Added SelectionModel changed args.

`SelectionModel` as ported from WinUI has no information about what changed in a `SelectionChanged` event. This adds that information along with unit tests.

src/Avalonia.Controls/SelectionModel.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Linq;
 using Avalonia.Controls.Utils;
 
 #nullable enable
@@ -19,7 +20,6 @@ namespace Avalonia.Controls
         private IReadOnlyList<IndexPath>? _selectedIndicesCached;
         private IReadOnlyList<object?>? _selectedItemsCached;
         private SelectionModelChildrenRequestedEventArgs? _childrenRequestedEventArgs;
-        private SelectionModelSelectionChangedEventArgs? _selectionChangedEventArgs;
 
         public event EventHandler<SelectionModelChildrenRequestedEventArgs>? ChildrenRequested;
         public event PropertyChangedEventHandler? PropertyChanged;
@@ -36,9 +36,12 @@ namespace Avalonia.Controls
             get => _rootNode?.Source;
             set
             {
-                ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
+                using (var operation = new Operation(this))
+                {
+                    ClearSelection(resetAnchor: true);
+                }
+
                 _rootNode.Source = value;
-                OnSelectionChanged();
                 RaisePropertyChanged("Source");
             }
         }
@@ -55,12 +58,13 @@ namespace Avalonia.Controls
 
                     if (value && selectedIndices != null && selectedIndices.Count > 0)
                     {
+                        using var operation = new Operation(this);
+
                         // We want to be single select, so make sure there is only 
                         // one selected item.
                         var firstSelectionIndexPath = selectedIndices[0];
-                        ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
-                        SelectWithPathImpl(firstSelectionIndexPath, select: true, raiseSelectionChanged: false);
-                        // Setting SelectedIndex will raise SelectionChanged event.
+                        ClearSelection(resetAnchor: true);
+                        SelectWithPathImpl(firstSelectionIndexPath, select: true);
                         SelectedIndex = firstSelectionIndexPath;
                     }
 
@@ -131,9 +135,9 @@ namespace Avalonia.Controls
 
                 if (!isSelected.HasValue || !isSelected.Value)
                 {
-                    ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
-                    SelectWithPathImpl(value, select: true, raiseSelectionChanged: false);
-                    OnSelectionChanged();
+                    using var operation = new Operation(this);
+                    ClearSelection(resetAnchor: true);
+                    SelectWithPathImpl(value, select: true);
                 }
             }
         }
@@ -289,7 +293,7 @@ namespace Avalonia.Controls
 
         public void Dispose()
         {
-            ClearSelection(resetAnchor: false, raiseSelectionChanged: false);
+            ClearSelection(resetAnchor: false);
             _rootNode?.Dispose();
             _selectedIndicesCached = null;
             _selectedItemsCached = null;
@@ -299,17 +303,41 @@ namespace Avalonia.Controls
 
         public void SetAnchorIndex(int groupIndex, int index) => AnchorIndex = new IndexPath(groupIndex, index);
 
-        public void Select(int index) => SelectImpl(index, select: true);
+        public void Select(int index)
+        {
+            using var operation = new Operation(this);
+            SelectImpl(index, select: true);
+        }
 
-        public void Select(int groupIndex, int itemIndex) => SelectWithGroupImpl(groupIndex, itemIndex, select: true);
+        public void Select(int groupIndex, int itemIndex)
+        {
+            using var operation = new Operation(this);
+            SelectWithGroupImpl(groupIndex, itemIndex, select: true);
+        }
 
-        public void SelectAt(IndexPath index) => SelectWithPathImpl(index, select: true, raiseSelectionChanged: true);
+        public void SelectAt(IndexPath index)
+        {
+            using var operation = new Operation(this);
+            SelectWithPathImpl(index, select: true);
+        }
 
-        public void Deselect(int index) => SelectImpl(index, select: false);
+        public void Deselect(int index)
+        {
+            using var operation = new Operation(this);
+            SelectImpl(index, select: false);
+        }
 
-        public void Deselect(int groupIndex, int itemIndex) => SelectWithGroupImpl(groupIndex, itemIndex, select: false);
+        public void Deselect(int groupIndex, int itemIndex)
+        {
+            using var operation = new Operation(this);
+            SelectWithGroupImpl(groupIndex, itemIndex, select: false);
+        }
 
