Added ContentPresenter.ChildChanging.

And listen for this event in `ContentControlMixin` in order to remove the logical child before setting the `DataContext`.

Fixes #1099

src/Avalonia.Controls/Mixins/ContentControlMixin.cs

@@ -19,8 +19,8 @@ namespace Avalonia.Controls.Mixins
     /// <para>
     /// The <see cref="ContentControlMixin"/> adds behavior to a control which acts as a content
     /// control such as <see cref="ContentControl"/> and <see cref="HeaderedItemsControl"/>. It
-    /// updates keeps the control's logical children in sync with the content being displayed by
-    /// the control.
+    /// keeps the control's logical children in sync with the content being displayed by the
+    /// control.
     /// </para>
     public class ContentControlMixin
     {
@@ -49,25 +49,42 @@ namespace Avalonia.Controls.Mixins
             Contract.Requires<ArgumentNullException>(content != null);
             Contract.Requires<ArgumentNullException>(logicalChildrenSelector != null);
 
+            void ChildChanging(object s, AvaloniaPropertyChangedEventArgs e)
+            {
+                if (s is IControl sender && sender?.TemplatedParent is TControl parent)
+                {
+                    UpdateLogicalChild(
+                        sender,
+                        logicalChildrenSelector(parent),
+                        e.OldValue,
+                        null);
+                }
+            }
+
             void TemplateApplied(object s, RoutedEventArgs ev)
             {
                 if (s is TControl sender)
                 {
                     var e = (TemplateAppliedEventArgs)ev;
-                    var presenter = (IControl)e.NameScope.Find(presenterName);
+                    var presenter = e.NameScope.Find(presenterName) as IContentPresenter;
 
                     if (presenter != null)
                     {
                         presenter.ApplyTemplate();
 
                         var logicalChildren = logicalChildrenSelector(sender);
-                        var subscription = presenter
+                        var subscription = new CompositeDisposable();
+
+                        presenter.ChildChanging += ChildChanging;
+                        subscription.Add(Disposable.Create(() => presenter.ChildChanging -= ChildChanging));
+
+                        subscription.Add(presenter
                             .GetPropertyChangedObservable(ContentPresenter.ChildProperty)
                             .Subscribe(c => UpdateLogicalChild(
                                 sender,
                                 logicalChildren,
-                                c.OldValue,
-                                c.NewValue));
+                                null,
+                                c.NewValue)));
 
                         UpdateLogicalChild(
                             sender,

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -5,6 +5,7 @@ using System;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
+using Avalonia.Data;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
@@ -82,6 +83,7 @@ namespace Avalonia.Controls.Presenters
 
         private IControl _child;
         private bool _createdChild;
+        EventHandler<AvaloniaPropertyChangedEventArgs> _childChanging;
         private IDataTemplate _dataTemplate;
         private readonly BorderRenderHelper _borderRenderer = new BorderRenderHelper();
 
@@ -188,6 +190,13 @@ namespace Avalonia.Controls.Presenters
             set { SetValue(PaddingProperty, value); }
         }
 
+        /// <inheritdoc/>
+        event EventHandler<AvaloniaPropertyChangedEventArgs> IContentPresenter.ChildChanging
+        {
+            add => _childChanging += value;
+            remove => _childChanging -= value;
+        }
+
         /// <inheritdoc/>
         public sealed override void ApplyTemplate()
         {
@@ -215,9 +224,30 @@ namespace Avalonia.Controls.Presenters
             var newChild = CreateChild();
 
             // Remove the old child if we're not recycling it.
-            if (oldChild != null && newChild != oldChild)
+            if (newChild != oldChild)
             {
-                VisualChildren.Remove(oldChild);
+                if (oldChild != null)
+                {
+                    VisualChildren.Remove(oldChild);
+                }
+
+                if (oldChild?.Parent == this)
+                {
+                    // If we're the child's parent then the presenter isn't in a ContentControl's
+                    // template.
+                    LogicalChildren.Remove(oldChild);
+                }
+                else
+                {
+                    // If we're in a ContentControl's template then invoke ChildChanging to let
+                    // ContentControlMixin handle removing the logical child.
+                    _childChanging?.Invoke(this, new AvaloniaPropertyChangedEventArgs(
+                        this,
+                        ChildProperty,
+                        oldChild,
+                        newChild,
+                        BindingPriority.LocalValue));
+                }
             }
 
             // Set the DataContext if the data isn't a control.
@@ -241,11 +271,9 @@ namespace Avalonia.Controls.Presenters
 
                 Child = newChild;
 
-                if (oldChild?.Parent == this)
-                {
-                    LogicalChildren.Remove(oldChild);
-                }
-
+                // If we're in a ContentControl's template then the child's parent will have been
+                // set by ContentControlMixin in response to Child changing. If not, then we're
+                // standalone and should make the control our own logical child.
                 if (newChild.Parent == null && TemplatedParent == null)
                 {
                     LogicalChildren.Add(newChild);

src/Avalonia.Controls/Presenters/IContentPresenter.cs

@@ -1,6 +1,8 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
+using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Primitives;
 
 namespace Avalonia.Controls.Presenters
@@ -20,5 +22,16 @@ namespace Avalonia.Controls.Presenters
         /// Gets or sets the content to be displayed by the presenter.
         /// </summary>
         object Content { get; set; }
+
+        /// <summary>
+        /// Raised when <see cref="Child"/> property is about to change.
+        /// </summary>
+        /// <remarks>
+        /// This event should be raised after the child has been removed from the visual tree,
+        /// but before the <see cref="Child"/> property has changed. It is intended for consumption
+        /// by <see cref="ContentControlMixin"/> in order to update the host control's logical
+        /// children.
+        /// </remarks>
+        event EventHandler<AvaloniaPropertyChangedEventArgs> ChildChanging;
     }
-}
+}