feat: Adding "previousValue" in NetworkListEvent (#1528) (#1577)

Author: SamuelBellomo

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -6,6 +6,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 Additional documentation and release notes are available at [Multiplayer Documentation](https://docs-multiplayer.unity3d.com).
 
+## [Unreleased]
+
+### Added
+
+- Added `PreviousValue` in NetworkListEvent, when `Value` has changed (#1528)
+
 ## [1.0.0-pre.4] - 2021-01-04
 
 ### Added

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

@@ -272,18 +272,22 @@ public override void ReadDelta(FastBufferReader reader, bool keepDirtyDelta)
                         {
                             reader.ReadValueSafe(out int index);
                             reader.ReadValueSafe(out T value);
-                            if (index < m_List.Length)
+                            if (index >= m_List.Length)
                             {
-                                m_List[index] = value;
+                                throw new Exception("Shouldn't be here, index is higher than list length");
                             }
 
+                            var previousValue = m_List[index];
+                            m_List[index] = value;
+
                             if (OnListChanged != null)
                             {
                                 OnListChanged(new NetworkListEvent<T>
                                 {
                                     Type = eventType,
                                     Index = index,
-                                    Value = value
+                                    Value = value,
+                                    PreviousValue = previousValue
                                 });
                             }
 
@@ -293,7 +297,8 @@ public override void ReadDelta(FastBufferReader reader, bool keepDirtyDelta)
                                 {
                                     Type = eventType,
                                     Index = index,
-                                    Value = value
+                                    Value = value,
+                                    PreviousValue = previousValue
                                 });
                             }
                         }
@@ -528,6 +533,11 @@ public enum EventType : byte
         /// </summary>
         public T Value;
 
+        /// <summary>
+        /// The previous value when "Value" has changed, if available.
+        /// </summary>
+        public T PreviousValue;
+
         /// <summary>
         /// the index changed, added or removed if available
         /// </summary>

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

@@ -328,6 +328,58 @@ public IEnumerator NetworkListArrayOperator([Values(true, false)] bool useHost)
             );
         }
 
+
+        [UnityTest]
+        public IEnumerator NetworkListValueUpdate([Values(true, false)] bool useHost)
+        {
+            m_TestWithHost = useHost;
+            yield return MultiInstanceHelpers.RunAndWaitForCondition(
+                () =>
+                {
+                    m_Player1OnServer.TheList.Add(k_TestVal1);
+                },
+                () =>
+                {
+                    return m_Player1OnServer.TheList.Count == 1 &&
+                           m_Player1OnClient1.TheList.Count == 1 &&
+                           m_Player1OnServer.TheList[0] == k_TestVal1 &&
+                           m_Player1OnClient1.TheList[0] == k_TestVal1;
+                }
+            );
+
+            var testSucceeded = false;
+
+            void TestValueUpdatedCallback(NetworkListEvent<int> changedEvent)
+            {
+                testSucceeded = changedEvent.PreviousValue == k_TestVal1 &&
+                                changedEvent.Value == k_TestVal3;
+            }
+
+            try
+            {
+                yield return MultiInstanceHelpers.RunAndWaitForCondition(
+                    () =>
+                    {
+                        m_Player1OnServer.TheList[0] = k_TestVal3;
+                        m_Player1OnClient1.TheList.OnListChanged += TestValueUpdatedCallback;
+                    },
+                    () =>
+                    {
+                        return m_Player1OnServer.TheList.Count == 1 &&
+                               m_Player1OnClient1.TheList.Count == 1 &&
+                               m_Player1OnServer.TheList[0] == k_TestVal3 &&
+                               m_Player1OnClient1.TheList[0] == k_TestVal3;
+                    }
+                );
+            }
+            finally
+            {
+                m_Player1OnClient1.TheList.OnListChanged -= TestValueUpdatedCallback;
+            }
+
+            Assert.That(testSucceeded);
+        }
+
         [Test]
         public void NetworkListIEnumerator([Values(true, false)] bool useHost)
         {