Make switching between flat and tree layouts easier.

samples/TreeDataGridDemo/MainWindow.axaml

@@ -65,6 +65,11 @@
                     DockPanel.Dock="Right">
             Cell Selection
           </CheckBox>
+          <CheckBox IsChecked="{Binding Files.FlatList}"
+                    Margin="4 0 0 0"
+                    DockPanel.Dock="Right">
+            Flat
+          </CheckBox>
           <TextBox Text="{Binding Files.SelectedPath, Mode=OneWay}"
                    Margin="4 0 0 0"
                    VerticalContentAlignment="Center"

samples/TreeDataGridDemo/ViewModels/FilesPageViewModel.cs

@@ -5,7 +5,6 @@
 using System.Linq;
 using System.Reactive.Linq;
 using System.Runtime.InteropServices;
-using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Models.TreeDataGrid;
 using Avalonia.Controls.Selection;
@@ -20,6 +19,9 @@ namespace TreeDataGridDemo.ViewModels
     public class FilesPageViewModel : ReactiveObject
     {
         private static IconConverter? s_iconConverter;
+        private readonly HierarchicalTreeDataGridSource<FileTreeNodeModel>? _treeSource;
+        private FlatTreeDataGridSource<FileTreeNodeModel>? _flatSource;
+        private ITreeDataGridSource<FileTreeNodeModel> _source;
         private bool _cellSelection;
         private FileTreeNodeModel? _root;
         private string _selectedDrive;
@@ -38,61 +40,17 @@ public FilesPageViewModel()
                 _selectedDrive = Drives.FirstOrDefault() ?? "/";
             }
 
-            Source = new HierarchicalTreeDataGridSource<FileTreeNodeModel>(Array.Empty<FileTreeNodeModel>())
-            {
-                Columns =
-                {
-                    new CheckBoxColumn<FileTreeNodeModel>(
-                        null,
-                        x => x.IsChecked,
-                        (o, v) => o.IsChecked = v,
-                        options: new()
-                        {
-                            CanUserResizeColumn = false,
-                        }),
-                    new HierarchicalExpanderColumn<FileTreeNodeModel>(
-                        new TemplateColumn<FileTreeNodeModel>(
-                            "Name",
-                            "FileNameCell",
-                            "FileNameEditCell",
-                            new GridLength(1, GridUnitType.Star),
-                            new()
-                            {
-                                CompareAscending = FileTreeNodeModel.SortAscending(x => x.Name),
-                                CompareDescending = FileTreeNodeModel.SortDescending(x => x.Name),
-                                IsTextSearchEnabled = true,
-                                TextSearchValueSelector = x => x.Name
-                            }),
-                        x => x.Children,
-                        x => x.HasChildren,
-                        x => x.IsExpanded),
-                    new TextColumn<FileTreeNodeModel, long?>(
-                        "Size",
-                        x => x.Size,
-                        options: new()
-                        {
-                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Size),
-                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Size),
-                        }),
-                    new TextColumn<FileTreeNodeModel, DateTimeOffset?>(
-                        "Modified",
-                        x => x.Modified,
-                        options: new()
-                        {
-                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Modified),
-                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Modified),
-                        }),
-                }
-            };
-
-            Source.RowSelection!.SingleSelect = false;
-            Source.RowSelection.SelectionChanged += SelectionChanged;
+            _source = _treeSource = CreateTreeSource();
 
             this.WhenAnyValue(x => x.SelectedDrive)
                 .Subscribe(x =>
                 {
                     _root = new FileTreeNodeModel(_selectedDrive, isDirectory: true, isRoot: true);
-                    Source.Items = new[] { _root };
+
+                    if (_treeSource is not null)
+                        _treeSource.Items = new[] { _root };
+                    else if (_flatSource is not null)
+                        _flatSource.Items = _root.Children;
                 });
         }
 
@@ -115,6 +73,16 @@ public bool CellSelection
 
         public IList<string> Drives { get; }
 
+        public bool FlatList
+        {
+            get => Source != _treeSource;
+            set
+            {
+                if (value != FlatList)
+                    Source = value ? _flatSource ??= CreateFlatSource() : _treeSource!;
+            }
+        }
+
         public string SelectedDrive
         {
             get => _selectedDrive;
@@ -127,7 +95,11 @@ public string SelectedDrive
             set => SetSelectedPath(value);
         }
 
-        public HierarchicalTreeDataGridSource<FileTreeNodeModel> Source { get; }
+        public ITreeDataGridSource<FileTreeNodeModel> Source
+        {
+            get => _source;
+            private set => this.RaiseAndSetIfChanged(ref _source, value);
+        }
 
         public static IMultiValueConverter FileIconConverter
         {
@@ -151,11 +123,115 @@ public static IMultiValueConverter FileIconConverter
             }
         }
 