-        public void DeselectAt(IndexPath index) => SelectWithPathImpl(index, select: false, raiseSelectionChanged: true);
+        public void DeselectAt(IndexPath index)
+        {
+            using var operation = new Operation(this);
+            SelectWithPathImpl(index, select: false);
+        }
 
         public bool? IsSelected(int index)
         {
@@ -383,46 +411,56 @@ namespace Avalonia.Controls
 
         public void SelectRangeFromAnchor(int index)
         {
+            using var operation = new Operation(this);
             SelectRangeFromAnchorImpl(index, select: true);
         }
 
         public void SelectRangeFromAnchor(int endGroupIndex, int endItemIndex)
         {
+            using var operation = new Operation(this);
             SelectRangeFromAnchorWithGroupImpl(endGroupIndex, endItemIndex, select: true);
         }
 
         public void SelectRangeFromAnchorTo(IndexPath index)
         {
+            using var operation = new Operation(this);
             SelectRangeImpl(AnchorIndex, index, select: true);
         }
 
         public void DeselectRangeFromAnchor(int index)
         {
+            using var operation = new Operation(this);
             SelectRangeFromAnchorImpl(index, select: false);
         }
 
         public void DeselectRangeFromAnchor(int endGroupIndex, int endItemIndex)
         {
+            using var operation = new Operation(this);
             SelectRangeFromAnchorWithGroupImpl(endGroupIndex, endItemIndex, false /* select */);
         }
 
         public void DeselectRangeFromAnchorTo(IndexPath index)
         {
+            using var operation = new Operation(this);
             SelectRangeImpl(AnchorIndex, index, select: false);
         }
 
         public void SelectRange(IndexPath start, IndexPath end)
         {
+            using var operation = new Operation(this);
             SelectRangeImpl(start, end, select: true);
         }
 
         public void DeselectRange(IndexPath start, IndexPath end)
         {
+            using var operation = new Operation(this);
             SelectRangeImpl(start, end, select: false);
         }
 
         public void SelectAll()
         {
+            using var operation = new Operation(this);
+
             SelectionTreeHelper.Traverse(
                 _rootNode,
                 realizeChildren: true,
@@ -433,13 +471,12 @@ namespace Avalonia.Controls
                         info.Node.SelectAll();
                     }
                 });
-
-            OnSelectionChanged();
         }
 
         public void ClearSelection()
         {
-            ClearSelection(resetAnchor: true, raiseSelectionChanged: true);
+            using var operation = new Operation(this);
+            ClearSelection(resetAnchor: true);
         }
 
         protected void OnPropertyChanged(string propertyName)
@@ -452,9 +489,15 @@ namespace Avalonia.Controls
             PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
         }
 
-        public void OnSelectionInvalidatedDueToCollectionChange()
+        public void OnSelectionInvalidatedDueToCollectionChange(
+            IEnumerable<object>? removedItems)
         {
-            OnSelectionChanged();
+            var e = new SelectionModelSelectionChangedEventArgs(
+                Enumerable.Empty<IndexPath>(),
+                Enumerable.Empty<IndexPath>(),
+                removedItems ?? Enumerable.Empty<object>(),
+                Enumerable.Empty<object>());
+            OnSelectionChanged(e);
         }
 
         internal object? ResolvePath(object data, SelectionNode sourceNode)
@@ -496,7 +539,7 @@ namespace Avalonia.Controls
             return resolved;
         }
 
-        private void ClearSelection(bool resetAnchor, bool raiseSelectionChanged)
+        private void ClearSelection(bool resetAnchor)
         {
             SelectionTreeHelper.Traverse(
                 _rootNode,
@@ -507,27 +550,17 @@ namespace Avalonia.Controls
             {
                 AnchorIndex = default;
             }
-
-            if (raiseSelectionChanged)
-            {
-                OnSelectionChanged();
-            }
         }
 
-        private void OnSelectionChanged()
+        private void OnSelectionChanged(SelectionModelSelectionChangedEventArgs? e = null)
         {
             _selectedIndicesCached = null;
             _selectedItemsCached = null;
 
             // Raise SelectionChanged event
-            if (SelectionChanged != null)
+            if (e != null)
             {
-                if (_selectionChangedEventArgs == null)
-                {
-                    _selectionChangedEventArgs = new SelectionModelSelectionChangedEventArgs();
-                }
-
-                SelectionChanged(this, _selectionChangedEventArgs);
+                SelectionChanged?.Invoke(this, e);
             }
 
