Merge pull request #4440 from AvaloniaUI/fixes/4272-listbox-initial-selection

Handle initial selection in SelectedItems.

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -11,8 +11,7 @@
               Spacing="16">
       <StackPanel Orientation="Vertical" Spacing="8">
         <ListBox Items="{Binding Items}"
-                 SelectedItem="{Binding SelectedItem}"
-                 SelectedItems="{Binding SelectedItems}"
+                 Selection="{Binding Selection}"
                  AutoScrollToSelectedItem="True"
                  SelectionMode="{Binding SelectionMode}"
                  Width="250"

samples/ControlCatalog/Pages/ListBoxPage.xaml.cs

@@ -1,10 +1,6 @@
-using System;
-using System.Collections.ObjectModel;
-using System.Linq;
-using System.Reactive;
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
-using ReactiveUI;
+using ControlCatalog.ViewModels;
 
 namespace ControlCatalog.Pages
 {
@@ -13,72 +9,12 @@ namespace ControlCatalog.Pages
         public ListBoxPage()
         {
             InitializeComponent();
-            DataContext = new PageViewModel();
+            DataContext = new ListBoxPageViewModel();
         }
 
         private void InitializeComponent()
         {
             AvaloniaXamlLoader.Load(this);
         }
-
-        private class PageViewModel : ReactiveObject
-        {
-            private int _counter;
-            private SelectionMode _selectionMode;
-
-            public PageViewModel()
-            {
-                Items = new ObservableCollection<string>(Enumerable.Range(1, 10000).Select(i => GenerateItem()));
-                SelectedItems = new ObservableCollection<string>();
-
-                AddItemCommand = ReactiveCommand.Create(() => Items.Add(GenerateItem()));
-
-                RemoveItemCommand = ReactiveCommand.Create(() =>
-                {
-                    while (SelectedItems.Count > 0)
-                    {
-                        Items.Remove(SelectedItems[0]);
-                    }
-                });
-
-                SelectRandomItemCommand = ReactiveCommand.Create(() =>
-                {
-                    var random = new Random();
-
-                    SelectedItem = Items[random.Next(Items.Count - 1)];
-                });
-            }
-
-            public ObservableCollection<string> Items { get; }
-
-            private string _selectedItem;
-
-            public string SelectedItem
-            {
-                get { return _selectedItem; }
-                set { this.RaiseAndSetIfChanged(ref _selectedItem, value); }
-            }
-
-
-            public ObservableCollection<string> SelectedItems { get; }
-
-            public ReactiveCommand<Unit, Unit> AddItemCommand { get; }
-
-            public ReactiveCommand<Unit, Unit> RemoveItemCommand { get; }
-
-            public ReactiveCommand<Unit, Unit> SelectRandomItemCommand { get; }
-
-            public SelectionMode SelectionMode
-            {
-                get => _selectionMode;
-                set
-                {
-                    SelectedItems.Clear();
-                    this.RaiseAndSetIfChanged(ref _selectionMode, value);
-                }
-            }
-
-            private string GenerateItem() => $"Item {_counter++.ToString()}";
-        }
     }
 }

