Fix DataContext notifications.

src/Avalonia.Base/AvaloniaObject.cs

@@ -596,32 +596,21 @@ namespace Avalonia
             BindingPriority priority,
             bool isEffectiveValue)
         {
-            if (isEffectiveValue)
-                property.Notifying?.Invoke(this, true);
-
-            try
-            {
-                var e = new AvaloniaPropertyChangedEventArgs<T>(
-                    this,
-                    property,
-                    oldValue,
-                    newValue,
-                    priority,
-                    isEffectiveValue);
+            var e = new AvaloniaPropertyChangedEventArgs<T>(
+                this,
+                property,
+                oldValue,
+                newValue,
+                priority,
+                isEffectiveValue);
 
-                OnPropertyChangedCore(e);
+            OnPropertyChangedCore(e);
 
-                if (isEffectiveValue)
-                {
-                    property.NotifyChanged(e);
-                    _propertyChanged?.Invoke(this, e);
-                    _inpcChanged?.Invoke(this, new PropertyChangedEventArgs(property.Name));
-                }
-            }
-            finally
+            if (isEffectiveValue)
             {
-                if (isEffectiveValue)
-                    property.Notifying?.Invoke(this, false);
+                property.NotifyChanged(e);
+                _propertyChanged?.Invoke(this, e);
+                _inpcChanged?.Invoke(this, new PropertyChangedEventArgs(property.Name));
             }
         }
 

src/Avalonia.Base/PropertyStore/EffectiveValue`1.cs

@@ -133,6 +133,7 @@ namespace Avalonia.PropertyStore
 
             if (valueChanged)
             {
+                using var notifying = PropertyNotifying.Start(owner.Owner, property);
                 owner.Owner.RaisePropertyChanged(property, oldValue, Value, Priority, true);
                 if (property.Inherits)
                     owner.OnInheritedEffectiveValueChanged(property, oldValue, this);
@@ -200,6 +201,7 @@ namespace Avalonia.PropertyStore
 
             if (valueChanged)
             {
+                using var notifying = PropertyNotifying.Start(owner.Owner, property);
                 owner.Owner.RaisePropertyChanged(property, oldValue, Value, Priority, true);
                 if (property.Inherits)
                     owner.OnInheritedEffectiveValueChanged(property, oldValue, this);

src/Avalonia.Base/PropertyStore/PropertyNotifying.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Diagnostics;
+
+namespace Avalonia.PropertyStore
+{
+    /// <summary>
+    /// Raises <see cref="AvaloniaProperty.Notifying"/> where necessary.
+    /// </summary>
+    /// <remarks>
+    /// Uses the disposable pattern to ensure that the closing Notifying call is made even in the
+    /// presence of exceptions. 
+    /// </remarks>
+    internal readonly struct PropertyNotifying : IDisposable
+    {
+        private readonly AvaloniaObject _owner;
+        private readonly AvaloniaProperty _property;
+
+        private PropertyNotifying(AvaloniaObject owner, AvaloniaProperty property)
+        {
+            Debug.Assert(property.Notifying is not null);
+            _owner = owner;
+            _property = property;
+            _property.Notifying!(owner, true);
+        }
+
+        public void Dispose() => _property.Notifying!(_owner, false);
+
+        public static PropertyNotifying? Start(AvaloniaObject owner, AvaloniaProperty property)
+        {
+            if (property.Notifying is null)
+                return null;
+            return new PropertyNotifying(owner, property);
+        }
+    }
+}

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -476,6 +476,8 @@ namespace Avalonia.PropertyStore
             if (_effectiveValues is not null && _effectiveValues.ContainsKey(property))
                 return;
 
+            using var notifying = PropertyNotifying.Start(Owner, property);
+
             Owner.RaisePropertyChanged(
                 property,
                 oldValue,
@@ -769,6 +771,8 @@ namespace Avalonia.PropertyStore
             if (_effectiveValues?.ContainsKey(property) == true)
                 return;
 
+            using var notifying = PropertyNotifying.Start(Owner, property);
+
             // Raise PropertyChanged on this object if necessary.
             (oldValue ?? newValue!).RaiseInheritedValueChanged(Owner, property, oldValue, newValue);
 

tests/Avalonia.Base.UnitTests/Styling/StyledElementTests.cs

@@ -470,6 +470,57 @@ namespace Avalonia.Base.UnitTests.Styling
                 called);
         }
 
+
+        [Fact]
+        public void DataContext_Notifications_Should_Be_Called_In_Correct_Order_When_Setting_Parent()
+        {
+            var root = new TestStackPanel
+            {
+                Name = "root",
+                DataContext = "foo",
+            };
+
+            var children = new[]
+            {
+                new TestControl
+                {
+                    Name = "a1",
+                    Child = new TestControl
+                    {
+                        Name = "b1",
+                    }
+                },
+                new TestControl
+                {
+                    Name = "a2",
+                    DataContext = "foo",
+                },
+            };
+
+            var called = new List<string>();
+
+            foreach (IDataContextEvents c in new[] { children[0], children[0].Child, children[1] })
+            {
+                c.DataContextBeginUpdate += (s, e) => called.Add("begin " + ((StyledElement)s).Name);
+                c.DataContextChanged += (s, e) => called.Add("changed " + ((StyledElement)s).Name);
+                c.DataContextEndUpdate += (s, e) => called.Add("end " + ((StyledElement)s).Name);
+            }
+
+            root.Children.AddRange(children);
+
+            Assert.Equal(
+                new[]
+                {
+                    "begin a1",
+                    "begin b1",
+                    "changed a1",
+                    "changed b1",
+                    "end b1",
+                    "end a1",
+                },
+                called);
+        }
+
         [Fact]
         public void Resources_Owner_Is_Set()
         {