# \[OnValueSynced] Attribute

It is often useful to know when a synchronized variable has changed its value. It can be easily achieved using the `OnValueSyncedAttribute`. This attribute lets you define a method that will be called each time a value of a synced member (field or property) changes in the non-simulated version of an entity.

### Usage

Let's start with a simple example:

```csharp
using Coherence.Toolkit;
using UnityEngine;
using UnityEngine.UI;

public class Player : MonoBehaviour
{
    [OnValueSynced(nameof(UpdateHealthLabel))]
    public float Health;

    public Text HealthLabel;

    public void UpdateHealthLabel(float oldHealth, float newHealth)
    {
        HealthLabel.text = newHealth.ToString();
        Debug.Log($"Player HP changed by: {newHealth - oldHealth}");
    }
}
```

Whenever the value of the `Health` field gets updated (synced with its simulated version) the `UpdateHealthLabel` will be called automatically, changing the health label text and printing a log with a health difference.

{% hint style="success" %}
For the OnValueSynced attribute to work, the given field or property has to be synchronized over the network, via the [Sync attribute](/manual/networking-state-changes/sync-and-command-attributes.md) or via the [configure window](/getting-started/setup-a-project/prefab-setup.md).
{% endhint %}

This comes in handy in projects that use authoritative [Simulators](/manual/simulation-server.md). The Client code can easily react to changes in the `Player` entity state introduced by the Simulator, updating the visual representation (which the Simulator doesn't need).

{% hint style="info" %}
Remember that the callback method will be called only for a **non-simulated** instance of an Entity. Use on a simulated (owned) instance requires calling the selected method manually whenever the value of a given field/member changes. We recommend using [properties with a backing field](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/properties#properties-with-backing-fields) for this.
{% endhint %}

### Limitations

The `OnValueSynced` feature can be used only on members of user-defined types, that is, there's no way to be notified about a change in the value of a Unity type member, like `transform.position`. This might however change in the future, so stay tuned!


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