fix: Fix SelectorExtensions not updating NumberOfPages on Selector lo…

…ad (#565)

* fix: Fix SelectorExtensions not updating NumberOfPages on Selector load

* chore: Fix build

src/Uno.Toolkit.RuntimeTests/Tests/SelectorExtensionsTests.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Uno.UI.RuntimeTests;
+using Uno.Toolkit.RuntimeTests.Helpers;
+using System.Threading.Tasks;
+using Uno.Toolkit.UI;
+using Microsoft.UI.Xaml.Controls;
+
+#if !IS_WINUI
+using Windows.UI.Xaml.Controls;
+#endif
+
+namespace Uno.Toolkit.RuntimeTests.Tests
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class SelectorExtensionsTests
+	{
+		[TestMethod]
+		public async Task TestAddingItemsBeforeLoadedAndAfterSetPipsPager()
+		{
+			var grid = new Grid();
+			var flipView = new FlipView();
+			var pipsPager = new PipsPager();
+			grid.Children.Add(flipView);
+			grid.Children.Add(pipsPager);
+			SelectorExtensions.SetPipsPager(flipView, pipsPager);
+			flipView.Items.Add("Test1");
+			flipView.Items.Add("Test2");
+			flipView.Items.Add("Test3");
+			await UnitTestUIContentHelperEx.SetContentAndWait(grid);
+			Assert.AreEqual(3, pipsPager.NumberOfPages);
+		}
+	}
+}

src/Uno.Toolkit.UI/Behaviors/SelectorExtensions.cs

@@ -116,6 +116,7 @@ private static void OnSelectorLoaded(object sender, RoutedEventArgs? e)
 		events.SetValue(selector, SelectorAttachedPropertyKey, eventHandler);
 
 		selector.Items.VectorChanged += eventHandler;
+		pipsPager.NumberOfPages = selector.Items.Count;
 
 		void OnItemsVectorChanged(IObservableVector<object> sender, IVectorChangedEventArgs @event) =>
 			pipsPager.NumberOfPages = selector.Items.Count;