             RaisePropertyChanged(nameof(SelectedIndex));
@@ -544,7 +577,7 @@ namespace Avalonia.Controls
         {
             if (_singleSelect)
             {
-                ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
+                ClearSelection(resetAnchor: true);
             }
 
             var selected = _rootNode.Select(index, select);
@@ -553,15 +586,13 @@ namespace Avalonia.Controls
             {
                 AnchorIndex = new IndexPath(index);
             }
-
-            OnSelectionChanged();
         }
 
         private void SelectWithGroupImpl(int groupIndex, int itemIndex, bool select)
         {
             if (_singleSelect)
             {
-                ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
+                ClearSelection(resetAnchor: true);
             }
 
             var childNode = _rootNode.GetAt(groupIndex, realizeChild: true);
@@ -571,17 +602,15 @@ namespace Avalonia.Controls
             {
                 AnchorIndex = new IndexPath(groupIndex, itemIndex);
             }
-
-            OnSelectionChanged();
         }
 
-        private void SelectWithPathImpl(IndexPath index, bool select, bool raiseSelectionChanged)
+        private void SelectWithPathImpl(IndexPath index, bool select)
         {
             bool selected = false;
             
             if (_singleSelect)
             {
-                ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
+                ClearSelection(resetAnchor: true);
             }
 
             SelectionTreeHelper.TraverseIndexPath(
@@ -601,11 +630,6 @@ namespace Avalonia.Controls
             {
                 AnchorIndex = index;
             }
-
-            if (raiseSelectionChanged)
-            {
-                OnSelectionChanged();
-            }
         }
 
         private void SelectRangeFromAnchorImpl(int index, bool select)
@@ -618,12 +642,7 @@ namespace Avalonia.Controls
                 anchorIndex = anchor.GetAt(0);
             }
 
-            bool selected = _rootNode.SelectRange(new IndexRange(anchorIndex, index), select);
-
-            if (selected)
-            {
-                OnSelectionChanged();
-            }
+            _rootNode.SelectRange(new IndexRange(anchorIndex, index), select);
         }
 
         private void SelectRangeFromAnchorWithGroupImpl(int endGroupIndex, int endItemIndex, bool select)
@@ -650,18 +669,12 @@ namespace Avalonia.Controls
                 endItemIndex = temp;
             }
 
-            var selected = false;
             for (int groupIdx = startGroupIndex; groupIdx <= endGroupIndex; groupIdx++)
             {
                 var groupNode = _rootNode.GetAt(groupIdx, realizeChild: true)!;
                 int startIndex = groupIdx == startGroupIndex ? startItemIndex : 0;
                 int endIndex = groupIdx == endGroupIndex ? endItemIndex : groupNode.DataCount - 1;
-                selected |= groupNode.SelectRange(new IndexRange(startIndex, endIndex), select);
-            }
-
-            if (selected)
-            {
-                OnSelectionChanged();
+                groupNode.SelectRange(new IndexRange(startIndex, endIndex), select);
             }
         }
 
@@ -691,8 +704,55 @@ namespace Avalonia.Controls
                         info.ParentNode!.Select(info.Path.GetAt(info.Path.GetSize() - 1), select);
                     }
                 });
+        }
+
+        private void BeginOperation()
+        {
+            if (SelectionChanged != null)
+            {
+                _rootNode.BeginOperation();
+            }
+        }
+
+        private void EndOperation()
+        {
+            static IEnumerable<T>? Concat<T>(IEnumerable<T>? a, IEnumerable<T> b)
+            {
+                return a == null ? b : a.Concat(b);
+            }
 
-            OnSelectionChanged();
+            SelectionModelSelectionChangedEventArgs? e = null;
+
+            if (SelectionChanged != null)
+            {
+                IEnumerable<IndexPath>? selectedIndices = null;
+                IEnumerable<IndexPath>? deselectedIndices = null;
+                IEnumerable<object>? selectedItems = null;
+                IEnumerable<object>? deselectedItems = null;
+
+                foreach (var changes in _rootNode.EndOperation())
+                {
+                    if (changes.HasChanges)
+                    {
+                        selectedIndices = Concat(selectedIndices, changes.SelectedIndices);
+                        deselectedIndices = Concat(deselectedIndices, changes.DeselectedIndices);
+                        selectedItems = Concat(selectedItems, changes.SelectedItems);
+                        deselectedItems = Concat(deselectedItems, changes.DeselectedItems);
+                    }
+                }
+
+                if (selectedIndices != null || deselectedIndices != null ||
+                    selectedItems != null || deselectedItems != null)
+                {
+                    e = new SelectionModelSelectionChangedEventArgs(
+                        deselectedIndices ?? Enumerable.Empty<IndexPath>(),
+                        selectedIndices ?? Enumerable.Empty<IndexPath>(),
+                        deselectedItems ?? Enumerable.Empty<object>(),
+                        selectedItems ?? Enumerable.Empty<object>());
+                }
+            }
+
+            OnSelectionChanged(e);
         }
 
