fix(statetriggers): evaluate StateTriggers before the first layout cycle

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_VisualStateManager.cs

@@ -1,13 +1,16 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
+using Windows.Foundation;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Controls.Primitives;
 using Microsoft.UI.Xaml.Media;
 using Microsoft.UI.Xaml.Shapes;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MUXControlsTestApp.Utilities;
+using SamplesApp.UITests;
 using Uno.Extensions;
 using Uno.UI.Extensions;
 using Uno.UI.RuntimeTests.Helpers;
@@ -79,4 +82,65 @@ public async Task SelectorItem_SelectedState()
 		var states = VisualStateHelper.GetCurrentVisualStateName(container2).ToArray();
 		Assert.IsTrue(states.Contains("MultiSelectEnabled"), $"container2 is not in 'MultiSelectEnabled' state: states={states.JoinBy(",")}");
 	}
+
+#if HAS_UNO
+	[TestMethod]
+	[UnoWorkItem("https://github.com/unoplatform/uno/issues/19364")]
+	public async Task When_StateTriggers_Evaluated_Before_First_Layout()
+	{
+		MyUserControl uc = new MyUserControl();
+		VisualStateManager.SetVisualStateGroups(uc, new List<VisualStateGroup>
+		{
+			new VisualStateGroup()
+			{
+				States =
+				{
+					new VisualState
+					{
+						Name = "MyVisualState1",
+					},
+					new VisualState
+					{
+						Name = "MyVisualState2",
+						StateTriggers =
+						{
+							new AdaptiveTrigger()
+							{
+								MinWindowWidth = 1
+							}
+						}
+					}
+				}
+			}
+		});
+
+		var contentControl = new ContentControl
+		{
+			Content = "0",
+			ContentTemplate = new DataTemplate(() =>
+			{
+				return uc;
+			})
+		};
+
+		await UITestHelper.Load(contentControl, control => control.IsLoaded);
+		Assert.AreEqual("MyVisualState2", uc.VisualStateOnFirstMeasure?.Name);
+	}
+
+	private partial class MyUserControl : UserControl
+	{
+		private bool _firstMeasure = true;
+		public VisualState VisualStateOnFirstMeasure { get; set; }
+
+		protected override Size MeasureOverride(Size availableSize)
+		{
+			if (_firstMeasure)
+			{
+				_firstMeasure = false;
+				VisualStateOnFirstMeasure = VisualStateManager.GetVisualStateGroups(this)[0].CurrentState;
+			}
+			return base.MeasureOverride(availableSize);
+		}
+	}
+#endif
 }

src/Uno.UI/UI/Xaml/Controls/Control/Control.crossruntime.cs

@@ -1,4 +1,6 @@
 using System.Linq;
+using Uno.Extensions;
+using Uno.UI.Xaml;
 
 namespace Microsoft.UI.Xaml.Controls;
 
@@ -25,4 +27,44 @@
 	{
 		return this.GetChildren()?.FirstOrDefault() as IFrameworkElement;
 	}
+
+	internal override void EnterImpl(EnterParams @params, int depth)
+	{
+		base.EnterImpl(@params, depth);
+
+		if (@params.IsLive)
+		{
+			// if (SupportsBuiltInStyles() && !m_fIsBuiltInStyleApplied)
+			// {
+			// 	// When we apply the built-in style, we may resolve theme resources in doing so
+			// 	// that haven't yet been resolved - for example, if a property value references
+			// 	// a resource that then references another resource.
+			// 	// We need to make sure we're operating under the correct theme during that resolution.
+			// 	bool removeRequestedTheme = false;
+			// 	const auto theme = GetTheme();
+			// 	const auto oldRequestedThemeForSubTree = GetRequestedThemeForSubTreeFromCore();
+			//
+			// 	if (theme != Theming::Theme::None && Theming::GetBaseValue(theme) != oldRequestedThemeForSubTree)
+			// 	{
+			// 		SetRequestedThemeForSubTreeOnCore(theme);
+			// 		removeRequestedTheme = true;
+			// 	}
+			//
+			// 	auto themeGuard = wil::scope_exit([&] {
+			// 		if (removeRequestedTheme)
+			// 		{
+			// 			SetRequestedThemeForSubTreeOnCore(oldRequestedThemeForSubTree);
+			// 		}
+			// 	});
+			//
+			// 	IFC_RETURN(ApplyBuiltInStyle());
+			// }
+
+			// Initialize StateTriggers at this time.  We need to wait for this to enter a visual tree
+			// since we need for it to be part of the main visual tree to know which visual tree's
+			// qualifier context to use.  We also need to force an update because our visual tree root
+			// may have changed since the last enter.
+			VisualStateManager.InitializeStateTriggers(this, true /* forceUpdate */);
+		}
+	}
 }

src/Uno.UI/UI/Xaml/FrameworkElement.Layout.crossruntime.cs

@@ -214,13 +214,15 @@
 		{
 			ApplyTemplate(out addedVisuals);
 
