Notify DataContextChanged down tree.

When a data context changes, it should be notified to all descendant controls who inherit the data context. This allows the `SelectingItemsControl` to defer setting the selected item until the `DataContext` has changed for the whole tree.

Renamed `OnDataContextChanging`/`OnDataContextChanged` to `OnDataContextBeginUpdate`/`OnDataContextEndUpdate` because `DataContextChanged` is now called from the property changed notification.

Fixes #507.

src/Avalonia.Controls/Control.cs

@@ -101,6 +101,7 @@ namespace Avalonia.Controls
         private Styles _styles;
         private bool _styled;
         private Subject<IStyleable> _styleDetach = new Subject<IStyleable>();
+        private bool _dataContextUpdating;
 
         /// <summary>
         /// Initializes static members of the <see cref="Control"/> class.
@@ -111,6 +112,7 @@ namespace Avalonia.Controls
             PseudoClass(IsEnabledCoreProperty, x => !x, ":disabled");
             PseudoClass(IsFocusedProperty, ":focus");
             PseudoClass(IsPointerOverProperty, ":pointerover");
+            DataContextProperty.Changed.AddClassHandler<Control>(x => x.OnDataContextChangedCore);
         }
 
         /// <summary>
@@ -681,18 +683,26 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Called before the <see cref="DataContext"/> property changes.
+        /// Called when the <see cref="DataContext"/> property changes.
         /// </summary>
-        protected virtual void OnDataContextChanging()
+        /// <param name="e">The event args.</param>
+        protected virtual void OnDataContextChanged(EventArgs e)
         {
+            DataContextChanged?.Invoke(this, EventArgs.Empty);
         }
 
         /// <summary>
-        /// Called after the <see cref="DataContext"/> property changes.
+        /// Called when the <see cref="DataContext"/> begins updating.
         /// </summary>
-        protected virtual void OnDataContextChanged()
+        protected virtual void OnDataContextBeginUpdate()
+        {
+        }
+
+        /// <summary>
+        /// Called when the <see cref="DataContext"/> finishes updating.
+        /// </summary>
+        protected virtual void OnDataContextEndUpdate()
         {
-            DataContextChanged?.Invoke(this, EventArgs.Empty);
         }
 
         /// <inheritdoc/>
@@ -745,24 +755,38 @@ namespace Avalonia.Controls
             }
         }
 
-        /// <summary>
-        /// Called when the <see cref="DataContext"/> property begins and ends being notified.
-        /// </summary>
-        /// <param name="o">The object on which the DataContext is changing.</param>
-        /// <param name="notifying">Whether the notifcation is beginning or ending.</param>
         private static void DataContextNotifying(IAvaloniaObject o, bool notifying)
         {
-            var control = o as Control;
+            if (o is Control control)
+            {
+                DataContextNotifying(control, notifying);
+            }
+        }
 
-            if (control != null)
+        private static void DataContextNotifying(Control control, bool notifying)
+        {
+            if (notifying)
             {
-                if (notifying)
+                if (!control._dataContextUpdating)
                 {
-                    control.OnDataContextChanging();
+                    control._dataContextUpdating = true;
+                    control.OnDataContextBeginUpdate();
+
+                    foreach (var child in control.LogicalChildren)
+                    {
+                        if (child is Control c && !c.IsSet(DataContextProperty))
+                        {
+                            DataContextNotifying(c, notifying);
+                        }
+                    }
                 }
-                else
+            }
+            else
+            {
+                if (control._dataContextUpdating)
                 {
-                    control.OnDataContextChanged();
+                    control.OnDataContextEndUpdate();
+                    control._dataContextUpdating = false;
                 }
             }
         }
@@ -881,6 +905,11 @@ namespace Avalonia.Controls
             }
         }
 
+        private void OnDataContextChangedCore(AvaloniaPropertyChangedEventArgs e)
+        {
+            OnDataContextChanged(EventArgs.Empty);
+        }
+
         private void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
             switch (e.Action)

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -414,16 +414,16 @@ namespace Avalonia.Controls.Primitives
         }
 
         /// <inheritdoc/>
-        protected override void OnDataContextChanging()
+        protected override void OnDataContextBeginUpdate()
         {
-            base.OnDataContextChanging();
+            base.OnDataContextBeginUpdate();
             ++_updateCount;
         }
 
