Skip initial assignments in property mapper (take 4)

src/Controls/src/Core/Label/Label.cs

@@ -12,7 +12,7 @@ namespace Microsoft.Maui.Controls
 {
 	/// <include file="../../docs/Microsoft.Maui.Controls/Label.xml" path="Type[@FullName='Microsoft.Maui.Controls.Label']/Docs/*" />
 	[ContentProperty(nameof(Text))]
-	public partial class Label : View, IFontElement, ITextElement, ITextAlignmentElement, ILineHeightElement, IElementConfiguration<Label>, IDecorableTextElement, IPaddingElement, ILabel
+	public partial class Label : View, IFontElement, ITextElement, ITextAlignmentElement, ILineHeightElement, IElementConfiguration<Label>, IDecorableTextElement, IPaddingElement, ILabel, ILabelCompat
 	{
 		/// <summary>Bindable property for <see cref="HorizontalTextAlignment"/>.</summary>
 		public static readonly BindableProperty HorizontalTextAlignmentProperty = TextAlignmentElement.HorizontalTextAlignmentProperty;

src/Core/src/Core/ILabelCompat.cs

@@ -0,0 +1,7 @@
+namespace Microsoft.Maui;
+
+internal interface ILabelCompat
+{
+	TextType TextType { get; }
+	TextTransform TextTransform { get; }
+}

src/Core/src/Core/IPropertyInitializerMapper.cs

@@ -0,0 +1,7 @@
+namespace Microsoft.Maui;
+
+// TODO: Potentially make public in NET10
+internal interface IPropertyInitializerMapper
+{
+	void InitializeProperties(IElementHandler viewHandler, IElement virtualView);
+}

src/Core/src/Handlers/Element/ElementHandler.cs

@@ -2,7 +2,7 @@
 
 namespace Microsoft.Maui.Handlers
 {
-	public abstract partial class ElementHandler : IElementHandler
+	public abstract partial class ElementHandler : IElementHandler, IPlatformPropertyDefaultsProvider
 	{
 		public static IPropertyMapper<IElement, IElementHandler> ElementMapper = new PropertyMapper<IElement, IElementHandler>()
 		{
@@ -16,6 +16,10 @@ public abstract partial class ElementHandler : IElementHandler
 		internal readonly CommandMapper? _commandMapper;
 		internal IPropertyMapper _mapper;
 
+
+		private protected IPlatformPropertyDefaults? PlatformPropertyDefaults { get; init; }
+		IPlatformPropertyDefaults? IPlatformPropertyDefaultsProvider.PlatformPropertyDefaults => PlatformPropertyDefaults;
+
 		protected ElementHandler(IPropertyMapper mapper, CommandMapper? commandMapper = null)
 		{
 			_ = mapper ?? throw new ArgumentNullException(nameof(mapper));
@@ -45,19 +49,29 @@ public virtual void SetVirtualView(IElement view)
 			var oldVirtualView = VirtualView;
 
 			bool setupPlatformView = oldVirtualView == null;
+			bool isNewPlatformView = false;
 
 			VirtualView = view;
-			PlatformView ??= CreatePlatformElement();
+
+			if (PlatformView is null)
+			{
+				PlatformView = CreatePlatformElement();
+				isNewPlatformView = true;
+			}
 
 			if (VirtualView.Handler != this)
+			{
 				VirtualView.Handler = this;
+			}
 
 			// We set the previous virtual view to null after setting it on the incoming virtual view.
 			// This makes it easier for the incoming virtual view to have influence
 			// on how the exchange of handlers happens.
 			// We will just set the handler to null ourselves as a last resort cleanup
 			if (oldVirtualView?.Handler != null)
+			{
 				oldVirtualView.Handler = null;
+			}
 
 			if (setupPlatformView)
 			{
@@ -76,6 +90,12 @@ public virtual void SetVirtualView(IElement view)
 				}
 			}
 
+			if (isNewPlatformView && _mapper is IPropertyInitializerMapper initializerMapper)
+			{
+				initializerMapper.InitializeProperties(this, VirtualView);
+				return;
+			}
+
 			_mapper.UpdateProperties(this, VirtualView);
 		}
 

src/Core/src/Handlers/IPlatformPropertyDefaultsProvider.cs

@@ -0,0 +1,7 @@
+namespace Microsoft.Maui;
+
+// TODO: Potentially make public in NET10
+internal interface IPlatformPropertyDefaultsProvider
+{
+	IPlatformPropertyDefaults? PlatformPropertyDefaults { get; }
+}

