From 2a87dbd6caf4e90bef242fb49562e494546d048f Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 6 Dec 2017 10:53:55 +0100
Subject: [PATCH] Fix opacity mask rendering.

We're potentially creating a new immutable version of the opacity mask each time the `VisualNode` is updated. Need to implement comparison between mutable and immutable brushes.
---
 .../Rendering/SceneGraph/SceneBuilder.cs      |  4 ++-
 .../Rendering/SceneGraph/VisualNode.cs        | 10 +++++++
 .../Rendering/DeferredRendererTests.cs        | 26 +++++++++++++++++++
 3 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs b/src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs
index 4f5f094e29..8f4f487e08 100644
--- a/src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs
+++ b/src/Avalonia.Visuals/Rendering/SceneGraph/SceneBuilder.cs
@@ -178,7 +178,9 @@ namespace Avalonia.Rendering.SceneGraph
                     node.ClipToBounds = clipToBounds;
                     node.GeometryClip = visual.Clip?.PlatformImpl;
                     node.Opacity = opacity;
-                    node.OpacityMask = visual.OpacityMask;
+
+                    // TODO: Check equality between node.OpacityMask and visual.OpacityMask before assigning.
+                    node.OpacityMask = visual.OpacityMask?.ToImmutable();
 
                     if (ShouldStartLayer(visual))
                     {
diff --git a/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs b/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
index e9e20557bd..ecff1a1077 100644
--- a/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
+++ b/src/Avalonia.Visuals/Rendering/SceneGraph/VisualNode.cs
@@ -231,6 +231,11 @@ namespace Avalonia.Rendering.SceneGraph
                 context.PushOpacity(Opacity);
             }
 
+            if (OpacityMask != null)
+            {
+                context.PushOpacityMask(OpacityMask, ClipBounds);
+            }
+
             context.Transform = Transform;
 
             if (GeometryClip != null)
@@ -247,6 +252,11 @@ namespace Avalonia.Rendering.SceneGraph
                 context.PopGeometryClip();
             }
 
+            if (OpacityMask != null)
+            {
+                context.PopOpacityMask();
+            }
+
             if (Opacity != 1 && !skipOpacity)
             {
                 context.PopOpacity();
diff --git a/tests/Avalonia.Visuals.UnitTests/Rendering/DeferredRendererTests.cs b/tests/Avalonia.Visuals.UnitTests/Rendering/DeferredRendererTests.cs
index 21065cb6b5..c97070a2aa 100644
--- a/tests/Avalonia.Visuals.UnitTests/Rendering/DeferredRendererTests.cs
+++ b/tests/Avalonia.Visuals.UnitTests/Rendering/DeferredRendererTests.cs
@@ -165,6 +165,32 @@ namespace Avalonia.Visuals.UnitTests.Rendering
             context.Verify(x => x.PopOpacity(), Times.Never);
         }
 
+        [Fact]
+        public void Should_Push_Opacity_Mask()
+        {
+            var root = new TestRoot
+            {
+                Width = 100,
+                Height = 100,
+                Child = new Border
+                {
+                    Background = Brushes.Red,
+                    OpacityMask = Brushes.Green,
+                }
+            };
+
+            root.Measure(Size.Infinity);
+            root.Arrange(new Rect(root.DesiredSize));
+
+            var target = CreateTargetAndRunFrame(root);
+            var context = GetLayerContext(target, root);
+            var animation = new BehaviorSubject<double>(0.5);
+
+            context.Verify(x => x.PushOpacityMask(Brushes.Green, new Rect(0, 0, 100, 100)), Times.Once);
+            context.Verify(x => x.FillRectangle(Brushes.Red, new Rect(0, 0, 100, 100), 0), Times.Once);
+            context.Verify(x => x.PopOpacityMask(), Times.Once);
+        }
+
         [Fact]
         public void Should_Create_Layer_For_Root()
         {