DataGrid: expose row index (#15909)

* feat: expose index.

* feat: make Index a direct property. obsolete get method. Update sample.

samples/ControlCatalog/Pages/DataGridPage.xaml

@@ -45,6 +45,11 @@
           </StackPanel>
           <DataGrid Name="dataGrid1" Margin="12" CanUserResizeColumns="True" CanUserReorderColumns="True" CanUserSortColumns="True" HeadersVisibility="All"
                     RowBackground="#1000">
+            <DataGrid.Styles>
+              <Style Selector="DataGridRow">
+                <Setter Property="Header" Value="{Binding $self.Index}"/>
+              </Style>
+            </DataGrid.Styles>
             <DataGrid.Columns>
               <!-- Using HeaderTemplate -->
               <DataGridTextColumn Header="Country or Region" HeaderTemplate="{StaticResource Demo.DataTemplates.CountryHeader}" Binding="{Binding Name}"  Width="6*" x:DataType="local:Country" />

samples/ControlCatalog/Pages/DataGridPage.xaml.cs

@@ -24,7 +24,6 @@ namespace ControlCatalog.Pages
             collectionView1.SortDescriptions.Add(dataGridSortDescription);
             var dg1 = this.Get<DataGrid>("dataGrid1");
             dg1.IsReadOnly = true;
-            dg1.LoadingRow += Dg1_LoadingRow;
             dg1.Sorting += (s, a) =>
             {
                 var binding = (a.Column as DataGridBoundColumn)?.Binding as Binding;
@@ -64,11 +63,6 @@ namespace ControlCatalog.Pages
         }
 
         public IEnumerable<Person> DataGrid3Source { get; }
-        
-        private void Dg1_LoadingRow(object? sender, DataGridRowEventArgs e)
-        {
-            e.Row.Header = e.Row.GetIndex() + 1;
-        }
 
         private void InitializeComponent()
         {

src/Avalonia.Controls.DataGrid/DataGridRow.cs

@@ -220,13 +220,18 @@ namespace Avalonia.Controls
             set;
         }
 
+        private int _index;
+
+        public static readonly DirectProperty<DataGridRow, int> IndexProperty = AvaloniaProperty.RegisterDirect<DataGridRow, int>(
+            nameof(Index), o => o.Index, (o, v) => o.Index = v);
+
         /// <summary>
         /// Index of the row
         /// </summary>
-        internal int Index
+        public int Index
         {
-            get;
-            set;
+            get => _index;
+            internal set => SetAndRaise(IndexProperty, ref _index, value);
         }
 
         internal double ActualBottomGridLineHeight
@@ -435,6 +440,7 @@ namespace Avalonia.Controls
         /// <returns>
         /// The index of the current row.
         /// </returns>
+        [Obsolete("This API is going to be removed in a future version. Use the Index property instead.")]
         public int GetIndex()
         {
             return Index;