workaround for regressions caused by ItemsPanelRoot resolution timing (#1294)

Author: Xiaoy312

src/Uno.Toolkit.UI/Controls/Chips/ChipGroup.cs

@@ -3,6 +3,8 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using Uno.UI.Extensions;
+
 
 #if IS_WINUI
 using Microsoft.UI.Xaml;
@@ -18,7 +20,6 @@ namespace Uno.Toolkit.UI
 {
 	public partial class ChipGroup : ItemsControl
 	{
-		private bool _isLoaded;
 		private bool _isSynchronizingSelection;
 
 		public ChipGroup()
@@ -31,11 +32,26 @@ public ChipGroup()
 		protected override void OnApplyTemplate()
 		{
 			base.OnApplyTemplate();
+
+			// workaround for #1287 ItemsPanelRoot resolution timing related issue
+			var presenter =
+				this.GetTemplateRoot() as ItemsPresenter ??
+				this.GetFirstDescendant<ItemsPresenter>();
+			if (presenter is { })
+			{
+				presenter.Loaded += OnItemsPresenterLoaded;
+			}
 		}
 
 		private void OnLoaded(object sender, RoutedEventArgs e)
 		{
-			_isLoaded = true;
+			SynchronizeInitialSelection();
+			EnforceSelectionMode();
+			ApplyIconTemplate(null, IconTemplate);
+		}
+
+		private void OnItemsPresenterLoaded(object sender, RoutedEventArgs e)
+		{
 			SynchronizeInitialSelection();
 			EnforceSelectionMode();
 			ApplyIconTemplate(null, IconTemplate);
@@ -58,6 +74,7 @@ private void OnSelectionMemberPathChanged(DependencyPropertyChangedEventArgs e)
 
 		private void ApplyIconTemplate(DataTemplate? oldTemplate, DataTemplate? newTemplate)
 		{
+			if (!IsReady) return;
 			if (oldTemplate == newTemplate) return;
 
 			foreach (var container in this.GetItemContainers<Chip>())
@@ -403,7 +420,7 @@ bool ShouldClearSelection(Chip container)
 			}
 		}
 
-		private bool IsReady => _isLoaded && HasItems && HasContainers;
+		private bool IsReady => IsLoaded && HasItems && HasContainers;
 
 		private bool HasItems => this.GetItems().OfType<object>().Any();
 

src/Uno.Toolkit.UI/Controls/TabBar/TabBar.cs

@@ -195,6 +195,14 @@ private void OnLoaded(object sender, RoutedEventArgs e)
 			UpdateOrientation();
 		}
 
+		internal void OnItemsPanelConnected(TabBarListPanel panel)
+		{
+			System.Diagnostics.Debug.Assert(ItemsPanelRoot != null, "ItemsPanelRoot is expected to be already set in here.");
+
+			SynchronizeInitialSelection();
+			UpdateOrientation();
+		}
+
 		private void OnTabBarItemClick(object sender, RoutedEventArgs e)
 		{
 			if (_isSynchronizingSelection)
@@ -441,7 +449,7 @@ private void RaiseSelectionChangedEvent(object? prevItem, object? nextItem)
 			return null;
 		}
 
-		private bool IsReady => _isLoaded && HasItems;
+		private bool IsReady => _isLoaded && HasItems && ItemsPanelRoot is { };
 
 		private bool HasItems => this.GetItems().Any();
 	}

src/Uno.Toolkit.UI/Controls/TabBar/TabBarListPanel.cs

@@ -17,23 +17,40 @@ namespace Uno.Toolkit.UI
 {
 	public partial class TabBarListPanel : Panel
 	{
-		public Orientation Orientation
-		{
-			get { return (Orientation)GetValue(OrientationProperty); }
-			set { SetValue(OrientationProperty, value); }
-		}
+		#region DependencyProperty: Orientation
 
 		public static DependencyProperty OrientationProperty { get; } = DependencyProperty.Register(
 			nameof(Orientation),
 			typeof(Orientation),
 			typeof(TabBarListPanel),
-			new PropertyMetadata(Orientation.Horizontal, (s, e) => ((TabBarListPanel)s).OnPropertyChanged(e)));
+			new PropertyMetadata(default(Orientation), OnOrientationChanged));
+
+		public Orientation Orientation
+		{
+			get => (Orientation)GetValue(OrientationProperty);
+			set => SetValue(OrientationProperty, value);
+		}
+
+		#endregion
+
+		public TabBarListPanel()
+		{
+			this.Loaded += OnLoaded;
+		}
+
+		private void OnLoaded(object sender, RoutedEventArgs e)
+		{
+			var owner = this.FindFirstParent<TabBar>();
+
+			// workaround for #1287 ItemsPanelRoot resolution timing related issue
+			owner?.OnItemsPanelConnected(this);
+		}
 
-		private void OnPropertyChanged(DependencyPropertyChangedEventArgs args)
+		private static void OnOrientationChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
 		{
-			if (args.Property == OrientationProperty)
+			if (sender is TabBar owner)
 			{
-				InvalidateMeasure();
+				owner.InvalidateMeasure();
 			}
 		}
 
@@ -132,6 +149,6 @@ protected override Size ArrangeOverride(Size finalSize)
 			return finalSize;
 		}
 
-		private bool IsVisible(UIElement x) => x.Visibility == Visibility.Visible;
+		private static bool IsVisible(UIElement x) => x.Visibility == Visibility.Visible;
 	}
 }

src/Uno.Toolkit.UI/Extensions/ItemsControlExtensions.cs

@@ -56,14 +56,6 @@ item as T ??
 		/// <remarks>An empty enumerable will returned if the <see cref="ItemsControl.ItemsPanelRoot"/> and the containers have not been materialized.</remarks>
 		public static IEnumerable<T> GetItemContainers<T>(this ItemsControl itemsControl) =>
 			itemsControl.ItemsPanelRoot?.Children.OfType<T>() ??
-			// #1281 workaround: ItemsPanelRoot would not be resolved until ItemsPanel is loaded,
-			// which will be the case between the ItemsControl::Loaded and ItemsPanel::Loaded.
-			// This is normally not a problem, as the container is typically created after that with ItemsSource.
-			// However, for xaml-defined items, this could cause a problem with initial selection synchronization,
-			// which happens on ItemsControl::Loaded. For this case, we will resort to using ItemsControl::Items.
-			(itemsControl.ItemsSource is null && itemsControl.Items is { }
-				? itemsControl.Items.OfType<T>()
-				: null) ??
 			Enumerable.Empty<T>();
 
 		/// <summary>