feat: Allow to pass custom data on Spawn calls

com.unity.netcode.gameobjects/Runtime/Connection/NetworkConnectionManager.cs

@@ -797,8 +797,9 @@ internal void HandleConnectionApproval(ulong ownerClientId, NetworkManager.Conne
                 // Server-side spawning (only if there is a prefab hash or player prefab provided)
                 if (!NetworkManager.DistributedAuthorityMode && response.CreatePlayerObject && (response.PlayerPrefabHash.HasValue || NetworkManager.NetworkConfig.PlayerPrefab != null))
                 {
-                    var playerObject = response.PlayerPrefabHash.HasValue ? NetworkManager.SpawnManager.GetNetworkObjectToSpawn(response.PlayerPrefabHash.Value, ownerClientId, response.Position ?? null, response.Rotation ?? null)
-                    : NetworkManager.SpawnManager.GetNetworkObjectToSpawn(NetworkManager.NetworkConfig.PlayerPrefab.GetComponent<NetworkObject>().GlobalObjectIdHash, ownerClientId, response.Position ?? null, response.Rotation ?? null);
+                    var instantiationPayloadWriter = new BufferSerializer<BufferSerializerWriter>(new BufferSerializerWriter(new FastBufferWriter(0,Allocator.Temp)));
+                    var playerObject = response.PlayerPrefabHash.HasValue ? NetworkManager.SpawnManager.GetNetworkObjectToSpawn(response.PlayerPrefabHash.Value, ownerClientId, ref instantiationPayloadWriter, response.Position ?? null, response.Rotation ?? null)
+                    : NetworkManager.SpawnManager.GetNetworkObjectToSpawn(NetworkManager.NetworkConfig.PlayerPrefab.GetComponent<NetworkObject>().GlobalObjectIdHash, ownerClientId, ref instantiationPayloadWriter, response.Position ?? null, response.Rotation ?? null);
 
                     // Spawn the player NetworkObject locally
                     NetworkManager.SpawnManager.SpawnNetworkObjectLocally(
@@ -950,7 +951,8 @@ internal void CreateAndSpawnPlayer(ulong ownerId)
                 if (playerPrefab != null)
                 {
                     var globalObjectIdHash = playerPrefab.GetComponent<NetworkObject>().GlobalObjectIdHash;
-                    var networkObject = NetworkManager.SpawnManager.GetNetworkObjectToSpawn(globalObjectIdHash, ownerId, playerPrefab.transform.position, playerPrefab.transform.rotation);
+                    var instantiationPayloadWriter = new BufferSerializer<BufferSerializerWriter>(new BufferSerializerWriter(new FastBufferWriter(0, Allocator.Temp)));
+                    var networkObject = NetworkManager.SpawnManager.GetNetworkObjectToSpawn(globalObjectIdHash, ownerId, ref instantiationPayloadWriter, playerPrefab.transform.position, playerPrefab.transform.rotation);
                     networkObject.IsSceneObject = false;
                     networkObject.SpawnAsPlayerObject(ownerId, networkObject.DestroyWithScene);
                 }

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

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using Unity.Collections;
 using Unity.Netcode.Components;
 #if UNITY_EDITOR
 using UnityEditor;
@@ -45,6 +46,13 @@ public sealed class NetworkObject : MonoBehaviour
         [SerializeField]
         internal uint InScenePlacedSourceGlobalObjectIdHash;
 
+        /// <summary>
+        /// Metadata sent during the instantiation process.
+        /// Retrieved in INetworkCustomSpawnDataSynchronizer before instantiation,
+        /// and available to INetworkPrefabInstanceHandler.Instantiate() for custom handling by user code.
+        /// </summary>
+        internal FastBufferReader InstantiationPayload;
+
         /// <summary>
         /// Gets the Prefab Hash Id of this object if the object is registerd as a prefab otherwise it returns 0
         /// </summary>
@@ -2812,6 +2820,7 @@ internal struct SceneObject
             public ulong NetworkObjectId;
             public ulong OwnerClientId;
             public ushort OwnershipFlags;
+            public FastBufferReader InstantiationPayload;
 
             public bool IsPlayerObject
             {
@@ -2882,6 +2891,12 @@ public bool SpawnWithObservers
                 set => ByteUtility.SetBit(ref m_BitField, 10, value);
             }
 
