avalonia-interop-rendering

Avalonia Advanced Rendering & Native Interop

Basic Render(DrawingContext), brushes, geometry and animations: see avalonia-custom-controls and avalonia-graphics-animation. Version labels: see avalonia-versions. This skill covers what sits below/beside them.

Which Tool?

Composition API

Namespace Avalonia.Rendering.Composition. Compositor objects live on the render thread; UI-thread properties are copied across.

// Entry point: the element's backing CompositionVisual (null until attached to a root)
var visual = ElementComposition.GetElementVisual(control);
var compositor = visual?.Compositor;

Use it from OnAttachedToVisualTree or later — never the constructor.

Custom visual with its own render-thread loop

sealed class SpinnerHandler : CompositionCustomVisualHandler
{
    public static readonly object Start = new(), Stop = new();
    bool _running; TimeSpan _last; double _angle;

    public override void OnMessage(object message)          // UI thread -> render thread
    {
        if (message == Start) { _running = true; _last = CompositionNow; RegisterForNextAnimationFrameUpdate(); }
        else if (message == Stop) _running = false;
    }

    public override void OnAnimationFrameUpdate()           // once per frame while registered
    {
        if (!_running) return;
        var now = CompositionNow;
        _angle = (_angle + (now - _last).TotalSeconds * 180) % 360;
        _last = now;
        Invalidate();                                       // or Invalidate(Rect) for precise dirty rects
        RegisterForNextAnimationFrameUpdate();              // must re-register every frame
    }

    public override void OnRender(ImmediateDrawingContext context)
    {
        // Draw from state computed in OnAnimationFrameUpdate (e.g. _angle -> position of a marker)
        var x = EffectiveSize.X / 2 + Math.Cos(_angle * Math.PI / 180) * 30;
        var y = EffectiveSize.Y / 2 + Math.Sin(_angle * Math.PI / 180) * 30;
        context.DrawEllipse(Brushes.DodgerBlue, null, new Point(x, y), 8, 8);   // as in ControlCatalog sample
    }
}

// In the control:
protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
{
    base.OnAttachedToVisualTree(e);
    var compositor = ElementComposition.GetElementVisual(this)?.Compositor;
    if (compositor is null) return;
    _custom = compositor.CreateCustomVisual(new SpinnerHandler());
    ElementComposition.SetElementChildVisual(this, _custom);
    _custom.Size = new Vector(Bounds.Width, Bounds.Height);   // keep in sync on size change
    _custom.SendHandlerMessage(SpinnerHandler.Start);
}