PreviousValue = previousValue
                 };
 
                 HandleAddListEvent(listEvent);

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

@@ -0,0 +1,82 @@
+using System.Collections;
+using UnityEngine;
+using UnityEngine.TestTools;
+using Unity.Netcode.TestHelpers.Runtime;
+
+namespace Unity.Netcode.RuntimeTests
+{
+    public class NetworkListChangedTestComponent : NetworkBehaviour
+    {
+
+    }
+
+    public class ListChangedObject : NetworkBehaviour
+    {
+        public int ExpectedPreviousValue = 0;
+        public int ExpectedValue = 0;
+        public bool AddDone = false;
+
+        public NetworkList<int> MyNetworkList = new NetworkList<int>();
+
+        public override void OnNetworkSpawn()
+        {
+            MyNetworkList.OnListChanged += Changed;
+            base.OnNetworkSpawn();
+        }
+
+        public void Changed(NetworkListEvent<int> listEvent)
+        {
+            if (listEvent.Type == NetworkListEvent<int>.EventType.Value)
+            {
+                if (listEvent.PreviousValue != ExpectedPreviousValue)
+                {
+                    Debug.Log($"Expected previous value mismatch {listEvent.PreviousValue} versus {ExpectedPreviousValue}");
+                    Debug.Assert(listEvent.PreviousValue == ExpectedPreviousValue);
+                }
+
+                if (listEvent.Value != ExpectedValue)
+                {
+                    Debug.Log($"Expected value mismatch {listEvent.Value} versus {ExpectedValue}");
+                    Debug.Assert(listEvent.Value == ExpectedValue);
+                }
+
+                AddDone = true;
+            }
+        }
+    }
+
+    public class NetworkListChangedTests : NetcodeIntegrationTest
+    {
+        protected override int NumberOfClients => 2;
+
+        private ulong m_ClientId0;
+        private GameObject m_PrefabToSpawn;
+
+        private NetworkObject m_NetSpawnedObject1;
+
+        protected override void OnServerAndClientsCreated()
+        {
+            m_PrefabToSpawn = CreateNetworkObjectPrefab("ListChangedObject");
+            m_PrefabToSpawn.AddComponent<ListChangedObject>();
+        }
+
+        [UnityTest]
+        public IEnumerator NetworkListChangedTest()
+        {
+            m_ClientId0 = m_ClientNetworkManagers[0].LocalClientId;
+
+            // create 3 objects
+            var spawnedObject1 = SpawnObject(m_PrefabToSpawn, m_ServerNetworkManager);
+            m_NetSpawnedObject1 = spawnedObject1.GetComponent<NetworkObject>();
+
+            m_NetSpawnedObject1.GetComponent<ListChangedObject>().MyNetworkList.Add(42);
+            m_NetSpawnedObject1.GetComponent<ListChangedObject>().ExpectedPreviousValue = 42;
+            m_NetSpawnedObject1.GetComponent<ListChangedObject>().ExpectedValue = 44;
+            m_NetSpawnedObject1.GetComponent<ListChangedObject>().MyNetworkList[0] = 44;
+
+            Debug.Assert(m_NetSpawnedObject1.GetComponent<ListChangedObject>().AddDone);
+
+            return null;
+        }
+    }
+}