+            public bool HasInstantiationPayload
+            {
+                get => ByteUtility.GetBit(m_BitField, 11);
+                set => ByteUtility.SetBit(ref m_BitField, 11, value);
+            }
+
             // When handling the initial synchronization of NetworkObjects,
             // this will be populated with the known observers.
             public ulong[] Observers;
@@ -2948,12 +2963,26 @@ public void Serialize(FastBufferWriter writer)
                 var writeSize = 0;
                 writeSize += HasTransform ? FastBufferWriter.GetWriteSize<TransformData>() : 0;
                 writeSize += FastBufferWriter.GetWriteSize<int>();
+                if (HasInstantiationPayload)
+                {
+                    writeSize += FastBufferWriter.GetWriteSize<int>();
+                    writeSize += InstantiationPayload.Length;
+                }
 
                 if (!writer.TryBeginWrite(writeSize))
                 {
                     throw new OverflowException("Could not serialize SceneObject: Out of buffer space.");
                 }
 
+                if (HasInstantiationPayload)
+                {
+                    writer.WriteValueSafe(InstantiationPayload.Length);
+                    unsafe
+                    {
+                        writer.WriteBytes(InstantiationPayload.GetUnsafePtr(), InstantiationPayload.Length);
+                    }
+                }
+
                 if (HasTransform)
                 {
                     writer.WriteValue(Transform);
@@ -3014,12 +3043,34 @@ public void Deserialize(FastBufferReader reader)
                 readSize += HasTransform ? FastBufferWriter.GetWriteSize<TransformData>() : 0;
                 readSize += FastBufferWriter.GetWriteSize<int>();
 
+                int preInstanceDataSize = 0;
+                if (HasInstantiationPayload)
+                {
+                    if (!reader.TryBeginRead(FastBufferWriter.GetWriteSize<int>()))
+                    {
+                        throw new OverflowException($"Could not deserialize SceneObject: Reading past the end of the buffer ({nameof(InstantiationPayload)} size)");
+                    }
+
+                    reader.ReadValueSafe(out preInstanceDataSize);
+                    readSize += FastBufferWriter.GetWriteSize<int>();
+                    readSize += preInstanceDataSize;
+                }
+
                 // Try to begin reading the remaining bytes
                 if (!reader.TryBeginRead(readSize))
                 {
                     throw new OverflowException("Could not deserialize SceneObject: Reading past the end of the buffer");
                 }
 
+                if (HasInstantiationPayload)
+                {
+                    unsafe
+                    {
+                        InstantiationPayload = new FastBufferReader(reader.GetUnsafePtrAtCurrentPosition(), Allocator.Persistent, preInstanceDataSize);
+                        reader.Seek(reader.Position + preInstanceDataSize);
+                    }
+                }
+
                 if (HasTransform)
                 {
                     reader.ReadValue(out Transform);
@@ -3148,7 +3199,9 @@ internal SceneObject GetMessageSceneObject(ulong targetClientId = NetworkManager
                 NetworkSceneHandle = NetworkSceneHandle,
                 Hash = CheckForGlobalObjectIdHashOverride(),
                 OwnerObject = this,
-                TargetClientId = targetClientId
+                TargetClientId = targetClientId,
+                HasInstantiationPayload = InstantiationPayload.IsInitialized,
+                InstantiationPayload = InstantiationPayload
             };
 
             // Handle Parenting
@@ -3243,6 +3296,11 @@ internal static NetworkObject AddSceneObject(in SceneObject sceneObject, FastBuf
             // in order to be able to determine which NetworkVariables the client will be allowed to read.
             networkObject.OwnerClientId = sceneObject.OwnerClientId;
 
+            // Even though the Instantiation Payload is typically consumed during the spawn message handling phase,
+            // we still assign it here to preserve the original spawn metadata for potential inspection, diagnostics,
+            // or in case future systems want to access it directly without relying on synchronization messages.
+            networkObject.InstantiationPayload = sceneObject.InstantiationPayload;
+
             // Special Case: Invoke NetworkBehaviour.OnPreSpawn methods here before SynchronizeNetworkBehaviours
             networkObject.InvokeBehaviourNetworkPreSpawn();
 

com.unity.netcode.gameobjects/Runtime/Messaging/CustomMessageManager.cs

@@ -180,18 +180,14 @@ internal void InvokeNamedMessage(ulong hash, ulong sender, FastBufferReader read
                 // We dont know what size to use. Try every (more collision prone)
                 if (m_NamedMessageHandlers32.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler32))
                 {
-                    // handler can remove itself, cache the name for metrics
-                    var messageName = m_MessageHandlerNameLookup32[hash];
                     messageHandler32(sender, reader);
-                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
+                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup32[hash], bytesCount);
                 }
 
