Fix duplicate state container view states

src/CommunityToolkit.Maui.UnitTests/Layouts/StateContainerTests.cs

@@ -514,10 +514,27 @@ public void StateContainer_CreatesControllerWithLayout()
 	}
 
 	[Fact]
-	public void Controller_ReturnsErrorLabelOnInvalidState()
+	public void Controller_ThrowsStateContainerExceptionInvalidStateKey()
 	{
 		Assert.Throws<StateContainerException>(() => controller.SwitchToState("InvalidStateKey"));
 	}
+
+	[Fact]
+	public void Controller_ThrowsStateContainerExceptionOnDuplicateStateKey()
+	{
+		// Arrange
+		var stackLayout = new StackLayout();
+		var label = new Label();
+		var button = new Button();
+
+		StateView.SetStateKey(label, StateKey.Anything);
+		StateView.SetStateKey(button, StateKey.Anything);
+		StateContainer.SetStateViews(stackLayout, [label, button]);
+
+		// Assert
+		var exception = Assert.Throws<StateContainerException>(() => StateContainer.SetCurrentState(stackLayout, StateKey.Anything));
+		exception.Message.Should().Contain("multiple");
+	}
 
 	[Fact]
 	public void Controller_SwitchesToStateFromContentSuccess()
@@ -594,12 +611,34 @@ public void EnsureDefaults()
 		var stackLayout = new StackLayout();
 
 		// Act Assert
-		Assert.Equal(StateContainerDefaults.StateViews, StateContainer.GetStateViews(stackLayout));
+		Assert.Equal([], StateContainer.GetStateViews(stackLayout));
+		Assert.Empty(StateContainer.GetStateViews(stackLayout));
 		Assert.Equal(StateContainerDefaults.CurrentState, StateContainer.GetCurrentState(stackLayout));
 		Assert.Equal(StateContainerDefaults.CanStateChange, StateContainer.GetCanStateChange(stackLayout));
 		Assert.Equal(StateViewDefaults.StateKey, StateView.GetStateKey(stackLayout));
 	}
 
+	[Fact]
+	public void EnsureLayoutControllerIsUniquePerLayout()
+	{
+		// Arrange
+		var grid1 = new Grid();
+		var grid2 = new Grid();
+
+		// Act
+		var grid1StateViews = StateContainer.GetStateViews(grid1);
+		var grid2StateViews = StateContainer.GetStateViews(grid2);
+		grid1StateViews.Add(new Label
+		{
+			Text = "Test",
+		});
+
+		// Assert
+		Assert.NotSame(grid1StateViews, grid2StateViews);
+		Assert.Single(grid1StateViews);
+		Assert.Empty(grid2StateViews);
+	}
+
 	sealed class ViewModel : INotifyPropertyChanged
 	{
 		public bool CanChangeState

src/CommunityToolkit.Maui/Layouts/StateContainer/StateContainer.shared.cs

@@ -192,7 +192,7 @@ static void ValidateCanStateChange(in BindableObject bindable)
 		}
 	}
 
-	static IList<View> CreateDefaultStateViewsProperty(BindableObject bindable) => StateContainerDefaults.StateViews;
+	static IList<View> CreateDefaultStateViewsProperty(BindableObject bindable) => [];
 }
 
 /// <summary>

src/CommunityToolkit.Maui/Layouts/StateContainer/StateContainerController.shared.cs

@@ -3,23 +3,21 @@
 /// <summary>
 /// StateContainer Controller
 /// </summary>
-sealed class StateContainerController
+/// <remarks>
+/// Initialize <see cref="StateContainerController"/> with a <see cref="Layout"/>
+/// </remarks>
+/// <param name="layout"></param>
+sealed class StateContainerController(Layout layout)
 {
-	readonly WeakReference<Layout> layoutWeakReference;
+	readonly WeakReference<Layout> layoutWeakReference = new(layout);
 
 	string? previousState;
 	List<IView> originalContent = Enumerable.Empty<IView>().ToList();
 
-	/// <summary>
-	/// Initialize <see cref="StateContainerController"/> with a <see cref="Layout"/>
-	/// </summary>
-	/// <param name="layout"></param>
-	public StateContainerController(Layout layout) => layoutWeakReference = new WeakReference<Layout>(layout);
-
 	/// <summary>
 	/// The StateViews defined in the StateContainer.
 	/// </summary>
-	public required IList<View> StateViews { get; set; }
+	public required IList<View> StateViews { get; init; }
 
 	/// <summary>
 	/// Display the default content.
@@ -84,7 +82,14 @@
 
 	View GetViewForState(string state)
 	{
-		var view = StateViews.FirstOrDefault(x => StateView.GetStateKey(x) == state);
-		return view ?? throw new StateContainerException($"View for {state} not defined.");
+		try
+		{
+			var view = StateViews.SingleOrDefault(x => StateView.GetStateKey(x) == state);
+			return view ?? throw new StateContainerException($"{nameof(StateView)} for {state} not defined.");
+		}
+		catch (InvalidOperationException e)
+		{
+			throw new StateContainerException($"Unable to determine {nameof(StateView)} for State: {state}. This State has been assigned to multiple {nameof(StateView)}s. Ensure each {nameof(StateView)} has a unique StateKey");
+		}
 	}
 }

src/CommunityToolkit.Maui/Primitives/StateContainerDefaults.shared.cs

@@ -4,5 +4,4 @@ static class StateContainerDefaults
 {
 	public const string? CurrentState = null;
 	public const bool CanStateChange = true;
-	public static IList<View> StateViews { get; } = [];
 }