         internal class SelectedItemInfo
@@ -706,5 +766,12 @@ namespace Avalonia.Controls
             public SelectionNode Node { get; }
             public IndexPath Path { get; }
         }
+
+        private struct Operation : IDisposable
+        {
+            private readonly SelectionModel _manager;
+            public Operation(SelectionModel manager) => (_manager = manager).BeginOperation();
+            public void Dispose() => _manager.EndOperation();
+        }
     }
 }

src/Avalonia.Controls/SelectionModelChangeSet.cs

@@ -0,0 +1,144 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+#nullable enable
+
+namespace Avalonia.Controls
+{
+    internal class SelectionModelChangeSet
+    {
+        private SelectionNode _owner;
+        private List<IndexRange>? _selected;
+        private List<IndexRange>? _deselected;
+
+        public SelectionModelChangeSet(SelectionNode owner) => _owner = owner;
+
+        public bool IsTracking { get; private set; }
+        public bool HasChanges => _selected?.Count > 0 || _deselected?.Count > 0;
+        public IEnumerable<IndexPath> SelectedIndices => EnumerateIndices(_selected);
+        public IEnumerable<IndexPath> DeselectedIndices => EnumerateIndices(_deselected);
+        public IEnumerable<object> SelectedItems => EnumerateItems(_selected);
+        public IEnumerable<object> DeselectedItems => EnumerateItems(_deselected);
+
+        public void BeginOperation()
+        {
+            if (IsTracking)
+            {
+                throw new AvaloniaInternalException("SelectionModel change operation already in progress.");
+            }
+
+            IsTracking = true;
+            _selected?.Clear();
+            _deselected?.Clear();
+        }
+
+        public void EndOperation() => IsTracking = false;
+
+        public void Selected(IndexRange range)
+        {
+            if (!IsTracking)
+            {
+                return;
+            }
+
+            Add(range, ref _selected, _deselected);
+        }
+
+        public void Selected(IEnumerable<IndexRange> ranges)
+        {
+            if (!IsTracking)
+            {
+                return;
+            }
+
+            foreach (var range in ranges)
+            {
+                Selected(range);
+            }
+        }
+
+        public void Deselected(IndexRange range)
+        {
+            if (!IsTracking)
+            {
+                return;
+            }
+
+            Add(range, ref _deselected, _selected);
+        }
+
+        public void Deselected(IEnumerable<IndexRange> ranges)
+        {
+            if (!IsTracking)
+            {
+                return;
+            }
+
+            foreach (var range in ranges)
+            {
+                Deselected(range);
+            }
+        }
+
+        private static void Add(
+            IndexRange range,
+            ref List<IndexRange>? add,
+            List<IndexRange>? remove)
+        {
+            if (remove != null)
+            {
+                var removed = new List<IndexRange>();
+                IndexRange.Remove(remove, range, removed);
+                var selected = IndexRange.Subtract(range, removed);
+
+                if (selected.Any())
+                {
+                    add ??= new List<IndexRange>();
+
+                    foreach (var r in selected)
+                    {
+                        IndexRange.Add(add, r);
+                    }
+                }
+            }
+            else
+            {
+                add ??= new List<IndexRange>();
+                IndexRange.Add(add, range);
+            }
+        }
+
+        private IEnumerable<IndexPath> EnumerateIndices(IEnumerable<IndexRange>? ranges)
+        {
+            var path = _owner.IndexPath;
+
+            if (ranges != null)
+            {
+                foreach (var range in ranges)
+                {
+                    for (var i = range.Begin; i <= range.End; ++i)
+                    {
+                        yield return path.CloneWithChildIndex(i);
+                    }
+                }
+            }
+        }
+
+        private IEnumerable<object> EnumerateItems(IEnumerable<IndexRange>? ranges)
+        {
+            var items = _owner.ItemsSourceView;
+
+            if (ranges != null && items != null)
+            {
+                foreach (var range in ranges)
+                {
+                    for (var i = range.Begin; i <= range.End; ++i)
+                    {
+                        yield return items.GetAt(i);
+                    }
+                }
+            }
+        }
+    }
+}

src/Avalonia.Controls/SelectionModelSelectionChangedEventArgs.cs

@@ -4,6 +4,7 @@
 // Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
 using System;
+using System.Collections.Generic;
 