+        private FlatTreeDataGridSource<FileTreeNodeModel> CreateFlatSource()
+        {
+            var result = new FlatTreeDataGridSource<FileTreeNodeModel>(_root!.Children)
+            {
+                Columns =
+                {
+                    new CheckBoxColumn<FileTreeNodeModel>(
+                        null,
+                        x => x.IsChecked,
+                        (o, v) => o.IsChecked = v,
+                        options: new()
+                        {
+                            CanUserResizeColumn = false,
+                        }),
+                    new TemplateColumn<FileTreeNodeModel>(
+                        "Name",
+                        "FileNameCell",
+                        "FileNameEditCell",
+                        new GridLength(1, GridUnitType.Star),
+                        new()
+                        {
+                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Name),
+                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Name),
+                            IsTextSearchEnabled = true,
+                            TextSearchValueSelector = x => x.Name
+                        }),
+                    new TextColumn<FileTreeNodeModel, long?>(
+                        "Size",
+                        x => x.Size,
+                        options: new()
+                        {
+                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Size),
+                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Size),
+                        }),
+                    new TextColumn<FileTreeNodeModel, DateTimeOffset?>(
+                        "Modified",
+                        x => x.Modified,
+                        options: new()
+                        {
+                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Modified),
+                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Modified),
+                        }),
+                }
+            };
+
+            result.RowSelection!.SingleSelect = false;
+            result.RowSelection.SelectionChanged += SelectionChanged;
+            return result;
+        }
+
+        private HierarchicalTreeDataGridSource<FileTreeNodeModel> CreateTreeSource()
+        {
+            var result = new HierarchicalTreeDataGridSource<FileTreeNodeModel>(Array.Empty<FileTreeNodeModel>())
+            {
+                Columns =
+                {
+                    new CheckBoxColumn<FileTreeNodeModel>(
+                        null,
+                        x => x.IsChecked,
+                        (o, v) => o.IsChecked = v,
+                        options: new()
+                        {
+                            CanUserResizeColumn = false,
+                        }),
+                    new HierarchicalExpanderColumn<FileTreeNodeModel>(
+                        new TemplateColumn<FileTreeNodeModel>(
+                            "Name",
+                            "FileNameCell",
+                            "FileNameEditCell",
+                            new GridLength(1, GridUnitType.Star),
+                            new()
+                            {
+                                CompareAscending = FileTreeNodeModel.SortAscending(x => x.Name),
+                                CompareDescending = FileTreeNodeModel.SortDescending(x => x.Name),
+                                IsTextSearchEnabled = true,
+                                TextSearchValueSelector = x => x.Name
+                            }),
+                        x => x.Children,
+                        x => x.HasChildren,
+                        x => x.IsExpanded),
+                    new TextColumn<FileTreeNodeModel, long?>(
+                        "Size",
+                        x => x.Size,
+                        options: new()
+                        {
+                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Size),
+                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Size),
+                        }),
+                    new TextColumn<FileTreeNodeModel, DateTimeOffset?>(
+                        "Modified",
+                        x => x.Modified,
+                        options: new()
+                        {
+                            CompareAscending = FileTreeNodeModel.SortAscending(x => x.Modified),
+                            CompareDescending = FileTreeNodeModel.SortDescending(x => x.Modified),
+                        }),
+                }
+            };
+
+            result.RowSelection!.SingleSelect = false;
+            result.RowSelection.SelectionChanged += SelectionChanged;
+            return result;
+        }
+
         private void SetSelectedPath(string? value)
         {
             if (string.IsNullOrEmpty(value))
             {
-                Source.RowSelection!.Clear();
+                GetRowSelection(Source).Clear();
                 return;
             }
 
@@ -204,12 +280,18 @@ private void SetSelectedPath(string? value)
                 }
             }
 
-            Source.RowSelection!.SelectedIndex = index;
+            GetRowSelection(Source).SelectedIndex = index;
+        }
+
+        private ITreeDataGridRowSelectionModel<FileTreeNodeModel> GetRowSelection(ITreeDataGridSource source)
+        {
+            return source.Selection as ITreeDataGridRowSelectionModel<FileTreeNodeModel> ??
+                throw new InvalidOperationException("Expected a row selection model.");
         }
 
         private void SelectionChanged(object? sender, TreeSelectionModelSelectionChangedEventArgs<FileTreeNodeModel> e)
         {
-            var selectedPath = Source.RowSelection?.SelectedItem?.Path;
+            var selectedPath = GetRowSelection(Source).SelectedItem?.Path;
             this.RaiseAndSetIfChanged(ref _selectedPath, selectedPath, nameof(SelectedPath));
 
             foreach (var i in e.DeselectedItems)

src/Avalonia.Controls.TreeDataGrid/ITreeDataGridSource.cs

@@ -23,9 +23,9 @@ public interface ITreeDataGridSource : INotifyPropertyChanged
         IRows Rows { get; }
 
         /// <summary>
-        /// Gets the selection model.
+        /// Gets or sets the selection model.
         /// </summary>
-        ITreeDataGridSelection? Selection { get; }
+        ITreeDataGridSelection? Selection { get; set; }
 
