Merge pull request #2689 from AvaloniaUI/fixes/2565-listbox-initial-selection

Fix ListBox initial selection

src/Avalonia.Controls/ListBox.cs

@@ -68,7 +68,13 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         public new IList SelectedItems => base.SelectedItems;
 
-        /// <inheritdoc/>
+        /// <summary>
+        /// Gets or sets the selection mode.
+        /// </summary>
+        /// <remarks>
+        /// Note that the selection mode only applies to selections made via user interaction.
+        /// Multiple selections can be made programatically regardless of the value of this property.
+        /// </remarks>
         public new SelectionMode SelectionMode
         {
             get { return base.SelectionMode; }

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -222,6 +222,10 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the selection mode.
         /// </summary>
+        /// <remarks>
+        /// Note that the selection mode only applies to selections made via user interaction.
+        /// Multiple selections can be made programatically regardless of the value of this property.
+        /// </remarks>
         protected SelectionMode SelectionMode
         {
             get { return GetValue(SelectionModeProperty); }
@@ -338,24 +342,36 @@ namespace Avalonia.Controls.Primitives
         {
             base.OnContainersMaterialized(e);
 
-            var selectedIndex = SelectedIndex;
-            var selectedContainer = e.Containers
-                .FirstOrDefault(x => (x.ContainerControl as ISelectable)?.IsSelected == true);
+            var resetSelectedItems = false;
 
-            if (selectedContainer != null)
+            foreach (var container in e.Containers)
             {
-                SelectedIndex = selectedContainer.Index;
-            }
-            else if (selectedIndex >= e.StartingIndex &&
-                     selectedIndex < e.StartingIndex + e.Containers.Count)
-            {
-                var container = e.Containers[selectedIndex - e.StartingIndex];
+                if ((container.ContainerControl as ISelectable)?.IsSelected == true)
+                {
+                    if (SelectedIndex == -1)
+                    {
+                        SelectedIndex = container.Index;
+                    }
+                    else
+                    {
+                        if (_selection.Add(container.Index))
+                        {
+                            resetSelectedItems = true;
+                        }
+                    }
 
-                if (container.ContainerControl != null)
+                    MarkContainerSelected(container.ContainerControl, true);
+                }
+                else if (_selection.Contains(container.Index))
                 {
                     MarkContainerSelected(container.ContainerControl, true);
                 }
             }
+
+            if (resetSelectedItems)
+            {
+                ResetSelectedItems();
+            }
         }
 
         /// <inheritdoc/>

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_Multiple.cs

@@ -53,7 +53,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
         }
 
         [Fact]
-        public void Assigning_SelectedItems_Should_Set_SelectedIndex()
+        public void Assigning_Single_SelectedItems_Should_Set_SelectedIndex()
         {
             var target = new TestSelector
             {
@@ -62,9 +62,51 @@ namespace Avalonia.Controls.UnitTests.Primitives
             };
 
             target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
             target.SelectedItems = new AvaloniaList<object>("bar");
 
             Assert.Equal(1, target.SelectedIndex);
+            Assert.Equal(new[] { "bar" }, target.SelectedItems);
+            Assert.Equal(new[] { 1 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Assigning_Multiple_SelectedItems_Should_Set_SelectedIndex()
+        {
+            // Note that we don't need SelectionMode = Multiple here. Multiple selections can always
+            // be made in code.
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar", "baz" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+            target.SelectedItems = new AvaloniaList<string>("foo", "bar", "baz");
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal(new[] { "foo", "bar", "baz" }, target.SelectedItems);
+            Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Selected_Items_Should_Be_Marked_When_Panel_Created_After_SelectedItems_Is_Set()
+        {
+            // Issue #2565.
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar", "baz" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.SelectedItems = new AvaloniaList<string>("foo", "bar", "baz");
+            target.Presenter.ApplyTemplate();
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal(new[] { "foo", "bar", "baz" }, target.SelectedItems);
+            Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
         }
 
         [Fact]
@@ -1026,6 +1068,31 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(1, target.SelectedItems.Count);
         }
 
+        [Fact]
+        public void Adding_Selected_ItemContainers_Should_Update_Selection()
+        {
+            var items = new AvaloniaList<ItemContainer>(new[]
+            {
+                new ItemContainer(),
+                new ItemContainer(),
+            });
+
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+            items.Add(new ItemContainer { IsSelected = true });
+            items.Add(new ItemContainer { IsSelected = true });
+
+            Assert.Equal(2, target.SelectedIndex);
+            Assert.Equal(items[2], target.SelectedItem);
+            Assert.Equal(new[] { items[2], items[3] }, target.SelectedItems);
+        }
+
         private IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
             return target.Presenter.Panel.Children
@@ -1078,5 +1145,11 @@ namespace Avalonia.Controls.UnitTests.Primitives
             public List<string> Items { get; } 
             public List<string> SelectedItems { get; }
         }
+
+        private class ItemContainer : Control, ISelectable
+        {
+            public string Value { get; set; }
+            public bool IsSelected { get; set; }
+        }
     }
 }