Added WritableBitmap DrawingContext support.

src/Avalonia.Visuals/ApiCompatBaseline.txt

@@ -63,4 +63,5 @@ InterfacesShouldHaveSameMembers : Interface member 'public System.Boolean Avalon
 InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Platform.IGlyphRunImpl Avalonia.Platform.IPlatformRenderInterface.CreateGlyphRun(Avalonia.Media.GlyphRun)' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Platform.IGlyphRunImpl Avalonia.Platform.IPlatformRenderInterface.CreateGlyphRun(Avalonia.Media.GlyphRun, System.Double)' is present in the contract but not in the implementation.
 MembersMustExist : Member 'public Avalonia.Platform.IGlyphRunImpl Avalonia.Platform.IPlatformRenderInterface.CreateGlyphRun(Avalonia.Media.GlyphRun, System.Double)' does not exist in the implementation but it does exist in the contract.
-Total Issues: 64
+InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Platform.IDrawingContextImpl Avalonia.Platform.IWriteableBitmapImpl.CreateDrawingContext(Avalonia.Rendering.IVisualBrushRenderer)' is present in the implementation but not in the contract.
+Total Issues: 65

src/Avalonia.Visuals/Media/Imaging/WriteableBitmap.cs

@@ -36,6 +36,14 @@ namespace Avalonia.Media.Imaging
 
         public ILockedFramebuffer Lock() => ((IWriteableBitmapImpl) PlatformImpl.Item).Lock();
 
+        /// <summary>
+        /// Creates a <see cref="DrawingContext"/> to render into the <see cref="WriteableBitmap"/>.
+        /// </summary>
+        public DrawingContext CreateDrawingContext()
+        {
+            return new DrawingContext(((IWriteableBitmapImpl)PlatformImpl.Item).CreateDrawingContext(null));
+        }
+
         private static IBitmapImpl CreatePlatformImpl(PixelSize size, in Vector dpi, PixelFormat? format, AlphaFormat? alphaFormat)
         {
             var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();

src/Avalonia.Visuals/Platform/IWriteableBitmapImpl.cs

@@ -1,10 +1,13 @@
-namespace Avalonia.Platform
+using Avalonia.Rendering;
+
+namespace Avalonia.Platform
 {
     /// <summary>
     /// Defines the platform-specific interface for a <see cref="Avalonia.Media.Imaging.WriteableBitmap"/>.
     /// </summary>
     public interface IWriteableBitmapImpl : IBitmapImpl
     {
+        IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer);
         ILockedFramebuffer Lock();
     }
 }

src/Skia/Avalonia.Skia/WriteableBitmapImpl.cs

@@ -1,7 +1,9 @@
 using System;
 using System.IO;
+using System.Reactive.Disposables;
 using System.Threading;
 using Avalonia.Platform;
+using Avalonia.Rendering;
 using Avalonia.Skia.Helpers;
 using SkiaSharp;
 
@@ -88,6 +90,28 @@ namespace Avalonia.Skia
             }
         }
 
+        public IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
+        {
+            Monitor.Enter(_lock);
+
+            var pixmap = _bitmap.PeekPixels();
+            var surface = SKSurface.Create(pixmap);
+            var createInfo = new DrawingContextImpl.CreateInfo
+            {
+                 Canvas = surface.Canvas,
+                 Dpi = Dpi,
+                 VisualBrushRenderer = visualBrushRenderer
+            };
+
+            return new DrawingContextImpl(createInfo, Disposable.Create(() =>
+            {
+                _bitmap.NotifyPixelsChanged();
+                surface.Dispose();
+                pixmap.Dispose();
+                Monitor.Exit(_lock);
+            }));
+        }
+
         /// <inheritdoc />
         public ILockedFramebuffer Lock() => new BitmapFramebuffer(this, _bitmap);
 

src/Windows/Avalonia.Direct2D1/Media/Imaging/WriteableWicBitmapImpl.cs

@@ -1,5 +1,7 @@
 using System;
 using Avalonia.Platform;
+using Avalonia.Rendering;
+using SharpDX.Direct2D1;
 using SharpDX.WIC;
 using PixelFormat = Avalonia.Platform.PixelFormat;
 