 #nullable enable
 
@@ -11,5 +12,49 @@ namespace Avalonia.Controls
 {
     public class SelectionModelSelectionChangedEventArgs : EventArgs
     {
+        private readonly IEnumerable<IndexPath> _selectedIndicesSource;
+        private readonly IEnumerable<IndexPath> _deselectedIndicesSource;
+        private readonly IEnumerable<object> _selectedItemsSource;
+        private readonly IEnumerable<object> _deselectedItemsSource;
+        private List<IndexPath>? _selectedIndices;
+        private List<IndexPath>? _deselectedIndices;
+        private List<object>? _selectedItems;
+        private List<object>? _deselectedItems;
+
+        public SelectionModelSelectionChangedEventArgs(
+            IEnumerable<IndexPath> deselectedIndices,
+            IEnumerable<IndexPath> selectedIndices,
+            IEnumerable<object> deselectedItems,
+            IEnumerable<object> selectedItems)
+        {
+            _selectedIndicesSource = selectedIndices;
+            _deselectedIndicesSource = deselectedIndices;
+            _selectedItemsSource = selectedItems;
+            _deselectedItemsSource = deselectedItems;
+        }
+
+        /// <summary>
+        /// Gets the indices of the items that were added to the selection.
+        /// </summary>
+        public IReadOnlyList<IndexPath> SelectedIndices =>
+            _selectedIndices ?? (_selectedIndices = new List<IndexPath>(_selectedIndicesSource));
+
+        /// <summary>
+        /// Gets the indices of the items that were removed from the selection.
+        /// </summary>
+        public IReadOnlyList<IndexPath> DeselectedIndices =>
+            _deselectedIndices ?? (_deselectedIndices = new List<IndexPath>(_deselectedIndicesSource));
+
+        /// <summary>
+        /// Gets the items that were added to the selection.
+        /// </summary>
+        public IReadOnlyList<object> SelectedItems =>
+            _selectedItems ?? (_selectedItems = new List<object>(_selectedItemsSource));
+
+        /// <summary>
+        /// Gets the items that were removed from the selection.
+        /// </summary>
+        public IReadOnlyList<object> DeselectedItems =>
+            _deselectedItems ?? (_deselectedItems = new List<object>(_deselectedItemsSource));
     }
 }

src/Avalonia.Controls/SelectionNode.cs

@@ -7,6 +7,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Linq;
 
 #nullable enable
 
@@ -28,6 +29,7 @@ namespace Avalonia.Controls
         private readonly SelectionNode? _parent;
         private readonly List<IndexRange> _selected = new List<IndexRange>();
         private readonly List<int> _selectedIndicesCached = new List<int>();
+        private SelectionModelChangeSet? _changes;
         private object? _source;
         private bool _selectedIndicesCacheIsValid;
 
@@ -134,6 +136,11 @@ namespace Avalonia.Controls
                         {
                             child = new SelectionNode(_manager, parent: this);
                             child.Source = resolvedChild;
+
+                            if (_changes?.IsTracking == true)
+                            {
+                                child.BeginOperation();
+                            }
                         }
                         else
                         {
@@ -276,12 +283,50 @@ namespace Avalonia.Controls
             }
         }
 
+        public IEnumerable<object> SelectedItems
+        {
+            get => SelectedIndices.Select(x => ItemsSourceView!.GetAt(x));
+        }
+
         public void Dispose()
         {
             ItemsSourceView?.Dispose();
             UnhookCollectionChangedHandler();
         }
 