-			//if (auto visualTree = VisualTree::GetForElementNoRef(pControl))
-			// {
-			//	// Create VisualState StateTriggers and perform evaulation to determine initial state,
-			//	// if we're in the visual tree (since we need it to get our qualifier context).
-			//	// If we're not in the visual tree, we'll do this when we enter it.
-			//	IFC(CVisualStateManager2::InitializeStateTriggers(this));
-			//}
+			var pControl = this as Control;
+
+			if (VisualTree.GetForElement(pControl) is { } visualTree)
+			{
+				// Create VisualState StateTriggers and perform evaulation to determine initial state,
+				// if we're in the visual tree (since we need it to get our qualifier context).
+				// If we're not in the visual tree, we'll do this when we enter it.
+				VisualStateManager.InitializeStateTriggers(pControl);
+			}
 
 			//var control = this as Control;
 

src/Uno.UI/UI/Xaml/VisualStateGroup.cs

@@ -150,11 +150,15 @@ private void OnParentChanged(object instance, object key, DependencyObjectParent
 			if (this.GetParent() is IFrameworkElement fe)
 			{
 				OnOwnerElementChanged();
+#if !__CROSSRUNTIME__
 				fe.Loaded += OnOwnerElementLoaded;
+#endif
 				fe.Unloaded += OnOwnerElementUnloaded;
 				_parentLoadedDisposable.Disposable = Disposable.Create(() =>
 				{
+#if !__CROSSRUNTIME__
 					fe.Loaded -= OnOwnerElementLoaded;
+#endif
 					fe.Unloaded -= OnOwnerElementUnloaded;
 				});
 			}
@@ -171,7 +175,7 @@ private void OnOwnerElementChanged()
 			ExecuteOnTriggers(t => t.OnOwnerElementChanged());
 		}
 
-		private void OnOwnerElementLoaded(object sender, RoutedEventArgs args)
+		internal void OnOwnerElementLoaded(object sender, RoutedEventArgs args)
 		{
 			if (_pendingOnOwnerElementChanged)
 			{

src/Uno.UI/UI/Xaml/VisualStateManager.cs

@@ -290,5 +290,74 @@
 
 			return (null, null);
 		}
+
+		internal static void InitializeStateTriggers(DependencyObject pDO, bool forceUpdate = false)
+		{
+			if (pDO is not Control pControl)
+			{
+				return;
+			}
+
+			var groupCollection = VisualStateManager.GetVisualStateGroups(pControl);
+			if (groupCollection is null)
+			{
+				return;
+			}
+
+			// auto& groupCollectionMap = groupCollection->GetStateTriggerVariantMaps();
+			// if (forceUpdate)
+			// {
+			// 	// Forcing an update, clear the map
+			// 	groupCollectionMap.clear();
+			//
+			// 	// If we're forcing an update, we should also clear the state trigger variant maps in each group.
+			// 	for (auto group : *groupCollection)
+			// 	{
+			// 		static_cast<CVisualStateGroup*>(group)->m_pStateTriggerVariantMap = nullptr;
+			// 	}
+			//
+			// 	for (auto& groupContext : groupCollection->GetGroupContext())
+			// 	{
+			// 		groupContext.GetStateTriggerVariantMap() = nullptr;
+			// 	}
+			// }
+			//
+			// if (!groupCollectionMap.empty())
+			// {
+			// 	// If the map isn't clear, then we are already initialized or we weren't forced by diagnostics to update.
+			// 	// Adding triggers isn't something usually supported at runtime.
+			// 	return S_OK;
+			// }
+			//
+			// auto dataSource = CreateVisualStateManagerDataSource(groupCollection);
+			// auto dataSourceMap = dataSource->GetStateTriggerVariantMaps();
+			//
+			// if (!dataSourceMap.empty())
+			// {
+			// 	if (forceUpdate)
+			// 	{
+			// 		// Forcing an update, clear the map
+			// 		dataSourceMap.clear();
+			// 	}
+			//
+			// 	if (!dataSourceMap.empty())
+			// 	{
+			// 		// If the map isn't clear, then we are already initialized or we weren't forced by diagnostics to update.
+			// 		// Adding triggers isn't something usually supported at runtime.
+			// 		return S_OK;
+			// 	}
+			// }
+			//
+			// VisualStateManagerActuator actuator(
+			// 	static_cast<CFrameworkElement*>(pControl->GetFirstChildNoAddRef()), dataSource.get());
+			// IFC_RETURN(actuator.InitializeStateTriggers(pControl));
+
+			// Uno Specific
+			foreach (var visualStateGroup in GetVisualStateGroups(pControl))
+			{
+				visualStateGroup.OnOwnerElementLoaded(pControl, null);
+				visualStateGroup.RefreshStateTriggers(forceUpdate);
+			}
+		}
 	}
 }