Add ItemsRepeaterPage scroll to item buttons.

To diagnose problems with `ItemsRepeater`.

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -16,12 +16,15 @@
       <Button Command="{Binding AddItem}">Add 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>
     </StackPanel>
     <Border BorderThickness="1" BorderBrush="{DynamicResource ThemeBorderMidBrush}" Margin="0 0 0 16">
       <ScrollViewer Name="scroller"
                     HorizontalScrollBarVisibility="Auto"
                     VerticalScrollBarVisibility="Auto">
-        <ItemsRepeater Name="repeater" Background="Transparent" Items="{Binding Items}">
+        <ItemsRepeater Name="repeater" Background="Transparent" Items="{Binding Items}"
+                       VerticalCacheLength="0" HorizontalCacheLength="0">
           <ItemsRepeater.ItemTemplate>
             <DataTemplate>
               <TextBlock Focusable="True" Height="{Binding Height}" Text="{Binding Text}"/>

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml.cs

@@ -5,23 +5,32 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Markup.Xaml;
+using Avalonia.VisualTree;
 using ControlCatalog.ViewModels;
 
 namespace ControlCatalog.Pages
 {
     public class ItemsRepeaterPage : UserControl
     {
+        private readonly ItemsRepeaterPageViewModel _viewModel;
         private ItemsRepeater _repeater;
         private ScrollViewer _scroller;
+        private Button _scrollToLast;
+        private Button _scrollToRandom;
+        private Random _random = new Random(0);
 
         public ItemsRepeaterPage()
         {
             this.InitializeComponent();
             _repeater = this.FindControl<ItemsRepeater>("repeater");
             _scroller = this.FindControl<ScrollViewer>("scroller");
+            _scrollToLast = this.FindControl<Button>("scrollToLast");
+            _scrollToRandom = this.FindControl<Button>("scrollToRandom");
             _repeater.PointerPressed += RepeaterClick;
             _repeater.KeyDown += RepeaterOnKeyDown;
-            DataContext = new ItemsRepeaterPageViewModel();
+            _scrollToLast.Click += scrollToLast_Click;
+            _scrollToRandom.Click += scrollToRandom_Click;
+            DataContext = _viewModel = new ItemsRepeaterPageViewModel();
         }
 
         private void InitializeComponent()
@@ -73,18 +82,36 @@ namespace ControlCatalog.Pages
             }
         }
 
+        private void ScrollTo(int index)
+        {
+            var layoutManager = ((Window)this.GetVisualRoot()).LayoutManager;
+            var element = _repeater.GetOrCreateElement(index);
+            layoutManager.ExecuteLayoutPass();
+            element.BringIntoView();
+        }
+
         private void RepeaterClick(object sender, PointerPressedEventArgs e)
         {
             var item = (e.Source as TextBlock)?.DataContext as ItemsRepeaterPageViewModel.Item;
-            ((ItemsRepeaterPageViewModel)DataContext).SelectedItem = item;
+            _viewModel.SelectedItem = item;
         }
 
         private void RepeaterOnKeyDown(object sender, KeyEventArgs e)
         {
             if (e.Key == Key.F5)
             {
-                ((ItemsRepeaterPageViewModel)DataContext).ResetItems();
+                _viewModel.ResetItems();
             }
         }
+
+        private void scrollToLast_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            ScrollTo(_viewModel.Items.Count - 1);
+        }
+
+        private void scrollToRandom_Click(object sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            ScrollTo(_random.Next(_viewModel.Items.Count - 1));
+        }
     }
 }