samples/ControlCatalog/ViewModels/ListBoxPageViewModel.cs

@@ -0,0 +1,65 @@
+using System;
+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Reactive;
+using Avalonia.Controls;
+using ReactiveUI;
+
+namespace ControlCatalog.ViewModels
+{
+    public class ListBoxPageViewModel : ReactiveObject
+    {
+        private int _counter;
+        private SelectionMode _selectionMode;
+
+        public ListBoxPageViewModel()
+        {
+            Items = new ObservableCollection<string>(Enumerable.Range(1, 10000).Select(i => GenerateItem()));
+            Selection = new SelectionModel();
+            Selection.Select(1);
+
+            AddItemCommand = ReactiveCommand.Create(() => Items.Add(GenerateItem()));
+
+            RemoveItemCommand = ReactiveCommand.Create(() =>
+            {
+                while (Selection.SelectedItems.Count > 0)
+                {
+                    Items.Remove((string)Selection.SelectedItems.First());
+                }
+            });
+
+            SelectRandomItemCommand = ReactiveCommand.Create(() =>
+            {
+                var random = new Random();
+
+                using (Selection.Update())
+                {
+                    Selection.ClearSelection();
+                    Selection.Select(random.Next(Items.Count - 1));
+                }
+            });
+        }
+
+        public ObservableCollection<string> Items { get; }
+
+        public SelectionModel Selection { get; }
+
+        public ReactiveCommand<Unit, Unit> AddItemCommand { get; }
+
+        public ReactiveCommand<Unit, Unit> RemoveItemCommand { get; }
+
+        public ReactiveCommand<Unit, Unit> SelectRandomItemCommand { get; }
+
+        public SelectionMode SelectionMode
+        {
+            get => _selectionMode;
+            set
+            {
+                Selection.ClearSelection();
+                this.RaiseAndSetIfChanged(ref _selectionMode, value);
+            }
+        }
+
+        private string GenerateItem() => $"Item {_counter++.ToString()}";
+    }
+}

src/Avalonia.Controls/Utils/SelectedItemsSync.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Specialized;
+using System.ComponentModel;
 using System.Linq;
 using Avalonia.Collections;
 
@@ -16,6 +17,7 @@ namespace Avalonia.Controls.Utils
         private IList? _items;
         private bool _updatingItems;
         private bool _updatingModel;
+        private bool _initializeOnSourceAssignment;
 
         public SelectedItemsSync(ISelectionModel model)
         {
@@ -63,10 +65,18 @@ namespace Avalonia.Controls.Utils
                 _updatingModel = true;
                 _items = items;
 
-                using (Model.Update())
+                if (Model.Source is object)
                 {
-                    Model.ClearSelection();
-                    Add(items);
+                    using (Model.Update())
+                    {
+                        Model.ClearSelection();
+                        Add(items);
+                    }
+                }
+                else if (!_initializeOnSourceAssignment)
+                {
+                    Model.PropertyChanged += SelectionModelPropertyChanged;
+                    _initializeOnSourceAssignment = true;
                 }
 
                 if (_items is INotifyCollectionChanged incc2)
@@ -86,9 +96,11 @@ namespace Avalonia.Controls.Utils
 
             if (_items != null)
             {
+                Model.PropertyChanged -= SelectionModelPropertyChanged;
                 Model.SelectionChanged -= SelectionModelSelectionChanged;
                 Model = model;
                 Model.SelectionChanged += SelectionModelSelectionChanged;
+                _initializeOnSourceAssignment = false;
 
                 try
                 {
@@ -175,6 +187,25 @@ namespace Avalonia.Controls.Utils
             }
         }
 
+        private void SelectionModelPropertyChanged(object sender, PropertyChangedEventArgs e)
+        {
+            if (_initializeOnSourceAssignment &&
+                _items != null &&
+                e.PropertyName == nameof(SelectionModel.Source))
+            {
+                try
+                {
+                    _updatingModel = true;
+                    Add(_items);
+                    _initializeOnSourceAssignment = false;
+                }
+                finally
+                {
+                    _updatingModel = false;
+                }
+            }
+        }
+
         private void SelectionModelSelectionChanged(object sender, SelectionModelSelectionChangedEventArgs e)
         {
             if (_updatingModel)

tests/Avalonia.Controls.UnitTests/Utils/SelectedItemsSyncTests.cs

@@ -208,6 +208,20 @@ namespace Avalonia.Controls.UnitTests.Utils
                 target.SetItems(new[] { "foo", "bar", "baz" }));
         }
 
+        [Fact]
+        public void Selected_Items_Can_Be_Set_Before_SelectionModel_Source()
+        {
+            var model = new SelectionModel();
+            var target = new SelectedItemsSync(model);
+            var items = new AvaloniaList<string> { "foo", "bar", "baz" };
+            var selectedItems = new AvaloniaList<string> { "bar" };
+
+            target.SetItems(selectedItems);
+            model.Source = items;
+
+            Assert.Equal(new IndexPath(1), model.SelectedIndex);
+        }
+
         private static SelectedItemsSync CreateTarget(
             IEnumerable<string> items = null)
         {