         /// <summary>
         /// Gets a value indicating whether the data source is hierarchical.
@@ -84,8 +84,8 @@ public interface ITreeDataGridSource : INotifyPropertyChanged
     public interface ITreeDataGridSource<TModel> : ITreeDataGridSource
     {
         /// <summary>
-        /// Gets the items in the data source.
+        /// Gets or sets the items in the data source.
         /// </summary>
-        new IEnumerable<TModel> Items { get; }
+        new IEnumerable<TModel> Items { get; set; }
     }
 }

src/Avalonia.Controls.TreeDataGrid/Models/TreeDataGrid/ColumnList.cs

@@ -9,6 +9,7 @@ namespace Avalonia.Controls.Models.TreeDataGrid
     /// </summary>
     public class ColumnList<TModel> : NotifyingListBase<IColumn<TModel>>, IColumns
     {
+        private bool _initialized;
         private double _viewportWidth;
 
         public event EventHandler? LayoutInvalidated;
@@ -22,6 +23,7 @@ public void AddRange(IEnumerable<IColumn<TModel>> items)
         public Size CellMeasured(int columnIndex, int rowIndex, Size size)
         {
             var column = (IUpdateColumnLayout)this[columnIndex];
+            _initialized = true;
             return new Size(column.CellMeasured(size.Width, rowIndex), size.Height);
         }
 
@@ -103,7 +105,8 @@ public void ViewportChanged(Rect viewport)
             if (_viewportWidth != viewport.Width)
             {
                 _viewportWidth = viewport.Width;
-                UpdateColumnSizes();
+                if (_initialized)
+                    UpdateColumnSizes();
             }
         }
 

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridPresenterBase.cs

@@ -427,7 +427,7 @@ protected virtual void OnEffectiveViewportChanged(object? sender, EffectiveViewp
             // viewport.
             Viewport = e.EffectiveViewport.Size == default ? 
                 s_invalidViewport :
-                e.EffectiveViewport.Intersect(new(Bounds.Size));
+                Intersect(e.EffectiveViewport, new(Bounds.Size));
 
             _isWaitingForViewportUpdate = false;
 
@@ -730,6 +730,24 @@ private void OnUnrealizedFocusedElementLostFocus(object? sender, RoutedEventArgs
 
         private static bool HasInfinity(Size s) => double.IsInfinity(s.Width) || double.IsInfinity(s.Height);
 
+        private static Rect Intersect(Rect a, Rect b)
+        {
+            // Hack fix for https://github.com/AvaloniaUI/Avalonia/issues/15075
+            var newLeft = (a.X > b.X) ? a.X : b.X;
+            var newTop = (a.Y > b.Y) ? a.Y : b.Y;
+            var newRight = (a.Right < b.Right) ? a.Right : b.Right;
+            var newBottom = (a.Bottom < b.Bottom) ? a.Bottom : b.Bottom;
+
+            if ((newRight >= newLeft) && (newBottom >= newTop))
+            {
+                return new Rect(newLeft, newTop, newRight - newLeft, newBottom - newTop);
+            }
+            else
+            {
+                return default;
+            }
+        }
+
         private struct MeasureViewport
         {
             public int anchorIndex;

tests/Avalonia.Controls.TreeDataGrid.Tests/TreeDataGridTests_Flat.cs

@@ -312,6 +312,46 @@ public void Does_Not_Realize_Columns_Outside_Viewport()
             Assert.True(double.IsNaN(columns[3].ActualWidth));
         }
 
+        [AvaloniaFact(Timeout = 10000)]
+        public void Columns_Are_Correctly_Sized_After_Changing_Source()
+        {
+            // Create the initial target with 2 columns and make sure our preconditions are correct.
+            var (target, items) = CreateTarget(columns: new IColumn<Model>[]
+            {
+                new TextColumn<Model, int>("ID", x => x.Id, width: new GridLength(1, GridUnitType.Star)),
+                new TextColumn<Model, string?>("Title1", x => x.Title, options: MinWidth(50)),
+            });
+
+            AssertColumnIndexes(target, 0, 2);
+
+            // Create a new source and assign it to the TreeDataGrid.
+            var newSource = new FlatTreeDataGridSource<Model>(items)
+            {
+                Columns =
+                {
+                    new TextColumn<Model, int>("ID", x => x.Id, width: new GridLength(1, GridUnitType.Star)),
+                    new TextColumn<Model, string?>("Title1", x => x.Title, options: MinWidth(20)),
+                    new TextColumn<Model, string?>("Title2", x => x.Title, options: MinWidth(20)),
+                }
+            };
+
+            target.Source = newSource;
+
+            // The columns should not have an ActualWidth yet.
+            Assert.True(double.IsNaN(newSource.Columns[0].ActualWidth));
+            Assert.True(double.IsNaN(newSource.Columns[1].ActualWidth));
+            Assert.True(double.IsNaN(newSource.Columns[2].ActualWidth));
+
+            // Do a layout pass and check that the columns have been correctly sized.
+            target.UpdateLayout();
+            AssertColumnIndexes(target, 0, 3);
+
+            var columns = (ColumnList<Model>)target.Columns!;
+            Assert.Equal(60, columns[0].ActualWidth);
+            Assert.Equal(20, columns[1].ActualWidth);
+            Assert.Equal(20, columns[2].ActualWidth);
+        }
+
         public class RemoveItems
         {
             [AvaloniaFact(Timeout = 10000)]