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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).
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using UnityEngine;
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using Mirror;
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public class PlayerController : NetworkBehaviour
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{
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[SyncVar(hook = nameof(SetColor))]
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Color playerColor = Color.black;
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// Unity makes a clone of the Material every time GetComponent().material is used.
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// Cache it here and Destroy it in OnDestroy to prevent a memory leak.
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Material cachedMaterial;
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public override void OnStartServer()
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{
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base.OnStartServer();
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playerColor = Random.ColorHSV(0f, 1f, 1f, 1f, 0.5f, 1f);
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}
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void SetColor(Color oldColor, Color newColor)
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{
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if (cachedMaterial == null)
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cachedMaterial = GetComponent().material;
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cachedMaterial.color = newColor;
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}
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void OnDestroy()
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{
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Destroy(cachedMaterial);
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}
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}
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Hook call order

Hooks are invoked in the order the syncvars are defined in the file.
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public class MyBehaviour : NetworkBehaviour
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{
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[SyncVar]
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int X;
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[SyncVar(hook = nameof(Hook1))]
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int Y;
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[SyncVar(hook = nameof(Hook2))]
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int Z;
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}
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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
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