+        public void BeginOperation()
+        {
+            _changes ??= new SelectionModelChangeSet(this);
+            _changes.BeginOperation();
+
+            for (var i = 0; i < _childrenNodes.Count; ++i)
+            {
+                _childrenNodes[i]?.BeginOperation();
+            }
+        }
+
+        public IEnumerable<SelectionModelChangeSet> EndOperation()
+        {
+            if (_changes != null)
+            {
+                _changes.EndOperation();
+                yield return _changes;
+
+                for (var i = 0; i < _childrenNodes.Count; ++i)
+                {
+                    var child = _childrenNodes[i];
+
+                    if (child != null)
+                    {
+                        foreach (var changes in child.EndOperation())
+                        {
+                            yield return changes;
+                        }
+                    }
+                }
+            }
+        }
+
         public bool Select(int index, bool select)
         {
             return Select(index, select, raiseOnSelectionChanged: true);
@@ -349,21 +394,13 @@ namespace Avalonia.Controls
 
         private void AddRange(IndexRange addRange, bool raiseOnSelectionChanged)
         {
-            // TODO: Check for duplicates (Task 14107720)
-            // TODO: Optimize by merging adjacent ranges (Task 14107720)
-            var oldCount = SelectedCount;
+            var selected = new List<IndexRange>();
 
-            for (int i = addRange.Begin; i <= addRange.End; i++)
-            {
-                if (!IsSelected(i))
-                {
-                    SelectedCount++;
-                }
-            }
+            SelectedCount += IndexRange.Add(_selected, addRange, selected);
 
-            if (oldCount != SelectedCount)
+            if (selected.Count > 0)
             {
-                _selected.Add(addRange);
+                _changes?.Selected(selected);
 
                 if (raiseOnSelectionChanged)
                 {
@@ -374,71 +411,17 @@ namespace Avalonia.Controls
 
         private void RemoveRange(IndexRange removeRange, bool raiseOnSelectionChanged)
         {
-            int oldCount = SelectedCount;
+            var removed = new List<IndexRange>();
 
-            // TODO: Prevent overlap of Ranges in _selected (Task 14107720)
-            for (int i = removeRange.Begin; i <= removeRange.End; i++)
-            {
-                if (IsSelected(i))
-                {
-                    SelectedCount--;
-                }
-            }
+            SelectedCount -= IndexRange.Remove(_selected, removeRange, removed);
 
-            if (oldCount != SelectedCount)
+            if (removed.Count > 0)
             {
-                // Build up a both a list of Ranges to remove and ranges to add
-                var toRemove = new List<IndexRange>();
-                var toAdd = new List<IndexRange>();
-
-                foreach (var range in _selected)
-                {
-                    // If this range intersects the remove range, we have to do something
-                    if (removeRange.Intersects(range))
-                    {
-                        // Intersection with the beginning of the range
-                        //  Anything to the left of the point (exclusive) stays
-                        //  Anything to the right of the point (inclusive) gets clipped
-                        if (range.Contains(removeRange.Begin - 1))
-                        {
-                            range.Split(removeRange.Begin - 1, out var before, out _);
-                            toAdd.Add(before);
-                        }
+                _changes?.Deselected(removed);
 
-                        // Intersection with the end of the range
-                        //  Anything to the left of the point (inclusive) gets clipped
-                        //  Anything to the right of the point (exclusive) stays
-                        if (range.Contains(removeRange.End))
-                        {
-                            if (range.Split(removeRange.End, out _, out var after))
-                            {
-                                toAdd.Add(after);
-                            }
-                        }
-
-                        // Remove this Range from the collection
-                        // New ranges will be added for any remaining subsections
-                        toRemove.Add(range);
-                    }
-                }
-
-                bool change = ((toRemove.Count > 0) || (toAdd.Count > 0));
-
-                if (change)
+                if (raiseOnSelectionChanged)
                 {
-                    // Remove tagged ranges
-                    foreach (var remove in toRemove)
-                    {
-                        _selected.Remove(remove);
-                    }
-
-                    // Add new ranges
-                    _selected.AddRange(toAdd);
-
-                    if (raiseOnSelectionChanged)
-                    {
-                        OnSelectionChanged();
-                    }
+                    OnSelectionChanged();
                 }
             }
         }
@@ -448,6 +431,7 @@ namespace Avalonia.Controls
             // Deselect all items
             if (_selected.Count > 0)
             {
+                _changes?.Deselected(_selected);
                 _selected.Clear();
                 OnSelectionChanged();
             }
@@ -496,6 +480,7 @@ namespace Avalonia.Controls
         private void OnSourceListChanged(object dataSource, NotifyCollectionChangedEventArgs args)
         {
             bool selectionInvalidated = false;
+            IList<object>? removed = null;
 
             switch (args.Action)
             {
@@ -507,7 +492,7 @@ namespace Avalonia.Controls
 
                 case NotifyCollectionChangedAction.Remove:
                 {
-                    selectionInvalidated = OnItemsRemoved(args.OldStartingIndex, args.OldItems.Count);
+                    (selectionInvalidated, removed) = OnItemsRemoved(args.OldStartingIndex, args.OldItems);
                     break;
                 }
 