         /// <inheritdoc/>
-        protected override void OnDataContextChanged()
+        protected override void OnDataContextEndUpdate()
         {
-            base.OnDataContextChanged();
+            base.OnDataContextEndUpdate();
 
             if (--_updateCount == 0)
             {

src/Avalonia.Styling/LogicalTree/LogicalExtensions.cs

@@ -50,6 +50,16 @@ namespace Avalonia.LogicalTree
             }
         }
 
+        public static IEnumerable<ILogical> GetSelfAndLogicalDescendants(this ILogical logical)
+        {
+            yield return logical;
+
+            foreach (var descendent in logical.GetLogicalDescendants())
+            {
+                yield return descendent;
+            }
+        }
+
         public static ILogical GetLogicalParent(this ILogical logical)
         {
             return logical.LogicalParent;

tests/Avalonia.Controls.UnitTests/ControlTests.cs

@@ -8,6 +8,7 @@ using Moq;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Xunit;
+using Avalonia.LogicalTree;
 
 namespace Avalonia.Controls.UnitTests
 {
@@ -328,9 +329,139 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(target.IsInitialized);
         }
 
-        private class TestControl : Control
+        [Fact]
+        public void DataContextChanged_Should_Be_Called()
+        {
+            var root = new TestStackPanel
+            {
+                Name = "root",
+                Children =
+                {
+                    new TestControl
+                    {
+                        Name = "a1",
+                        Child = new TestControl
+                        {
+                            Name = "b1",
+                        }
+                    },
+                    new TestControl
+                    {
+                        Name = "a2",
+                        DataContext = "foo",
+                    },
+                }
+            };
+
+            var called = new List<string>();
+            void Record(object sender, EventArgs e) => called.Add(((Control)sender).Name);
+
+            root.DataContextChanged += Record;
+
+            foreach (TestControl c in root.GetLogicalDescendants())
+            {
+                c.DataContextChanged += Record;
+            }
+
+            root.DataContext = "foo";
+
+            Assert.Equal(new[] { "root", "a1", "b1", }, called);
+        }
+
+        [Fact]
+        public void DataContext_Notifications_Should_Be_Called_In_Correct_Order()
         {
+            var root = new TestStackPanel
+            {
+                Name = "root",
+                Children =
+                {
+                    new TestControl
+                    {
+                        Name = "a1",
+                        Child = new TestControl
+                        {
+                            Name = "b1",
+                        }
+                    },
+                    new TestControl
+                    {
+                        Name = "a2",
+                        DataContext = "foo",
+                    },
+                }
+            };
+
+            var called = new List<string>();
+
+            foreach (IDataContextEvents c in root.GetSelfAndLogicalDescendants())
+            {
+                c.DataContextBeginUpdate += (s, e) => called.Add("begin " + ((Control)s).Name);
+                c.DataContextChanged += (s, e) => called.Add("changed " + ((Control)s).Name);
+                c.DataContextEndUpdate += (s, e) => called.Add("end " + ((Control)s).Name);
+            }
+
+            root.DataContext = "foo";
+
+            Assert.Equal(
+                new[] 
+                {
+                    "begin root",
+                    "begin a1",
+                    "begin b1",
+                    "changed root",
+                    "changed a1",
+                    "changed b1",
+                    "end b1",
+                    "end a1",
+                    "end root",
+                },
+                called);
+        }
+
+        private interface IDataContextEvents
+        {
+            event EventHandler DataContextBeginUpdate;
+            event EventHandler DataContextChanged;
+            event EventHandler DataContextEndUpdate;
+        }
+
+        private class TestControl : Decorator, IDataContextEvents
+        {
+            public event EventHandler DataContextBeginUpdate;
+            public event EventHandler DataContextEndUpdate;
+
             public new IAvaloniaObject InheritanceParent => base.InheritanceParent;
+
+            protected override void OnDataContextBeginUpdate()
+            {
+                DataContextBeginUpdate?.Invoke(this, EventArgs.Empty);
+                base.OnDataContextBeginUpdate();
+            }
+
+            protected override void OnDataContextEndUpdate()
+            {
+                DataContextEndUpdate?.Invoke(this, EventArgs.Empty);
+                base.OnDataContextEndUpdate();
+            }
+        }
+
+        private class TestStackPanel : StackPanel, IDataContextEvents
+        {
+            public event EventHandler DataContextBeginUpdate;
+            public event EventHandler DataContextEndUpdate;
+
+            protected override void OnDataContextBeginUpdate()
+            {
+                DataContextBeginUpdate?.Invoke(this, EventArgs.Empty);
+                base.OnDataContextBeginUpdate();
+            }
+
+            protected override void OnDataContextEndUpdate()
+            {
+                DataContextEndUpdate?.Invoke(this, EventArgs.Empty);
+                base.OnDataContextEndUpdate();
+            }
         }
     }
 }