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SyncVar Hooks

The hook attribute can be used to specify a function to be called when the SyncVar changes value on the client.
  • The Hook method must have two parameters of the same type as the SyncVar property. One for the old value, one for the new value.
  • The Hook is always called after the property value is set. You don't need to set it yourself.
  • The Hook only fires for changed values, and changing a value in the inspector will not trigger an update.
  • As of version 11.1.4 (March 2020) and later, hooks can be virtual methods and overriden in a derived class.
Below is a simple example of assigning a random color to each player when they're spawned on the server. All clients will see all players in the correct colors, even if they join later.
Note: The signature for hook methods was changed in version 9.0 (Feb 2020) to having 2 parameters (old and new values). If you're on an older version, hook methods just have one parameter (new value).
using UnityEngine;
using Mirror;
public class PlayerController : NetworkBehaviour
{
[SyncVar(hook = nameof(SetColor))]
Color playerColor = Color.black;
// Unity makes a clone of the Material every time GetComponent().material is used.
// Cache it here and Destroy it in OnDestroy to prevent a memory leak.
Material cachedMaterial;
public override void OnStartServer()
{
base.OnStartServer();
playerColor = Random.ColorHSV(0f, 1f, 1f, 1f, 0.5f, 1f);
}
void SetColor(Color oldColor, Color newColor)
{
if (cachedMaterial == null)
cachedMaterial = GetComponent().material;
cachedMaterial.color = newColor;
}
void OnDestroy()
{
Destroy(cachedMaterial);
}
}

Hook call order

Hooks are invoked in the order the syncvars are defined in the file.
public class MyBehaviour : NetworkBehaviour
{
[SyncVar]
int X;
[SyncVar(hook = nameof(Hook1))]
int Y;
[SyncVar(hook = nameof(Hook2))]
int Z;
}
if X, Y, and Z are all set on the server at the same time then the call order will be:
  1. 1.
    X value is set
  2. 2.
    Y value is set
  3. 3.
    Hook1 is called
  4. 4.
    Z value is set
  5. 5.
    Hook2 is called