                 if (m_NamedMessageHandlers64.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler64))
                 {
-                    // handler can remove itself, cache the name for metrics
-                    var messageName = m_MessageHandlerNameLookup64[hash];
                     messageHandler64(sender, reader);
-                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
+                    m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup64[hash], bytesCount);
                 }
             }
             else
@@ -202,19 +198,15 @@ internal void InvokeNamedMessage(ulong hash, ulong sender, FastBufferReader read
                     case HashSize.VarIntFourBytes:
                         if (m_NamedMessageHandlers32.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler32))
                         {
-                            // handler can remove itself, cache the name for metrics
-                            var messageName = m_MessageHandlerNameLookup32[hash];
                             messageHandler32(sender, reader);
-                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
+                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup32[hash], bytesCount);
                         }
                         break;
                     case HashSize.VarIntEightBytes:
                         if (m_NamedMessageHandlers64.TryGetValue(hash, out HandleNamedMessageDelegate messageHandler64))
                         {
-                            // handler can remove itself, cache the name for metrics
-                            var messageName = m_MessageHandlerNameLookup64[hash];
                             messageHandler64(sender, reader);
-                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, messageName, bytesCount);
+                            m_NetworkManager.NetworkMetrics.TrackNamedMessageReceived(sender, m_MessageHandlerNameLookup64[hash], bytesCount);
                         }
                         break;
                 }

com.unity.netcode.gameobjects/Runtime/Spawning/INetworkCustomSpawnDataReceiver.cs

@@ -0,0 +1,20 @@
+namespace Unity.Netcode
+{
+    /// <summary>
+    /// Interface for synchronizing custom instantiation payloads during NetworkObject spawning.
+    /// Used alongside <see cref="INetworkPrefabInstanceHandler"/> to extend instantiation behavior.
+    /// </summary>
+    public interface INetworkInstantiationPayloadSynchronizer
+	{
+        /// <summary>
+        /// Provides a method for synchronizing instantiation payload data during the spawn process.
+        /// Extends <see cref="INetworkPrefabInstanceHandler"/> to allow passing additional data prior to instantiation
+        /// to help identify or configure the local object instance that should be linked to the spawned NetworkObject.
+        ///
+        /// This method is invoked immediately before <see cref="INetworkPrefabInstanceHandler.Instantiate"/> is called,
+        /// allowing you to cache or prepare information needed during instantiation.
+        /// </summary>
+        /// <param name="serializer">The buffer serializer used to read or write custom instantiation data.</param>
+        void OnSynchronize<T>(ref BufferSerializer<T> serializer) where T : IReaderWriter;
+	}
+}

com.unity.netcode.gameobjects/Runtime/Spawning/NetworkPrefabHandler.cs

@@ -252,12 +252,33 @@ internal uint GetSourceGlobalObjectIdHash(uint networkPrefabHash)
         /// <param name="position"></param>
         /// <param name="rotation"></param>
         /// <returns></returns>
-        internal NetworkObject HandleNetworkPrefabSpawn(uint networkPrefabAssetHash, ulong ownerClientId, Vector3 position, Quaternion rotation)
+        internal NetworkObject HandleNetworkPrefabSpawn<T>(uint networkPrefabAssetHash, ulong ownerClientId, ref BufferSerializer<T> preInstanceDataSerializer, Vector3 position, Quaternion rotation) where T : IReaderWriter
         {
             if (m_PrefabAssetToPrefabHandler.TryGetValue(networkPrefabAssetHash, out var prefabInstanceHandler))
             {
+                if (prefabInstanceHandler is INetworkInstantiationPayloadSynchronizer synchronizer)
+                {
+                    synchronizer.OnSynchronize(ref preInstanceDataSerializer);
+                }
                 var networkObjectInstance = prefabInstanceHandler.Instantiate(ownerClientId, position, rotation);
-
+                if (networkObjectInstance != null)
+                {
+                    if (preInstanceDataSerializer.IsReader)
+                    {
+                        networkObjectInstance.InstantiationPayload = preInstanceDataSerializer.GetFastBufferReader();
+                    }
+                    else
+                    {
+                        var writer = preInstanceDataSerializer.GetFastBufferWriter();
+                        if (writer.Length > 0)
+                        {
+                            unsafe
+                            {
+                                networkObjectInstance.InstantiationPayload = new FastBufferReader(writer.GetUnsafePtr(), Collections.Allocator.Persistent, writer.Length);
+                            }
+                        }
+                    }
+                }
                 //Now we must make sure this alternate PrefabAsset spawned in place of the prefab asset with the networkPrefabAssetHash (GlobalObjectIdHash)
                 //is registered and linked to the networkPrefabAssetHash so during the HandleNetworkPrefabDestroy process we can identify the alternate prefab asset.
                 if (networkObjectInstance != null && !m_PrefabInstanceToPrefabAsset.ContainsKey(networkObjectInstance.GlobalObjectIdHash))