Custom skia gpu support.

src/Skia/Avalonia.Skia/CustomRenderTarget.cs

@@ -0,0 +1,39 @@
+using Avalonia.Platform;
+using Avalonia.Rendering;
+
+namespace Avalonia.Skia
+{
+    /// <summary>
+    /// Adapts <see cref="ICustomSkiaRenderTarget"/> to be used within Skia rendering pipeline.
+    /// </summary>
+    internal class CustomRenderTarget : IRenderTarget
+    {
+        private readonly ICustomSkiaRenderTarget _renderTarget;
+
+        public CustomRenderTarget(ICustomSkiaRenderTarget renderTarget)
+        {
+            _renderTarget = renderTarget;
+        }
+
+        public void Dispose()
+        {
+            _renderTarget.Dispose();
+        }
+
+        public IDrawingContextImpl CreateDrawingContext(IVisualBrushRenderer visualBrushRenderer)
+        {
+            ICustomSkiaRenderSession session = _renderTarget.BeginRendering();
+
+            var nfo = new DrawingContextImpl.CreateInfo
+            {
+                GrContext = session.GrContext,
+                Canvas = session.Canvas,
+                Dpi = SkiaPlatform.DefaultDpi * session.ScaleFactor,
+                VisualBrushRenderer = visualBrushRenderer,
+                DisableTextLcdRendering = true
+            };
+
+            return new DrawingContextImpl(nfo, session);
+        }
+    }
+}

src/Skia/Avalonia.Skia/ICustomSkiaGpu.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using SkiaSharp;
+
+namespace Avalonia.Skia
+{
+    /// <summary>
+    /// Custom Skia gpu instance.
+    /// </summary>
+    public interface ICustomSkiaGpu
+    {
+        /// <summary>
+        /// Skia GrContext used.
+        /// </summary>
+        GRContext GrContext { get; }
+
+        /// <summary>
+        /// Attempts to create custom render target from given surfaces.
+        /// </summary>
+        /// <param name="surfaces">Surfaces.</param>
+        /// <returns>Created render target or <see langword="null"/> if it fails.</returns>
+        ICustomSkiaRenderTarget TryCreateRenderTarget(IEnumerable<object> surfaces);
+    }
+}

src/Skia/Avalonia.Skia/ICustomSkiaRenderSession.cs

@@ -0,0 +1,26 @@
+using System;
+using SkiaSharp;
+
+namespace Avalonia.Skia
+{
+    /// <summary>
+    /// Custom render session for Skia render target.
+    /// </summary>
+    public interface ICustomSkiaRenderSession : IDisposable
+    {
+        /// <summary>
+        /// GrContext used by this session.
+        /// </summary>
+        GRContext GrContext { get; }
+
+        /// <summary>
+        /// Canvas that will be used to render.
+        /// </summary>
+        SKCanvas Canvas { get; }
+
+        /// <summary>
+        /// Scaling factor.
+        /// </summary>
+        double ScaleFactor { get; }
+    }
+}

src/Skia/Avalonia.Skia/ICustomSkiaRenderTarget.cs

@@ -0,0 +1,16 @@
+using System;
+
+namespace Avalonia.Skia
+{
+    /// <summary>
+    /// Custom Skia render target.
+    /// </summary>
+    public interface ICustomSkiaRenderTarget : IDisposable
+    {
+        /// <summary>
+        /// Start rendering to this render target.
+        /// </summary>
+        /// <returns></returns>
+        ICustomSkiaRenderSession BeginRendering();
+    }
+}

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -15,14 +15,25 @@ namespace Avalonia.Skia
     /// <summary>
     /// Skia platform render interface.
     /// </summary>
-    public class PlatformRenderInterface : IPlatformRenderInterface
+    internal class PlatformRenderInterface : IPlatformRenderInterface
     {
+        private readonly ICustomSkiaGpu _customSkiaGpu;
+
         private GRContext GrContext { get; }
 
         public IEnumerable<string> InstalledFontNames => SKFontManager.Default.FontFamilies;
 
-        public PlatformRenderInterface()
+        public PlatformRenderInterface(ICustomSkiaGpu customSkiaGpu)
         {
+            if (customSkiaGpu != null)
+            {
+                _customSkiaGpu = customSkiaGpu;
+
+                GrContext = _customSkiaGpu.GrContext;
+
+                return;
+            }
+
             var gl = AvaloniaLocator.Current.GetService<IWindowingPlatformGlFeature>();
             if (gl != null)
             {
@@ -32,12 +43,11 @@ namespace Avalonia.Skia
                     ? GRGlInterface.AssembleGlInterface((_, proc) => display.GlInterface.GetProcAddress(proc))
                     : GRGlInterface.AssembleGlesInterface((_, proc) => display.GlInterface.GetProcAddress(proc)))
                 {
-                    
                     GrContext = GRContext.Create(GRBackend.OpenGL, iface);
                 }
             }
         }
-        
+
         /// <inheritdoc />
         public IFormattedTextImpl CreateFormattedText(
             string text,
@@ -98,13 +108,23 @@ namespace Avalonia.Skia
                 DisableTextLcdRendering = false,
                 GrContext = GrContext
             };
-            
+
             return new SurfaceRenderTarget(createInfo);
         }
 
         /// <inheritdoc />
-        public virtual IRenderTarget CreateRenderTarget(IEnumerable<object> surfaces)
+        public IRenderTarget CreateRenderTarget(IEnumerable<object> surfaces)
         {
+            if (_customSkiaGpu != null)
+            {
+                ICustomSkiaRenderTarget customRenderTarget = _customSkiaGpu.TryCreateRenderTarget(surfaces);
+
+                if (customRenderTarget != null)
+                {
+                    return new CustomRenderTarget(customRenderTarget);
+                }
+            }
+
             foreach (var surface in surfaces)
             {
                 if (surface is IGlPlatformSurface glSurface && GrContext != null)

src/Skia/Avalonia.Skia/SkiaApplicationExtensions.cs

@@ -1,6 +1,7 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using Avalonia.Controls;
 using Avalonia.Skia;
 
@@ -18,9 +19,11 @@ namespace Avalonia
         /// <typeparam name="T">Builder type.</typeparam>
         /// <param name="builder">Builder.</param>
         /// <returns>Configure builder.</returns>
-        public static T UseSkia<T>(this T builder) where T : AppBuilderBase<T>, new()
+        public static T UseSkia<T>(this T builder, Func<ICustomSkiaGpu> gpuFactory = null) where T : AppBuilderBase<T>, new()
         {
-            builder.UseRenderingSubsystem(() => SkiaPlatform.Initialize(), "Skia");
+            var customGpu = gpuFactory?.Invoke();
+
+            builder.UseRenderingSubsystem(() => SkiaPlatform.Initialize(customGpu), "Skia");
             return builder;
         }
     }

src/Skia/Avalonia.Skia/SkiaPlatform.cs

@@ -13,10 +13,10 @@ namespace Avalonia.Skia
         /// <summary>
         /// Initialize Skia platform.
         /// </summary>
-        public static void Initialize()
+        public static void Initialize(ICustomSkiaGpu customGpu)
         {
-            var renderInterface = new PlatformRenderInterface();
+            var renderInterface = new PlatformRenderInterface(customGpu);
-            
+
             AvaloniaLocator.CurrentMutable
                 .Bind<IPlatformRenderInterface>().ToConstant(renderInterface);
         }