Trying to create custom drawn control

[FraserElectronics]

I am attempting to render some graphics using SkiaSharp using the example here: https://github.com/AvaloniaUI/Avalonia/tree/master/samples/RenderDemo

I am using Visual Studio 2022 on Windows 11 and have put a Trace.WriteLine in ICustomDrawOperation.Render method.
When my application runs and I just leave it on the main view, I see endless messages appear in the Output window in visual studio.
Nothing should be invalidating the view so I don't understand why render is being called continuously !

My custom control code:

    public class GraphicsControl : Control
    {
        public GraphicsControl()
        {
            ClipToBounds = true;
        }

        public override void Render( DrawingContext context )
        {
            context.Custom( new CustomDrawOp( new Rect( 0, 0, Bounds.Width, Bounds.Height ) ) );
            Dispatcher.UIThread.InvokeAsync( InvalidateVisual, DispatcherPriority.Background );
        }

        class CustomDrawOp : ICustomDrawOperation
        {
            public Rect Bounds { get; }

            public CustomDrawOp( Rect bounds )
            {
                Bounds = bounds;
            }

            public void Dispose()
            {
            }

            public bool HitTest( Avalonia.Point p )
            {
                return false;
            }

            public bool Equals( ICustomDrawOperation other )
            {
                return false;
            }

            /// <summary>
            /// This method handles the rendering of the chart using SkiaSharp.
            /// </summary>
            /// <param name="context">The drawing context.</param>
            public void Render( ImmediateDrawingContext context )
            {
                var leaseFeature = context.TryGetFeature<ISkiaSharpApiLeaseFeature>();
                if ( leaseFeature != null )
                {
                    Trace.WriteLine( "Render" );

                    using var lease = leaseFeature.Lease();
                    var canvas = lease.SkCanvas;
                    canvas.Save();

                    SKRect bounds = new SKRect( Convert.ToSingle( Bounds.Left ), Convert.ToSingle( Bounds.Top ), Convert.ToSingle( Bounds.Right ), Convert.ToSingle( Bounds.Bottom ) );

                    SKPaint brush = new SKPaint
                    {
                        Color = new SKColor(32,32,32),
                        Style = SKPaintStyle.Fill,
                    };
                    canvas.DrawRect( bounds, brush );

                    SKPaint pen = new SKPaint
                    {
                        Color = new SKColor(0, 255, 0 ),
                        Style = SKPaintStyle.Stroke,
                        IsAntialias = true,
                        StrokeWidth = 2,
                    };
                    bounds.Inflate( -10, -10 );
                    canvas.DrawRect( bounds, pen );

                    SKPaint textPaint = new SKPaint
                    {
                        Color = new SKColor(255, 255, 255),
                        Style = SKPaintStyle.Fill,
                        IsAntialias = true,
                        TextSize = 20,
                    };
                    canvas.DrawText( "Hello World!", bounds.Left + 10, bounds.Top + 40, textPaint );

                    canvas.Restore();
                }
            }
        }
    }

You can see the Trace.WriteLine( "Render" ); that I see in the output window.
Am I missing something or have I copied the example code wrong somewhere?

Andy

[maxkatz6]

Nothing should be invalidating the view so I don't understand why render is being called continuously

In this code:

        public override void Render( DrawingContext context )
        {
            context.Custom( new CustomDrawOp( new Rect( 0, 0, Bounds.Width, Bounds.Height ) ) );
            Dispatcher.UIThread.InvokeAsync( InvalidateVisual, DispatcherPriority.Background );
        }

You invalidate visual after each Render call. Basically, creating a control that requests a new render on each frame.

[FraserElectronics]

Okay, I thought that code had come from an Avalonia example but it seems I copied it from some other code I found online :-(
Taking out the InvalidateVisual call sorts the problem.

Thanks for your help 👍

Andy

[cyraid]

ISkiaSharpApiLeaseFeature doesn't seem to exist in newer versions?

[stevemonaco]

It still exists. See https://github.com/AvaloniaUI/Avalonia/blob/d39cb62a5d87fc38f70d41878ba8cd4abda6d2ce/samples/RenderDemo/Pages/CustomSkiaPage.cs

Note that you need to be in an ICustomDrawOperation to get the ImmediateDrawingContext.