feat(NetworkIdentity): Reuse Network IDs

- Uses a Queue (FIFO) with delay time in seconds
- Configurable in Network Manager (Enabled & Delay)
- Pool shown in NI Info Panel

Assets/Mirror/Core/NetworkIdentity.cs

@@ -264,17 +264,52 @@ internal static void ResetClientStatics()
 
         internal static void ResetServerStatics()
         {
+            poolNetworkIds = true;
+            reuseDelay = 1;
+
+            netIdPool.Clear();
             nextNetworkId = 1;
         }
 
         /// <summary>Gets the NetworkIdentity from the sceneIds dictionary with the corresponding id</summary>
         public static NetworkIdentity GetSceneIdentity(ulong id) => sceneIds[id];
 
+        #region NetworkID Handling
+
+        internal static bool poolNetworkIds = true;
+        internal static byte reuseDelay = 1;
+
+        internal struct NetworkIdPool
+        {
+            public uint poolNetId;
+            public double timeAvailable;
+        }
+
+        // pool of NetworkIds that can be reused
+        internal static readonly Queue<NetworkIdPool> netIdPool = new Queue<NetworkIdPool>();
         static uint nextNetworkId = 1;
-        internal static uint GetNextNetworkId() => nextNetworkId++;
+
+        internal static uint GetNextNetworkId()
+        {
+            if (poolNetworkIds && netIdPool.TryPeek(out NetworkIdPool entry) && entry.timeAvailable < NetworkTime.time)
+            {
+                //Debug.LogFormat(LogType.Log, LogOption.NoStacktrace, null, $"[GetNextNetworkId] Reusing NetworkId {entry.poolNetId}.");
+
+                netIdPool.Dequeue();
+                return entry.poolNetId;
+            }
+
+            return nextNetworkId++;
+        }
 
         /// <summary>Resets nextNetworkId = 1</summary>
-        public static void ResetNextNetworkId() => nextNetworkId = 1;
+        public static void ResetNextNetworkId()
+        {
+            netIdPool.Clear();
+            nextNetworkId = 1;
+        }
+
+        #endregion
 
         /// <summary>The delegate type for the clientAuthorityCallback.</summary>
         public delegate void ClientAuthorityCallback(NetworkConnectionToClient conn, NetworkIdentity identity, bool authorityState);
@@ -628,6 +663,9 @@ void OnDestroy()
                 NetworkServer.Destroy(gameObject);
             }
 
+            if (isServer && poolNetworkIds && netId > 0)
+                netIdPool.Enqueue(new NetworkIdPool { poolNetId = netId, timeAvailable = NetworkTime.time + reuseDelay });
+
             if (isLocalPlayer)
             {
                 // previously there was a bug where isLocalPlayer was
@@ -1298,7 +1336,14 @@ internal void Reset()
             isOwned = false;
             NotifyAuthority();
 
+            if (netId > 0)
+            {
+                if (poolNetworkIds)
+                    netIdPool.Enqueue(new NetworkIdPool { poolNetId = netId, timeAvailable = NetworkTime.time + reuseDelay });
+
                 netId = 0;
+            }
+
             connectionToServer = null;
             connectionToClient = null;
 

Assets/Mirror/Core/NetworkManager.cs

@@ -63,6 +63,13 @@ public class NetworkManager : MonoBehaviour
         // [Tooltip("Client broadcasts 'sendRate' times per second. Use around 60Hz for fast paced games like Counter-Strike to minimize latency. Use around 30Hz for games like WoW to minimize computations. Use around 1-10Hz for slow paced games like EVE.")]
         // public int clientSendRate = 30; // 33 ms
 
+        [Header("Network ID Pooling")]
+        [Tooltip("Reuse network IDs when objects are unspawned or destroyed?")]
+        public bool poolNetworkIds = true;
+        [Range(1, 30)]
+        [Tooltip("Delay in seconds before network IDs are reused")]
+        public byte reuseDelay = 1;
+
         /// <summary>Automatically switch to this scene upon going offline (on start / on disconnect / on shutdown).</summary>
         [Header("Scene Management")]
         [Scene]
@@ -321,6 +328,10 @@ void SetupServer()
             NetworkServer.disconnectInactiveTimeout = disconnectInactiveTimeout;
             NetworkServer.exceptionsDisconnect = exceptionsDisconnect;
 
+            // Setup reuseable network IDs
+            NetworkIdentity.poolNetworkIds = poolNetworkIds;
+            NetworkIdentity.reuseDelay = reuseDelay;
+
             if (runInBackground)
                 Application.runInBackground = true;
 

Assets/Mirror/Editor/NetworkInformationPreview.cs

@@ -113,6 +113,8 @@ public override void OnPreviewGUI(Rect r, GUIStyle background)
 
             Y = DrawObservers(identity, initialX, Y);
 
+            Y = DrawNetworkIDPool(initialX, Y);
+
             _ = DrawOwner(identity, initialX, Y);
 
         }
@@ -191,6 +193,25 @@ float DrawObservers(NetworkIdentity identity, float initialX, float Y)
             return Y;
         }
 
+        float DrawNetworkIDPool(float initialX, float Y)
+        {
+            if (NetworkIdentity.netIdPool.Count > 0)
+            {
+                Rect poolRect = new Rect(initialX, Y + 10, 200, 20);
+                GUI.Label(poolRect, new GUIContent("Network ID Pool"), styles.componentName);
+                poolRect.x += 20;
+                poolRect.y += poolRect.height;
+                foreach (var entry in NetworkIdentity.netIdPool)
+                {
+                    GUI.Label(poolRect, $"[{entry.poolNetId}] {entry.timeAvailable:0.000}", styles.labelStyle);
+                    poolRect.y += poolRect.height;
+                    Y = poolRect.y;
+                }
+            }
+
+            return Y;
+        }
+
         float DrawOwner(NetworkIdentity identity, float initialX, float Y)
         {
             if (identity.connectionToClient != null)