From 959d0cce1ea2afbbad7e56db76d30830bfd1e862 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 28 Apr 2020 10:59:53 +0200
Subject: [PATCH] Don't notify AvaloniaProperty.Changed on non-effective value
 change.

---
 src/Avalonia.Base/AvaloniaObject.cs                |  6 +++++-
 .../AvaloniaPropertyChangedEventArgs.cs            |  2 +-
 src/Avalonia.Base/PropertyStore/PriorityValue.cs   |  2 +-
 .../AvaloniaPropertyTests.cs                       | 14 ++++++++++++++
 4 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Base/AvaloniaObject.cs b/src/Avalonia.Base/AvaloniaObject.cs
index 073cedbb1c..479ce54ea0 100644
--- a/src/Avalonia.Base/AvaloniaObject.cs
+++ b/src/Avalonia.Base/AvaloniaObject.cs
@@ -769,7 +769,11 @@ namespace Avalonia
             try
             {
                 OnPropertyChangedCore(change);
-                change.Property.NotifyChanged(change);
+
+                if (change.IsEffectiveValueChange && !change.IsOutdated)
+                {
+                    change.Property.NotifyChanged(change);
+                }
 
                 if (_listeners is object && _listeners.TryGetValue(change.Property, out var listener))
                 {
diff --git a/src/Avalonia.Base/AvaloniaPropertyChangedEventArgs.cs b/src/Avalonia.Base/AvaloniaPropertyChangedEventArgs.cs
index 94aa8baa32..3bdcbf8ff4 100644
--- a/src/Avalonia.Base/AvaloniaPropertyChangedEventArgs.cs
+++ b/src/Avalonia.Base/AvaloniaPropertyChangedEventArgs.cs
@@ -80,7 +80,7 @@ namespace Avalonia
         public bool IsOutdated { get; private set; }
 
         internal void MarkOutdated() => IsOutdated = true;
-        internal void MarkInactiveValue() => IsEffectiveValueChange = false;
+        internal void MarkNonEffectiveValue() => IsEffectiveValueChange = false;
         protected abstract AvaloniaProperty GetProperty();
         protected abstract object? GetOldValue();
         protected abstract object? GetNewValue();
diff --git a/src/Avalonia.Base/PropertyStore/PriorityValue.cs b/src/Avalonia.Base/PropertyStore/PriorityValue.cs
index 599dfdb8ad..5e223cad60 100644
--- a/src/Avalonia.Base/PropertyStore/PriorityValue.cs
+++ b/src/Avalonia.Base/PropertyStore/PriorityValue.cs
@@ -250,7 +250,7 @@ namespace Avalonia.PropertyStore
             }
             else if (change is object)
             {
-                change.MarkInactiveValue();
+                change.MarkNonEffectiveValue();
                 change.SetOldValue(default);
                 _sink.ValueChanged(change);
             }
diff --git a/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs b/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
index b235655e61..0da35ac65c 100644
--- a/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
+++ b/tests/Avalonia.Base.UnitTests/AvaloniaPropertyTests.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Avalonia.Data;
 using Avalonia.Utilities;
 using Xunit;
@@ -88,6 +89,19 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal("newvalue", value);
         }
 
+        [Fact]
+        public void Changed_Observable_Fired_Only_On_Effective_Value_Change()
+        {
+            var target = new Class1();
+            var result = new List<string>();
+
+            Class1.FooProperty.Changed.Subscribe(x => result.Add((string)x.NewValue));
+            target.SetValue(Class1.FooProperty, "animated", BindingPriority.Animation);
+            target.SetValue(Class1.FooProperty, "local");
+
+            Assert.Equal(new[] { "animated" }, result);
+        }
+
         [Fact]
         public void Property_Equals_Should_Handle_Null()
         {