@@ -520,7 +505,7 @@ namespace Avalonia.Controls
 
                 case NotifyCollectionChangedAction.Replace:
                 {
-                    selectionInvalidated = OnItemsRemoved(args.OldStartingIndex, args.OldItems.Count);
+                    (selectionInvalidated, removed) = OnItemsRemoved(args.OldStartingIndex, args.OldItems);
                     selectionInvalidated |= OnItemsAdded(args.NewStartingIndex, args.NewItems.Count);
                     break;
                 }
@@ -529,7 +514,7 @@ namespace Avalonia.Controls
             if (selectionInvalidated)
             {
                 OnSelectionChanged();
-                _manager.OnSelectionInvalidatedDueToCollectionChange();
+                _manager.OnSelectionInvalidatedDueToCollectionChange(removed);
             }
         }
 
@@ -609,21 +594,23 @@ namespace Avalonia.Controls
             return selectionInvalidated;
         }
 
-        private bool OnItemsRemoved(int index, int count)
+        private (bool, IList<object>) OnItemsRemoved(int index, IList items)
         {
-            bool selectionInvalidated = false;
+            var selectionInvalidated = false;
+            var removed = new List<object>();
+            var count = items.Count;
             
             // Remove the items from the selection for leaf
             if (ItemsSourceView!.Count > 0)
             {
                 bool isSelected = false;
 
-                for (int i = index; i <= index + count - 1; i++)
+                for (int i = 0; i <= count - 1; i++)
                 {
-                    if (IsSelected(i))
+                    if (IsSelected(index + i))
                     {
                         isSelected = true;
-                        break;
+                        removed.Add(items[i]);
                     }
                 }
 
@@ -654,6 +641,7 @@ namespace Avalonia.Controls
                     {
                         if (_childrenNodes[index] != null)
                         {
+                            removed.AddRange(_childrenNodes[index]!.SelectedItems);
                             RealizedChildrenNodeCount--;
                         }
                         _childrenNodes.RemoveAt(index);
@@ -696,7 +684,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            return selectionInvalidated;
+            return (selectionInvalidated, removed);
         }
 
         private void OnSelectionChanged()

