From d5f0110ca6a8e7a15308811ef3e314db3743a42e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 12 May 2020 12:57:58 +0200
Subject: [PATCH] Raise SelectionChanged when nested children change.

---
 src/Avalonia.Controls/SelectionNode.cs        |  22 ++-
 .../SelectionModelTests.cs                    | 148 +++++++++++-------
 2 files changed, 115 insertions(+), 55 deletions(-)

diff --git a/src/Avalonia.Controls/SelectionNode.cs b/src/Avalonia.Controls/SelectionNode.cs
index e25f88ff29..2fa7c5f697 100644
--- a/src/Avalonia.Controls/SelectionNode.cs
+++ b/src/Avalonia.Controls/SelectionNode.cs
@@ -8,6 +8,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Linq;
+using Avalonia.Controls.Utils;
 
 #nullable enable
 
@@ -214,7 +215,21 @@ namespace Avalonia.Controls
 
         public void SetChildrenObservable(IObservable<object?> resolver)
         {
-            _childrenSubscription = resolver.Subscribe(x => Source = x);
+            _childrenSubscription = resolver.Subscribe(x =>
+            {
+                if (Source != null)
+                {
+                    using (_manager.Update())
+                    {
+                        SelectionTreeHelper.Traverse(
+                            this,
+                            realizeChildren: false,
+                            info => info.Node.Clear());
+                    }
+                }
+
+                Source = x;
+            });
         }
 
         public int SelectedCount { get; private set; }
@@ -544,11 +559,14 @@ namespace Avalonia.Controls
 
         private void ClearChildNodes()
         {
-            foreach (var child in _childrenNodes)
+            for (int i = 0; i < _childrenNodes.Count; i++)
             {
+                var child = _childrenNodes[i];
+
                 if (child != null && child != _manager.SharedLeafNode)
                 {
                     child.Dispose();
+                    _childrenNodes[i] = null;
                 }
             }
 
diff --git a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
index 5d5669189e..337c0e6c5f 100644
--- a/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs
@@ -1929,19 +1929,61 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Can_Replace_Children_Collection()
+        public void Can_Replace_Parent_Children_Collection()
         {
             var root = new Node("Root");
             var target = new SelectionModel { Source = new[] { root } };
+            var raised = 0;
+
             target.ChildrenRequested += (s, e) => e.Children = ((Node)e.Source).WhenAnyValue(x => x.Children);
 
             target.Select(0, 9);
 
-            Assert.Equal("Child 9", ((Node)target.SelectedItem).Header);
+            var selected = (Node)target.SelectedItem;
+            Assert.Equal("Child 9", selected.Header);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Equal(new[] { Path(0, 9) }, e.DeselectedIndices);
+                Assert.Equal(new[] { selected }, e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            root.ReplaceChildren();
+
+            Assert.Null(target.SelectedItem);
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Can_Replace_Grandparent_Children_Collection()
+        {
+            var root = new Node("Root");
+            var target = new SelectionModel { Source = new[] { root } };
+            var raised = 0;
+
+            target.ChildrenRequested += (s, e) => e.Children = ((Node)e.Source).WhenAnyValue(x => x.Children);
+
+            target.SelectAt(Path(0, 9, 1));
+
+            var selected = (Node)target.SelectedItem;
+            Assert.Equal("Child 1", selected.Header);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Equal(new[] { Path(0, 9, 1) }, e.DeselectedIndices);
+                Assert.Equal(new[] { selected }, e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
 
             root.ReplaceChildren();
 
             Assert.Null(target.SelectedItem);
+            Assert.Equal(1, raised);
         }
 
         [Fact]
@@ -1979,57 +2021,6 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(0, node.PropertyChangedSubscriptions);
         }
 
-        private class Node : INotifyPropertyChanged
-        {
-            private ObservableCollection<Node> _children;
-            private PropertyChangedEventHandler _propertyChanged;
-
-            public Node(string header)
-            {
-                Header = header;
-            }
-
-            public string Header { get; }
-
-            public ObservableCollection<Node> Children
-            {
-                get => _children ??= CreateChildren(10);
-                private set
-                {
-                    _children = value;
-                    _propertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Children)));
-                }
-            }
-
-            public event PropertyChangedEventHandler PropertyChanged
-            {
-                add
-                {
-                    _propertyChanged += value;
-                    ++PropertyChangedSubscriptions;
-                }
-
-                remove
-                {
-                    _propertyChanged -= value;
-                    --PropertyChangedSubscriptions;
-                }
-            }
-
-            public int PropertyChangedSubscriptions { get; private set; }
-
-            public void ReplaceChildren()
-            {
-                Children = CreateChildren(5);
-            }
-
-            private ObservableCollection<Node> CreateChildren(int count)
-            {
-                return new ObservableCollection<Node>(
-                    Enumerable.Range(0, count).Select(x => new Node("Child " + x)));
-            }
-        }
-
         private int GetSubscriberCount(AvaloniaList<object> list)
         {
             return ((INotifyCollectionChangedDebug)list).GetCollectionChangedSubscribers()?.Length ?? 0;
@@ -2396,6 +2387,57 @@ namespace Avalonia.Controls.UnitTests
                     new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
             }
         }
+
+        private class Node : INotifyPropertyChanged
+        {
+            private ObservableCollection<Node> _children;
+            private PropertyChangedEventHandler _propertyChanged;
+
+            public Node(string header)
+            {
+                Header = header;
+            }
+
+            public string Header { get; }
+
+            public ObservableCollection<Node> Children
+            {
+                get => _children ??= CreateChildren(10);
+                private set
+                {
+                    _children = value;
+                    _propertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Children)));
+                }
+            }
+
+            public event PropertyChangedEventHandler PropertyChanged
+            {
+                add
+                {
+                    _propertyChanged += value;
+                    ++PropertyChangedSubscriptions;
+                }
+
+                remove
+                {
+                    _propertyChanged -= value;
+                    --PropertyChangedSubscriptions;
+                }
+            }
+
+            public int PropertyChangedSubscriptions { get; private set; }
+
+            public void ReplaceChildren()
+            {
+                Children = CreateChildren(5);
+            }
+
+            private ObservableCollection<Node> CreateChildren(int count)
+            {
+                return new ObservableCollection<Node>(
+                    Enumerable.Range(0, count).Select(x => new Node("Child " + x)));
+            }
+        }
     }
 
     class CustomSelectionModel : SelectionModel