Request the server to spawn an object, then return a reference to that object to the client?

Hey everyone, sorry if this is a simple question but I've tried googling everything and cannot find an answer. Using Netcode for GameObjects, how do you request the host/server to spawn some items, then return a reference those items the client that requested them?

Some more info: I am learning Netcode and am trying to make a simple scene where players can build tileable building pieces. This includes a "ghost" tile that shows where a player is about to build. At network spawn, I (think) I need to request the server to spawn this ghost tiles in, then return them to the client. The client then controls their position using a ClientNetworkTransform and their visibility using an animator. I am not sure if I am approaching this correctly, so any tips would really help me out.

 

Right after spawning the new object, the server can take the spawned NetworkObjectId and send it back to the client via ClientRPC. The client can then look up the NetworkSpawnManager's SpawnedObjects(List) in order to find the local instance of that network object.

 

Ayy I appreciate the response! I thought so, but in practice I am running in to an issue where the host is throwing exceptions when I try to then use those ghost references from the client.

Here is my code if that helps. This is where it is initialized:

Code (CSharp):

1.  [ServerRpc]
2.     private void IntializeServerRpc(ulong id)
3.     {
4.         print("intialize server rpc");
5.  
6.         ClientRpcParams clientRpcParams = new ClientRpcParams
7.         {
8.             Send = new ClientRpcSendParams
9.             {
10.                 TargetClientIds = new ulong[] { id }
11.             }
12.         };
13.  
14.         List<ulong> spawned = new();
15.  
16.         foreach (Transform item in m_GhostPrefabs)
17.         {
18.             Transform newObj = Instantiate(item);
19.  
20.             NetworkObject newNetworkObj = newObj.GetComponent<NetworkObject>();
21.          
22.             newNetworkObj.SpawnWithOwnership(id);
23.             spawned.Add(newNetworkObj.NetworkObjectId);
24.         }
25.  
26.         UpdateClientInitializedClientRpc(spawned.ToArray(), clientRpcParams);
27.     }
28.  
29.     [ClientRpc]
30.     private void UpdateClientInitializedClientRpc(ulong[] ghostTiles, ClientRpcParams clientRpcParams)
31.     {
32.         m_Ghosts.Clear();
33.  
34.         foreach (ulong item in ghostTiles)
35.         {
36.             NetworkObject foundObj = NetworkManager.Singleton.SpawnManager.SpawnedObjects[item];
37.             m_Ghosts.Add(foundObj.transform);
38.             Animator foundAnimator = foundObj.GetComponent<Animator>();
39.             m_GhostAnimatorsDictionary.Add(foundObj.transform, foundAnimator);
40.  
41.             print("RETURNED TO CLIENT: " + foundObj.gameObject.name);
42.         }
43.     }

and this is where I try to use that to spawn in objects:

Code (CSharp):

1. [ServerRpc]
2.     private void SpawnWallServerRpc(ulong id, PosAndRotData posAndRotData)
3.     {
4.         if (m_Ghosts.Count == 0)
5.         {
6.             Debug.LogWarning("GHOSTS NOT INTIALIZED. ", gameObject);
7.         }
8.  
9.         string[] ghostExpaned = m_Ghosts[m_CurrentGhostInt].name.Split('_');
10.         Transform foundTransform = null;
11.         foreach (Transform item in m_FinishedPrefabs)
12.         {
13.             string[] foundExpanded = item.name.Split('_');
14.  
15.             if (foundExpanded[0].Equals(ghostExpaned[0]) && foundExpanded[1].Equals(ghostExpaned[1]))
16.             {
17.                 foundTransform = item;
18.             }
19.         }
20.  
21.         Transform newWall = Instantiate(foundTransform);
22.         newWall.GetComponent<NetworkObject>().SpawnWithOwnership(id, true);
23.  
24.         Vector3 newPos = new(posAndRotData._xPos, posAndRotData._yPos, posAndRotData._zPos);
25.         Quaternion newRot = new(posAndRotData._xRot, posAndRotData._yRot, posAndRotData._zRot, posAndRotData._wRot);
26.  
27.         //newWall.SetPositionAndRotation(newPos, newRot);
28.         NetworkTransform foundNetworkTransform = newWall.GetComponent<NetworkTransform>();
29.  
30.         foundNetworkTransform.SetState(newPos, newRot);
31.     }

The issue I am having is the host can spawn in things fine, but when the client tries the same thing, the host hits the Debug.LogWarning at the start of SpwnWallServerRpc, saying it has nothing in the list of ghost objects. Checking in editor shows that on the host side, the host's list is serialized correct, but the client has nothing serialized in the m_Ghosts list, and when I check the Client side, it's the opposite. it shows nothing in the host's serialized m_Ghosts list but the client's list is indeed serialized correctly. I must be missing something that is causing the desync, but i can't figure out what.

I hope that makes sense, thanks again for the help!

EDIT: Also, these methods are all in the same script, which extends NetworkBehaviour.

 

Check if the UpdateClientInitializedClientRpc runs in both situation, ie host spawning and client spawning. I think it will not run for the host if the client spawns because you instruct the ClientRpc to only run for the spawning client, thus the host-client won't fill its m_Ghosts list.

For a similar reason the code will fail for pure server instances since the server will never run ClientRPC methods.

It's good practice to keep the RPC method body to a minimum. In this case I would move the entire body of UpdateClientInitializedClientRpc to a separate method which is called from IntializeServerRpc if the NetworkManager's IsServer flag is true and always called from the UpdateClientInitializedClientRpc. That way you avoid duplicating a lot of code because both server and client need to run it to fill their respective m_Ghosts lists.

 

Ahhh I thiiiiink things are starting to get clearer now. I will test out everything you mentioned and update the thread after. Thanks a TON for looking over all that, really appreciated.

So if I understand you the pseudocode for the correct process is:

• Create a new, non-rpc method method to initialize the

  m_Ghost 

  list, called

  InitializeGhostList

  
  
• Call

  IntializeServerRpc 

  from the

  OnNetworkSpawn()

• Inside

  InitalizeServerRpc

  , its important to determine whether this is a client or host using

  IsServer

• If

  IsServer

  , call

  InitailizeGhostList()

  , because this will not execute inside ClientRpc if

  IsServer

  .
• if

  (!IsServer)

  : Skip calling

  InitailizeGhostList()

  directly, and continue on.
• end of

  InitializeServerRpc()

  , call the

  UpdateClientInitializedClientRpc()

• If this

  IsServer

  , code execution stops.
• if (

  !IsServer

  ), the ClientRpc method calls

  InitializeGhostList 

Does that sound correct to you?

 

Sounds good so far!

 

That seemed to do the trick! Really appreciate the help

 

You‘re welcome!