Allow adding items anywhere in list.

By clicking an item in `ItemsRepeater` to select the insertion point.

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml

@@ -19,7 +19,7 @@
       <ScrollViewer Name="scroller"
                     HorizontalScrollBarVisibility="Auto"
                     VerticalScrollBarVisibility="Auto">
-        <ItemsRepeater Name="repeater" Items="{Binding Items}"/>
+        <ItemsRepeater Name="repeater" Background="Transparent" Items="{Binding Items}"/>
       </ScrollViewer>
     </Border>
   </DockPanel>

samples/ControlCatalog/Pages/ItemsRepeaterPage.xaml.cs

@@ -1,6 +1,8 @@
+using System;
 using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
+using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Markup.Xaml;
 using ControlCatalog.ViewModels;
@@ -17,6 +19,7 @@ namespace ControlCatalog.Pages
             this.InitializeComponent();
             _repeater = this.FindControl<ItemsRepeater>("repeater");
             _scroller = this.FindControl<ScrollViewer>("scroller");
+            _repeater.PointerPressed += RepeaterClick;
             DataContext = new ItemsRepeaterPageViewModel();
         }
 
@@ -68,5 +71,11 @@ namespace ControlCatalog.Pages
                     break;
             }
         }
+
+        private void RepeaterClick(object sender, PointerPressedEventArgs e)
+        {
+            var item = (e.Source as TextBlock)?.DataContext as string;
+            ((ItemsRepeaterPageViewModel)DataContext).SelectedItem = item;
+        }
     }
 }

samples/ControlCatalog/ViewModels/ItemsRepeaterPageViewModel.cs

@@ -16,9 +16,12 @@ namespace ControlCatalog.ViewModels
 
         public ObservableCollection<string> Items { get; }
 
+        public string SelectedItem { get; set; }
+
         public void AddItem()
         {
-            Items.Insert(0, $"New Item {newItemIndex++}");
+            var index = SelectedItem != null ? Items.IndexOf(SelectedItem) : -1;
+            Items.Insert(index + 1, $"New Item {newItemIndex++}");
         }
     }
 }