source/SkiaSharp.Views.Uno/SkiaSharp.Views.Uno.WinUI.Skia/SKXamlCanvas.Skia.cs

using System;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.WindowsRuntime;

#if WINUI
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Media;
using Microsoft.UI.Xaml.Media.Imaging;
#else
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;
#endif

#if WINDOWS || WINUI
namespace SkiaSharp.Views.Windows
#else
namespace SkiaSharp.Views.UWP
#endif
{
	public partial class SKXamlCanvas : Canvas
	{
		private byte[] pixels;
		private GCHandle pixelsHandle;
		private int pixelWidth;
		private int pixelHeight;
		private WriteableBitmap bitmap;

		public SKXamlCanvas()
		{
			Initialize();
		}

		partial void DoUnloaded() =>
			FreeBitmap();

		private void DoInvalidate()
		{
			if (designMode)
				return;

			if (!isVisible)
				return;

			if (ActualWidth <= 0 || ActualHeight <= 0)
			{
				CanvasSize = SKSize.Empty;
				return;
			}

			var info = CreateBitmap(out var unscaledSize, out var dpi);

			using (var surface = SKSurface.Create(info, pixelsHandle.AddrOfPinnedObject(), info.RowBytes))
			{
				var userVisibleSize = IgnorePixelScaling ? unscaledSize : info.Size;
				CanvasSize = userVisibleSize;

				if (IgnorePixelScaling)
				{
					var canvas = surface.Canvas;
					canvas.Scale(dpi);
					canvas.Save();
				}

				OnPaintSurface(new SKPaintSurfaceEventArgs(surface, info.WithSize(userVisibleSize), info));
			}

			// This implementation is not fast enough, and providing the original pixel buffer
			// is needed, yet the internal `IBufferByteAccess` interface is not yet available in Uno.
			// Once it is, we can replace this implementation and provide the pinned array directly
			// to skia.
			using (var data = bitmap.PixelBuffer.AsStream())
			{
				data.Write(pixels, 0, pixels.Length);
				data.Flush();
			}

			bitmap.Invalidate();
		}

		private SKImageInfo CreateBitmap(out SKSizeI unscaledSize, out float dpi)
		{
			var size = CreateSize(out unscaledSize, out dpi);
			var info = new SKImageInfo(size.Width, size.Height, SKColorType.Bgra8888, SKAlphaType.Premul);

			if (bitmap?.PixelWidth != info.Width || bitmap?.PixelHeight != info.Height)
				FreeBitmap();

			if (bitmap == null && info.Width > 0 && info.Height > 0)
			{
				bitmap = new WriteableBitmap(info.Width, info.Height);

				var brush = new ImageBrush
				{
					ImageSource = bitmap,
					AlignmentX = AlignmentX.Left,
					AlignmentY = AlignmentY.Top,
					Stretch = Stretch.Fill
				};

				Background = brush;
			}

			if (pixels == null || pixelWidth != info.Width || pixelHeight != info.Height)
			{
				FreeBitmap();

				pixels = new byte[info.BytesSize];
				pixelsHandle = GCHandle.Alloc(pixels, GCHandleType.Pinned);

				pixelWidth = info.Width;
				pixelHeight = info.Height;
			}

			return info;
		}

		private void FreeBitmap()
		{
			if (pixels != null)
			{
				pixelsHandle.Free();
				pixels = null;
				bitmap = null;
			}
		}
	}
}