Headless Avalonia

AvaloniaHeadlessConsoleApp.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Avalonia.Headless" Version="11.0.0-preview3" />
    <PackageReference Include="Avalonia" Version="11.0.0-preview3" />
    <PackageReference Include="Avalonia.Skia" Version="11.0.0-preview3" />
    <PackageReference Include="SkiaSharp" Version="2.88.3" />
  </ItemGroup>

</Project>

Program.cs

//#r "nuget: Avalonia.Headless, 11.0.0-preview3"
//#r "nuget: Avalonia, 11.0.0-preview3"
//#r "nuget: Avalonia.Skia, 11.0.0-preview3"
//#r "nuget: SkiaSharp, 2.88.3"

using Avalonia.Themes.Fluent;
using SkiaSharp;

var builder = AppBuilder
    .Configure<App>()
    .UseSkia()
    .UseHeadless(new() { UseCompositor = true, UseHeadlessDrawing = false });
    // This initialization logic should be part of kernel probably. I canot execute this method just once inside single AppContext
    builder.SetupWithoutStarting();
var window = CreateWindow();
window.Show();

// Get render target.
var renderRoot = (IRenderRoot)window;
var renderTarget = renderRoot.CreateRenderTarget();

ImmediateRenderer.Render(window, renderTarget);

var headlessWindow = (IHeadlessWindow)window.PlatformImpl!;
var lastRenderedFrame = headlessWindow.GetLastRenderedFrame();
var lockedFramebuffer = lastRenderedFrame.Item.Lock();
WriteImage(lockedFramebuffer, "test.png");

void WriteImage(ILockedFramebuffer framebuffer, string fileName)
{
    var imageInfo = new SKImageInfo(framebuffer.Size.Width, framebuffer.Size.Height, SKColorType.Rgba8888, SKAlphaType.Opaque);
    var _bitmap = SKImage.FromPixelCopy(imageInfo, framebuffer.Address);
    var data = _bitmap.Encode();
    using var file = File.OpenWrite(fileName);
    data.SaveTo(file);
    file.Flush();
}

Window CreateWindow()
{
    var window = new HeadlessWindow();
    var grid = new Grid();
    grid.HorizontalAlignment = Avalonia.Layout.HorizontalAlignment.Center;
    grid.VerticalAlignment = Avalonia.Layout.VerticalAlignment.Center;
    grid.Children.Add(new Button() { Content = "Hello Headless Avalonia" });
    window.Content = grid;
    window.Resize(new Size(200, 100));
    return window;
}

class HeadlessWindow : Window
{
    public void Resize(Size newSize)
    {
        ClientSize = newSize;
        PlatformImpl!.GetType().GetMethod("DoResize", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.InvokeMethod)!
            .Invoke(PlatformImpl, new object[] { newSize });
    }
}

class App : Application
{
    public override void Initialize()
    {
        var theme = new FluentTheme(new Uri("avares://AvaloniaHeadlessConsoleApp"));
        theme.Mode = FluentThemeMode.Light;
        Styles.Add(theme);
        base.Initialize();
    }
}