From c872cc005d37529017f7311d5c180708e73ad9b2 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Thu, 30 Apr 2020 15:15:31 +0200
Subject: [PATCH] Only call property notify on effective value change.

---
 src/Avalonia.Base/AvaloniaObject.cs           | 10 ++++++++--
 .../AvaloniaPropertyTests.cs                  | 20 ++++++++++++++++++-
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Base/AvaloniaObject.cs b/src/Avalonia.Base/AvaloniaObject.cs
index 546ab78e31..f387d7e0b6 100644
--- a/src/Avalonia.Base/AvaloniaObject.cs
+++ b/src/Avalonia.Base/AvaloniaObject.cs
@@ -716,7 +716,10 @@ namespace Avalonia
         {
             VerifyAccess();
 
-            change.Property.Notifying?.Invoke(this, true);
+            if (change.IsEffectiveValueChange)
+            {
+                change.Property.Notifying?.Invoke(this, true);
+            }
 
             try
             {
@@ -747,7 +750,10 @@ namespace Avalonia
             }
             finally
             {
-                change.Property.Notifying?.Invoke(this, false);
+                if (change.IsEffectiveValueChange)
+                {
+                    change.Property.Notifying?.Invoke(this, false);
+                }
             }
         }
 
diff --git a/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs b/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
index 3819e715f3..d7f927372e 100644
--- a/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
+++ b/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
@@ -102,6 +102,17 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal(new[] { "animated" }, result);
         }
 
+        [Fact]
+        public void Notify_Fired_Only_On_Effective_Value_Change()
+        {
+            var target = new Class1();
+
+            target.SetValue(Class1.FooProperty, "animated", BindingPriority.Animation);
+            target.SetValue(Class1.FooProperty, "local");
+
+            Assert.Equal(2, target.NotifyCount);
+        }
+
         [Fact]
         public void Property_Equals_Should_Handle_Null()
         {
@@ -180,7 +191,14 @@ namespace Avalonia.Base.UnitTests
         private class Class1 : AvaloniaObject
         {
             public static readonly StyledProperty<string> FooProperty =
-                AvaloniaProperty.Register<Class1, string>("Foo", "default");
+                AvaloniaProperty.Register<Class1, string>("Foo", "default", notifying: FooNotifying);
+
+            public int NotifyCount { get; private set; }
+
+            private static void FooNotifying(IAvaloniaObject o, bool n)
+            {
+                ++((Class1)o).NotifyCount;
+            }
         }
 
         private class Class2 : Class1