# How to update the UI when a callback resolves?

When working with Unity, all Unity APIs, for example APIs you use to do **UI updates**, need to happen on the **main thread**. In the code we'll write however, we get **callbacks** on other threads. We want to update UI in these callbacks, so we need a way to go from a side thread onto the main thread. To execute code on the main thread from a side thread, we'll use the [dispatcher pattern](/mixed-reality-docs/glossary/mixed-reality.md#dispatcher-pattern).

* Let's add a [member variable](/mixed-reality-docs/glossary/mixed-reality.md#member-variable), **dispatchQueue**, which is a **Queue of Actions**. We will push Actions onto the queue, and then **dequeue** and run the Actions on the main thread.

```csharp
/// <summary>
/// Set this string to the Spatial Anchors account key provided in the Spatial Anchors resource.
/// </summary>
protected string SpatialAnchorsAccountKey = "Set me";

/// <summary>
/// Our queue of actions that will be executed on the main thread.
/// </summary>
private readonly Queue<Action> dispatchQueue = new Queue<Action>();

/// <summary>
/// Use the recognizer to detect air taps.
/// </summary>
private GestureRecognizer recognizer;
```

* Next, let's add **method** to **add an Action to the Queue**. Add `QueueOnUpdate()` right after `Update()` :

```csharp
/// <summary>
/// Queues the specified <see cref="Action"/> on update.
/// </summary>
/// <param name="updateAction">The update action.</param>
protected void QueueOnUpdate(Action updateAction)
{
    lock (dispatchQueue)
    {
        dispatchQueue.Enqueue(updateAction);
    }
}
```

* Let's now use the **Update()** loop to check if there is an Action queued. If so, we will dequeue the action and run it.

```csharp
// Update is called once per frame
void Update()
{
    lock (dispatchQueue)
    {
        if (dispatchQueue.Count > 0)
        {
            dispatchQueue.Dequeue()();
        }
    }
}
```


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