@@ -12,6 +14,27 @@ namespace Avalonia.Direct2D1.Media.Imaging
         {
         }
 
+        public IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
+        {
+            if (WicImpl.PixelFormat != SharpDX.WIC.PixelFormat.Format32bppPBGRA &&
+                WicImpl.PixelFormat != SharpDX.WIC.PixelFormat.Format32bppPRGBA)
+                throw new NotSupportedException("Direct2D only supports drawing to bitmaps with premultiplied alpha.");
+
+            var renderTarget = new WicRenderTarget(
+                Direct2D1Platform.Direct2D1Factory,
+                WicImpl,
+                new RenderTargetProperties
+                {
+                    DpiX = (float)Dpi.X,
+                    DpiY = (float)Dpi.Y,
+                });
+
+            return new DrawingContextImpl(visualBrushRenderer, null, renderTarget, finishedCallback: () =>
+            {
+                Version++;
+            });
+        }
+ 
         class LockedBitmap : ILockedFramebuffer
         {
             private readonly WriteableWicBitmapImpl _parent;

tests/Avalonia.RenderTests/Media/WriteableBitmapTests.cs

@@ -0,0 +1,81 @@
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+using Avalonia.Controls;
+using Avalonia.Controls.Platform.Surfaces;
+using Avalonia.Controls.Shapes;
+using Avalonia.Layout;
+using Avalonia.Media;
+using Avalonia.Media.Imaging;
+using Avalonia.Platform;
+using Xunit;
+
+#if AVALONIA_SKIA
+namespace Avalonia.Skia.RenderTests
+#else
+namespace Avalonia.Direct2D1.RenderTests.Media
+#endif
+{
+    public class WriteableBitmapTests : TestBase
+    {
+        public WriteableBitmapTests()
+            : base(@"Media\WriteableBitmap")
+        {
+            Directory.CreateDirectory(OutputPath);
+        }
+
+        [Fact]
+        public void WriteableBitmap_DrawingContext()
+        {
+            using var target = new WriteableBitmap(
+                new PixelSize(100, 100),
+                new Vector(96, 96),
+                PixelFormat.Bgra8888,
+                AlphaFormat.Premul);
+
+            using (var context = target.CreateDrawingContext())
+            {
+                var geometry = new PolylineGeometry(
+                    new[] { new Point(5, 0), new Point(8, 8), new Point(0, 3), new Point(10, 3), new Point(2, 8) },
+                    true);
+                context.FillRectangle(Brushes.White, new Rect(0, 0, 100, 100));
+                context.PushPostTransform(Matrix.CreateScale(10, 12));
+                context.DrawGeometry(Brushes.Violet, null, geometry);
+            }
+
+            var testName = nameof(WriteableBitmap_DrawingContext);
+            target.Save(System.IO.Path.Combine(OutputPath, testName + ".out.png"));
+            CompareImagesNoRenderer(testName);
+        }
+
+        [Fact]
+        public void WriteableBitmap_DrawingContext_Two_Draws()
+        {
+            using var target = new WriteableBitmap(
+                new PixelSize(100, 100),
+                new Vector(96, 96),
+                PixelFormat.Bgra8888,
+                AlphaFormat.Premul);
+
+            using (var context = target.CreateDrawingContext())
+            {
+                var geometry = new PolylineGeometry(
+                    new[] { new Point(5, 0), new Point(8, 8), new Point(0, 3), new Point(10, 3), new Point(2, 8) },
+                    true);
+                context.FillRectangle(Brushes.White, new Rect(0, 0, 100, 100));
+                context.PushPostTransform(Matrix.CreateScale(10, 12));
+                context.DrawGeometry(Brushes.Violet, null, geometry);
+            }
+
+            using (var context = target.CreateDrawingContext())
+            {
+                var geometry = new EllipseGeometry(new Rect(40, 40, 20, 20));
+                context.DrawGeometry(Brushes.Red, null, geometry);
+            }
+
+            var testName = nameof(WriteableBitmap_DrawingContext_Two_Draws);
+            target.Save(System.IO.Path.Combine(OutputPath, testName + ".out.png"));
+            CompareImagesNoRenderer(testName);
+        }
+    }
+}