How To Access the UI Thread

This guide will show you how to access the UI thread in your Avalonia UI application.

Avalonia UI applications have one main thread, and this handles the UI. When you have a process that is intensive, or long running, then you will usually opt to run it on a different thread. Then you may have scenarios where you want to update them main UI thread (for example with progress updates).

A dispatcher provides services for managing work items on any specific thread. In Avalonia UI you will already have the dispatcher that handles the UI thread. When you need to update the UI from a different thread, you access it through this dispatcher, as follows:

Dispatcher.UIThread

You can use either the Post method or the InvokeAsync method to run a process on the UI thread.

Use Post when you just want to start a job, but you do not need to wait for the job to be finished, and you do not need the result: this is the 'fire-and-forget' dispatcher method.

Use InvokeAsync when you need to wait for the result, and potentially want to receive the result.

Dispatcher Priority

Both of the above methods have a dispatcher priority parameter. You can use this with the DispatcherPriority enumeration to specify the queue priority that the given job should be given.

info

For the possible values of the DispatcherPriority enumeration, see here.

Example

This example shows how to access the ui thread from a worker thread to update or get the text of a TextBlock. Create a new Avalonia project and replace the content of the following two files:

MainView.axaml:

XAML

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:vm="clr-namespace:AvaloniaApplication1.ViewModels"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="AvaloniaApplication1.Views.MainView"
             x:DataType="vm:MainViewModel">
  <Design.DataContext>
    <!-- This only sets the DataContext for the previewer in an IDE,
         to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
    <vm:MainViewModel />
  </Design.DataContext>

	<StackPanel Margin="20">
		<TextBlock Name="TextBlock1" />
	</StackPanel>
</UserControl>

MainView.axaml.cs:

MainView C#

using Avalonia.Controls;
using Avalonia.Threading;
using System;
using System.Threading.Tasks;

namespace AvaloniaApplication1.Views;

public partial class MainView : UserControl
{
    public MainView()
    {
        InitializeComponent();

        // Execute OnTextFromAnotherThread on the thread pool
        // to demonstrate how to access the UI thread from
        // there.
        _ = Task.Run(() => OnTextFromAnotherThread("test"));
    }

    private void SetText(string text) => TextBlock1.Text = text;
    private string GetText() => TextBlock1.Text ?? "";

    private async void OnTextFromAnotherThread(string text)
    {
        try
        {
            // Start the job on the ui thread and return immediately.
            Dispatcher.UIThread.Post(() => SetText(text));

            // Start the job on the ui thread and wait for the result.
            var result = await Dispatcher.UIThread.InvokeAsync(GetText);

            // This invocation would cause an exception because we are
            // running on a worker thread:
            // System.InvalidOperationException: 'Call from invalid thread'
            //SetText(text);
        }
        catch (Exception)
        {
            throw; // Todo: Handle exception.
        }
    }
}

More Information

info

For the complete API documentation about the dispatcher, see here.

info

View the source code on GitHub Dispatcher.cs