Mirror/docs/Concepts/GameObjects/SpawnObjectCustom.md

# Custom Spawn Functions

You can use spawn handler functions to customize the default behavior when creating spawned GameObjects on the client. Spawn handler functions ensure you have full control of how you spawn the GameObject, as well as how you destroy it.

Use ClientScene.RegisterSpawnHandler to register functions to spawn and destroy client GameObjects. The server creates GameObjects directly, and then spawns them on the clients through this functionality. This function takes the asset ID of the GameObject and two function delegates: one to handle creating GameObjects on the client, and one to handle destroying GameObjects on the client. The asset ID can be a dynamic one, or just the asset ID found on the prefab GameObject you want to spawn (if you have one).

The spawn / un-spawner need to have this GameObject signature. This is defined in the high level API.

```cs
// Handles requests to spawn GameObjects on the client
public delegate GameObject SpawnDelegate(Vector3 position, System.Guid assetId);

// Handles requests to unspawn GameObjects on the client
public delegate void UnSpawnDelegate(GameObject spawned);
```

The asset ID passed to the spawn function can be found on NetworkIdentity.assetId for prefabs, where it is populated automatically. The registration for a dynamic asset ID is handled like this:

```cs
// generate a new unique assetId 
System.Guid creatureAssetId = System.Guid.NewGuid();

// register handlers for the new assetId
ClientScene.RegisterSpawnHandler(creatureAssetId, SpawnCreature, UnSpawnCreature);

// get assetId on an existing prefab
System.Guid coinAssetId = coinPrefab.GetComponent<NetworkIdentity>().assetId;

// register handlers for an existing prefab you'd like to custom spawn
ClientScene.RegisterSpawnHandler(coinAssetId, SpawnCoin, UnSpawnCoin);

// spawn a coin - SpawnCoin is called on client
NetworkServer.Spawn(gameObject, coinAssetId);
```

The spawn functions themselves are implemented with the delegate signature. Here is the coin spawner. The SpawnCreature would look the same, but have different spawn logic:

```cs
public GameObject SpawnCoin(Vector3 position, System.Guid assetId)
{
    return (GameObject)Instantiate(m_CoinPrefab, position, Quaternion.identity);
}
public void UnSpawnCoin(GameObject spawned)
{
    Destroy(spawned);
}
```

When using custom spawn functions, it is sometimes useful to be able to unspawn GameObjects without destroying them. This can be done by calling [NetworkServer.UnSpawn]. This causes a message to be sent to clients to un-spawn the GameObject, so that the custom un-spawn function will be called on the clients. The GameObject is not destroyed when this function is called.

Note that on the host, GameObjects are not spawned for the local client, because they already exist on the server. This also means that no spawn handler functions are called.

## Setting Up a GameObject Pool with Custom Spawn Handlers

Here is an example of how you might set up a very simple GameObject pooling system with custom spawn handlers. Spawning and unspawning then puts GameObjects in or out of the pool.

```cs
using UnityEngine;
using Mirror;
using System.Collections;

public class SpawnManager : MonoBehaviour
{
    public int m_ObjectPoolSize = 5;
    public GameObject m_Prefab;
    public GameObject[] m_Pool;

    public System.Guid assetId { get; set;
}
    
    public delegate GameObject SpawnDelegate(Vector3 position, System.Guid assetId);
    public delegate void UnSpawnDelegate(GameObject spawned);

    void Start()
    {
        assetId = m_Prefab.GetComponent<NetworkIdentity> ().assetId;
        m_Pool = new GameObject[m_ObjectPoolSize];
        for (int i = 0; i < m_ObjectPoolSize; ++i)
        {
            m_Pool[i] = (GameObject)Instantiate(m_Prefab, Vector3.zero, Quaternion.identity);
            m_Pool[i].name = "PoolObject" + i;
            m_Pool[i].SetActive(false);
        }
        
        ClientScene.RegisterSpawnHandler(assetId, SpawnObject, UnSpawnObject);
    }

    public GameObject GetFromPool(Vector3 position)
    {
        foreach (var obj in m_Pool)
        {
            if (!obj.activeInHierarchy)
            {
                Debug.Log("Activating GameObject " + obj.name + " at " + position);
                obj.transform.position = position;
                obj.SetActive (true);
                return obj;
            }
        }
        Debug.LogError ("Could not grab GameObject from pool, nothing available");
        return null;
    }
    
    public GameObject SpawnObject(Vector3 position, System.Guid assetId)
    {
        return GetFromPool(position);
    }
    
    public void UnSpawnObject(GameObject spawned)
    {
        Debug.Log ("Re-pooling GameObject " + spawned.name);
        spawned.SetActive (false);
    }
}
```

To use this manager, create a new empty GameObject and name it “SpawnManager”. Create a new script called *SpawnManager,* copy in the code sample above, and attach it to the\* \*new SpawnManager GameObject. Next, drag a prefab you want to spawn multiple times to the Prefab field, and set the Object Pool Size (default is 5).

Finally, set up a reference to the SpawnManager in the script you are using for player movement:

```cs
SpawnManager spawnManager;

void Start()
{
    spawnManager = GameObject.Find("SpawnManager").GetComponent<SpawnManager> ();
}
```

Your player logic might contain something like this, which moves and fires coins:

```cs
void Update()
{
    if (!isLocalPlayer)
        return;
    
    var x = Input.GetAxis("Horizontal")*0.1f;
    var z = Input.GetAxis("Vertical")*0.1f;
    
    transform.Translate(x, 0, z);

    if (Input.GetKeyDown(KeyCode.Space))
    {
        // Command function is called on the client, but invoked on the server
        CmdFire();
    }
}
```

In the fire logic on the player, make it use the GameObject pool:

```cs
[Command]
void CmdFire()
{
    // Set up coin on server
    var coin = spawnManager.GetFromPool(transform.position + transform.forward);  
    coin.GetComponent<Rigidbody>().velocity = transform.forward*4;
    
    // spawn coin on client, custom spawn handler is called
    NetworkServer.Spawn(coin, spawnManager.assetId);
    
    // when the coin is destroyed on the server, it is automatically destroyed on clients
    StartCoroutine (Destroy (coin, 2.0f));
}

public IEnumerator Destroy(GameObject go, float timer)
{
    yield return new WaitForSeconds (timer);
    spawnManager.UnSpawnObject(go);
    NetworkServer.UnSpawn(go);
}
```

The automatic destruction shows how the GameObjects are returned to the pool and re-used when you fire again.