Merge pull request #7892 from 0x0ade/recycledanchors

Fix IsRegisteredAsAnchorCandidate desync, mainly affecting recycled anchors

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -53,10 +53,12 @@
         <ComboBoxItem>UniformGrid - Horizontal</ComboBoxItem>
       </ComboBox>
       <Button Command="{Binding AddItem}">Add Item</Button>
+      <Button Command="{Binding RemoveItem}">Remove Item</Button>
       <Button Command="{Binding RandomizeHeights}">Randomize Heights</Button>
       <Button Command="{Binding ResetItems}">Reset items</Button>
       <Button x:Name="scrollToLast">Scroll to Last</Button>
       <Button x:Name="scrollToRandom">Scroll to Random</Button>
+      <Button x:Name="scrollToSelected">Scroll to Selected</Button>
     </StackPanel>
     <Border BorderThickness="1" BorderBrush="{DynamicResource SystemControlHighlightBaseMediumLowBrush}" Margin="0 0 0 16">
       <ScrollViewer Name="scroller"

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml.cs

@@ -15,8 +15,10 @@ namespace ControlCatalog.Pages
         private readonly ItemsRepeaterPageViewModel _viewModel;
         private ItemsRepeater _repeater;
         private ScrollViewer _scroller;
+        private int _selectedIndex;
         private Button _scrollToLast;
         private Button _scrollToRandom;
+        private Button _scrollToSelected;
         private Random _random = new Random(0);
 
         public ItemsRepeaterPage()
@@ -26,10 +28,12 @@ namespace ControlCatalog.Pages
             _scroller = this.FindControl<ScrollViewer>("scroller");
             _scrollToLast = this.FindControl<Button>("scrollToLast");
             _scrollToRandom = this.FindControl<Button>("scrollToRandom");
+            _scrollToSelected = this.FindControl<Button>("scrollToSelected");
             _repeater.PointerPressed += RepeaterClick;
             _repeater.KeyDown += RepeaterOnKeyDown;
             _scrollToLast.Click += scrollToLast_Click;
             _scrollToRandom.Click += scrollToRandom_Click;
+            _scrollToSelected.Click += scrollToSelected_Click;
             DataContext = _viewModel = new ItemsRepeaterPageViewModel();
         }
 
@@ -121,6 +125,7 @@ namespace ControlCatalog.Pages
         {
             var item = (e.Source as TextBlock)?.DataContext as ItemsRepeaterPageViewModel.Item;
             _viewModel.SelectedItem = item;
+            _selectedIndex = _viewModel.Items.IndexOf(item);
         }
 
         private void RepeaterOnKeyDown(object sender, KeyEventArgs e)
@@ -140,5 +145,10 @@ namespace ControlCatalog.Pages
         {
             ScrollTo(_random.Next(_viewModel.Items.Count - 1));
         }
+
+        private void scrollToSelected_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            ScrollTo(_selectedIndex);
+        }
     }
 }

samples/ControlCatalog/ViewModels/ItemsRepeaterPageViewModel.cs

@@ -31,6 +31,19 @@ namespace ControlCatalog.ViewModels
             Items.Insert(index + 1, new Item(index + 1) { Text = $"New Item {_newItemIndex++}" });
         }
 
+        public void RemoveItem()
+        {
+            if (SelectedItem is not null)
+            {
+                Items.Remove(SelectedItem);
+                SelectedItem = null;
+            }
+            else if (Items.Count > 0)
+            {
+                Items.RemoveAt(Items.Count - 1);
+            }
+        }
+
         public void RandomizeHeights()
         {
             var random = new Random();

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -401,11 +401,7 @@ namespace Avalonia.Controls
                         var newBounds = element.Bounds;
                         virtInfo.ArrangeBounds = newBounds;
 
-                        if (!virtInfo.IsRegisteredAsAnchorCandidate)
-                        {
-                            _viewportManager.RegisterScrollAnchorCandidate(element);
-                            virtInfo.IsRegisteredAsAnchorCandidate = true;
-                        }
+                        _viewportManager.RegisterScrollAnchorCandidate(element, virtInfo);
                     }
                 }
 
@@ -490,7 +486,7 @@ namespace Avalonia.Controls
                     _processingItemsSourceChange.Action == NotifyCollectionChangedAction.Reset);
 
             _viewManager.ClearElement(element, isClearedDueToCollectionChange);
-            _viewportManager.OnElementCleared(element);
+            _viewportManager.OnElementCleared(element, GetVirtualizationInfo(element));
         }
 
         private int GetElementIndexImpl(IControl element)

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -249,9 +249,10 @@ namespace Avalonia.Controls
             virtInfo.IsRegisteredAsAnchorCandidate = false;
         }
 
-        public void OnElementCleared(IControl element)
+        public void OnElementCleared(IControl element, VirtualizationInfo virtInfo)
         {
             _scroller?.UnregisterAnchorCandidate(element);
+            virtInfo.IsRegisteredAsAnchorCandidate = false;
         }
 
         public void OnOwnerMeasuring()
@@ -358,9 +359,12 @@ namespace Avalonia.Controls
                 {
                     foreach (var child in _owner.Children)
                     {
-                        if (child != targetChild)
+                        var info = ItemsRepeater.GetVirtualizationInfo(child);
+
+                        if (child != targetChild && info.IsRegisteredAsAnchorCandidate)
                         {
                             _scroller.UnregisterAnchorCandidate(child);
+                            info.IsRegisteredAsAnchorCandidate = false;
                         }
                     }
                 }
@@ -377,9 +381,13 @@ namespace Avalonia.Controls
             }
         }
 
-        public void RegisterScrollAnchorCandidate(IControl element)
+        public void RegisterScrollAnchorCandidate(IControl element, VirtualizationInfo virtInfo)
         {
-            _scroller?.RegisterAnchorCandidate(element);
+            if (!virtInfo.IsRegisteredAsAnchorCandidate)
+            {
+                _scroller?.RegisterAnchorCandidate(element);
+                virtInfo.IsRegisteredAsAnchorCandidate = true;
+            }
         }
 
         private IControl? GetImmediateChildOfRepeater(IControl descendant)
@@ -405,15 +413,18 @@ namespace Avalonia.Controls
             _isBringIntoViewInProgress = false;
             _makeAnchorElement = null;
 
+            // Undo the anchor deregistrations done by OnBringIntoViewRequested.
             if (_scroller is object)
             {
                 foreach (var child in _owner.Children)
                 {
                     var info = ItemsRepeater.GetVirtualizationInfo(child);
 
-                    if (info.IsRealized && info.IsHeldByLayout)
+                    // The item brought into view is still registered - don't register it more than once.
+                    if (info.IsRealized && info.IsHeldByLayout && !info.IsRegisteredAsAnchorCandidate)
                     {
                         _scroller.RegisterAnchorCandidate(child);
+                        info.IsRegisteredAsAnchorCandidate = true;
                     }
                 }
             }
@@ -430,7 +441,13 @@ namespace Avalonia.Controls
             {
                 foreach (var child in _owner.Children)
                 {
-                    _scroller.UnregisterAnchorCandidate(child);
+                    var info = ItemsRepeater.GetVirtualizationInfo(child);
+
+                    if (info.IsRegisteredAsAnchorCandidate)
+                    {
+                        _scroller.UnregisterAnchorCandidate(child);
+                        info.IsRegisteredAsAnchorCandidate = false;
+                    }
                 }
 
                 _scroller = null;