tests/Avalonia.Controls.UnitTests/SelectionModelTests.cs

@@ -898,6 +898,422 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
+        [Fact]
+        public void Selecting_Item_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(4) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 4 }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.Select(4);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Selecting_Already_Selected_Item_Doesnt_Raise_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+            target.SelectionChanged += (s, e) => ++raised;
+            target.Select(4);
+
+            Assert.Equal(0, raised);
+        }
+
+        [Fact]
+        public void SingleSelecting_Item_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel { SingleSelect = true };
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(3);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Equal(new[] { new IndexPath(3) }, e.DeselectedIndices);
+                Assert.Equal(new object[] { 3 }, e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(4) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 4 }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.Select(4);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SingleSelecting_Already_Selected_Item_Doesnt_Raise_SelectionChanged()
+        {
+            var target = new SelectionModel { SingleSelect = true };
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+            target.SelectionChanged += (s, e) => ++raised;
+            target.Select(4);
+
+            Assert.Equal(0, raised);
+        }
+
+        [Fact]
+        public void Selecting_Item_With_Group_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = CreateNestedData(1, 2, 3);
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(1, 1) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 4 }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.Select(1, 1);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectAt_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = CreateNestedData(1, 2, 3);
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(1, 1) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 4 }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.SelectAt(new IndexPath(1, 1));
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectAll_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel { SingleSelect = true };
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(0, 10);
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(expected.Select(x => new IndexPath(x)), e.SelectedIndices);
+                Assert.Equal(expected, e.SelectedItems.Cast<int>());
+                ++raised;
+            };
+
+            target.SelectAll();
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectAll_With_Already_Selected_Items_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel { SingleSelect = true };
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(0, 10).Except(new[] { 4 });
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(expected.Select(x => new IndexPath(x)), e.SelectedIndices);
+                Assert.Equal(expected, e.SelectedItems.Cast<int>());
+                ++raised;
+            };
+
+            target.SelectAll();
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectRangeFromAnchor_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(4, 3);
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(expected.Select(x => new IndexPath(x)), e.SelectedIndices);
+                Assert.Equal(expected, e.SelectedItems.Cast<int>());
+                ++raised;
+            };
+
+            target.AnchorIndex = new IndexPath(4);
+            target.SelectRangeFromAnchor(6);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectRangeFromAnchor_With_Group_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = CreateNestedData(1, 2, 10);
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(11, 6);
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(expected.Select(x => new IndexPath(x / 10, x % 10)), e.SelectedIndices);
+                Assert.Equal(expected, e.SelectedItems.Cast<int>());
+                ++raised;
+            };
+
+            target.AnchorIndex = new IndexPath(1, 1);
+            target.SelectRangeFromAnchor(1, 6);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void SelectRangeFromAnchorTo_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = CreateNestedData(1, 2, 10);
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(11, 6);
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Equal(expected.Select(x => new IndexPath(x / 10, x % 10)), e.SelectedIndices);
+                Assert.Equal(expected, e.SelectedItems.Cast<int>());
+                ++raised;
+            };
+
+            target.AnchorIndex = new IndexPath(1, 1);
+            target.SelectRangeFromAnchorTo(new IndexPath(1, 6));
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void ClearSelection_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+            target.Select(5);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(4, 2);
+                Assert.Equal(expected.Select(x => new IndexPath(x)), e.DeselectedIndices);
+                Assert.Equal(expected, e.DeselectedItems.Cast<int>());
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            target.ClearSelection();
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Changing_Source_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+            target.Select(5);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                var expected = Enumerable.Range(4, 2);
+                Assert.Equal(expected.Select(x => new IndexPath(x)), e.DeselectedIndices);
+                Assert.Equal(expected, e.DeselectedItems.Cast<int>());
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            target.Source = Enumerable.Range(20, 10).ToList();
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Setting_SelectedIndex_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var raised = 0;
+
+            target.Source = Enumerable.Range(0, 10).ToList();
+            target.Select(4);
+            target.Select(5);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Equal(new[] { new IndexPath(4), new IndexPath(5) }, e.DeselectedIndices);
+                Assert.Equal(new object[] { 4, 5 }, e.DeselectedItems);
+                Assert.Equal(new[] { new IndexPath(6) }, e.SelectedIndices);
+                Assert.Equal(new object[] { 6 }, e.SelectedItems);
+                ++raised;
+            };
+
+            target.SelectedIndex = new IndexPath(6);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Removing_Selected_Item_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var data = new ObservableCollection<int>(Enumerable.Range(0, 10));
+            var raised = 0;
+
+            target.Source = data;
+            target.Select(4);
+            target.Select(5);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Equal(new object[] { 4 }, e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            data.Remove(4);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Removing_Selected_Child_Item_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var data = CreateNestedData(1, 2, 3);
+            var raised = 0;
+
+            target.Source = data;
+            target.SelectRange(new IndexPath(0), new IndexPath(1, 1));
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Equal(new object[] { 1}, e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            ((AvaloniaList<object>)data[0]).RemoveAt(1);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Removing_Selected_Item_With_Children_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var data = CreateNestedData(1, 2, 3);
+            var raised = 0;
+
+            target.Source = data;
+            target.SelectRange(new IndexPath(0), new IndexPath(1, 1));
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Equal(new object[] { 0, 1, 2 }, e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            data.RemoveAt(0);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Removing_Unselected_Item_Before_Selected_Item_Raises_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var data = new ObservableCollection<int>(Enumerable.Range(0, 10));
+            var raised = 0;
+
+            target.Source = data;
+            target.Select(8);
+
+            target.SelectionChanged += (s, e) =>
+            {
+                Assert.Empty(e.DeselectedIndices);
+                Assert.Empty(e.DeselectedItems);
+                Assert.Empty(e.SelectedIndices);
+                Assert.Empty(e.SelectedItems);
+                ++raised;
+            };
+
+            data.Remove(6);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void Removing_Unselected_Item_After_Selected_Item_Doesnt_Raise_SelectionChanged()
+        {
+            var target = new SelectionModel();
+            var data = new ObservableCollection<int>(Enumerable.Range(0, 10));
+            var raised = 0;
+
+            target.Source = data;
+            target.Select(4);
+
+            target.SelectionChanged += (s, e) => ++raised;
+
+            data.Remove(6);
+
+            Assert.Equal(0, raised); 
+        }
 
         [Fact]
         public void Disposing_Unhooks_CollectionChanged_Handlers()