Merge pull request #4771 from donandren/issues/skiadrawingcontextctor

allow to create Skia drawing context from existing canvas

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -42,6 +42,11 @@ namespace Avalonia.Skia
         /// </summary>
         public struct CreateInfo
         {
+            /// <summary>
+            /// Canvas to draw to.
+            /// </summary>
+            public SKCanvas Canvas;
+
             /// <summary>
             /// Surface to draw to.
             /// </summary>
@@ -89,7 +94,7 @@ namespace Avalonia.Skia
             if (_grContext != null)
                 Monitor.Enter(_grContext);
             Surface = createInfo.Surface;
-            Canvas = createInfo.Surface.Canvas;
+            Canvas = createInfo.Canvas ?? createInfo.Surface?.Canvas;
 
             if (Canvas == null)
             {

src/Skia/Avalonia.Skia/Helpers/DrawingContextHelper.cs

@@ -0,0 +1,31 @@
+using Avalonia.Platform;
+using Avalonia.Rendering;
+using SkiaSharp;
+
+namespace Avalonia.Skia.Helpers
+{
+    public class DrawingContextHelper
+    {
+        /// <summary>
+        /// Wrap Skia canvas in drawing context so we can use Avalonia api to render to external skia canvas
+        /// this is useful in scenarios where canvas is not controlled by application, but received from another non avalonia api
+        /// like: SKCanvas canvas = SKDocument.BeginPage(...);
+        /// </summary>
+        /// <param name="canvas"></param>
+        /// <param name="dpi"></param>
+        /// <param name="visualBrushRenderer"></param>
+        /// <returns>DrawingContext</returns>
+        public static IDrawingContextImpl WrapSkiaCanvas(SKCanvas canvas, Vector dpi, IVisualBrushRenderer visualBrushRenderer = null)
+        {
+            var createInfo = new DrawingContextImpl.CreateInfo
+            {
+                Canvas = canvas,
+                Dpi = dpi,
+                VisualBrushRenderer = visualBrushRenderer,
+                DisableTextLcdRendering = true,
+            };
+
+            return new DrawingContextImpl(createInfo);
+        }
+    }
+}