AtomUI/tests/AtomUI.Core.Tests/HeadlessTestApp.cs

using Avalonia;
using Avalonia.Headless;

namespace AtomUI.Core.Tests;

internal static class HeadlessTestApp
{
    internal static void Run(Action action)
    {
        ArgumentNullException.ThrowIfNull(action);
        using var session = HeadlessUnitTestSession.StartNew(
            typeof(TestAppBuilder),
            AvaloniaTestIsolationLevel.PerTest);
        session.Dispatch(action, CancellationToken.None).GetAwaiter().GetResult();
    }

    internal static async Task RunAsync(Func<Task> action)
    {
        ArgumentNullException.ThrowIfNull(action);
        var session = HeadlessUnitTestSession.StartNew(
            typeof(TestAppBuilder),
            AvaloniaTestIsolationLevel.PerTest);
        try
        {
            await session.Dispatch(
                async () =>
                {
                    await action();
                    return true;
                },
                CancellationToken.None).ConfigureAwait(false);
        }
        finally
        {
            await Task.Run(session.Dispose).ConfigureAwait(false);
        }
    }
}

public static class TestAppBuilder
{
    public static AppBuilder BuildAvaloniaApp()
    {
        return AppBuilder.Configure<TestApplication>()
                         .UseHeadless(new AvaloniaHeadlessPlatformOptions());
    }
}

internal sealed partial class TestApplication : Application;