src/Core/src/Handlers/Label/LabelHandler.cs

@@ -15,6 +15,24 @@ namespace Microsoft.Maui.Handlers
 {
 	public partial class LabelHandler : ILabelHandler
 	{
+		internal static readonly IPlatformPropertyDefaults<ILabel> LabelPropertyDefaults = new PlatformPropertyDefaults<ILabel>(ViewHandler.ViewPropertyDefaults)
+		{
+			[nameof(ILabel.CharacterSpacing)] = HasDefaultCharacterSpacing,
+
+#if !IOS && !MACATALYST
+			// TODO: It is not clear if this can be enabled on iOS-like platforms.
+			[nameof(ILabel.HorizontalTextAlignment)] = HasDefaultHorizontalTextAlignment,
+
+			// Test Issue21325 fails on Mac Catalyst if this is on as a label is not placed on the top but in the center. 
+			[nameof(ILabel.VerticalTextAlignment)] = HasDefaultVerticalTextAlignment,
+#endif
+
+			[nameof(ILabel.TextDecorations)] = HasDefaultTextDecorations,
+
+			[nameof(ILabelCompat.TextType)] = HasDefaultTextType,
+			[nameof(ILabelCompat.TextTransform)] = HasDefaultTextTransform,
+		};
+
 		public static IPropertyMapper<ILabel, ILabelHandler> Mapper = new PropertyMapper<ILabel, ILabelHandler>(ViewHandler.ViewMapper)
 		{
 #if IOS || TIZEN
@@ -48,20 +66,41 @@ public partial class LabelHandler : ILabelHandler
 
 		public LabelHandler() : base(Mapper, CommandMapper)
 		{
+			PlatformPropertyDefaults = LabelPropertyDefaults;
 		}
 
 		public LabelHandler(IPropertyMapper? mapper)
 			: base(mapper ?? Mapper, CommandMapper)
 		{
+			PlatformPropertyDefaults = LabelPropertyDefaults;
 		}
 
 		public LabelHandler(IPropertyMapper? mapper, CommandMapper? commandMapper)
 			: base(mapper ?? Mapper, commandMapper ?? CommandMapper)
 		{
+			PlatformPropertyDefaults = LabelPropertyDefaults;
 		}
 
 		ILabel ILabelHandler.VirtualView => VirtualView;
 
 		PlatformView ILabelHandler.PlatformView => PlatformView;
+
+		/// <summary>This is a legacy property automatically remapped to `Text`, so it can always be considered as a default value.</summary>
+		internal static bool HasDefaultTextTransform(ILabel arg) => true;
+
+		/// <summary>This is a legacy property automatically remapped to `Text`, so it can always be considered as a default value.</summary>
+		internal static bool HasDefaultTextType(ILabel arg) => true;
+
+		internal static bool HasDefaultTextDecorations(ILabel label)
+			=> label.TextDecorations == TextDecorations.None;
+
+		internal static bool HasDefaultCharacterSpacing(ILabel label)
+			=> label.CharacterSpacing == 0;
+
+		internal static bool HasDefaultHorizontalTextAlignment(ILabel label)
+			=> label.HorizontalTextAlignment == TextAlignment.Start;
+
+		internal static bool HasDefaultVerticalTextAlignment(ILabel label)
+			=> label.VerticalTextAlignment == TextAlignment.Start;
 	}
 }

src/Core/src/Handlers/View/ViewHandler.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using Microsoft.Maui.Graphics;
 #if __IOS__ || MACCATALYST
 using PlatformView = UIKit.UIView;
@@ -20,6 +22,52 @@ namespace Microsoft.Maui.Handlers
 	/// Handlers are also responsible for instantiating the underlying platform view, and mapping the cross-platform control API to the platform view API. </remarks>
 	public abstract partial class ViewHandler : ElementHandler, IViewHandler
 	{
+		internal static readonly IPlatformPropertyDefaults<IView> ViewPropertyDefaults = new PlatformPropertyDefaults<IView>
+		{
+			[nameof(IView.AutomationId)] = HasDefaultAutomationId,
+			[nameof(IView.Clip)] = HasDefaultClip,
+
+			// Uncommenting this leads to failing of the test Issue25887 (ContentPresenter just rendering component string in .Net9).
+			// [nameof(IPadding.Padding)] = HasDefaultPadding,
+
+			[nameof(IView.Shadow)] = HasDefaultShadow,
+			[nameof(IView.Visibility)] = HasDefaultVisibility,
+			[nameof(IView.FlowDirection)] = HasDefaultFlowDirection,
+
+			// TODO: Can this be enabled?
+			// [nameof(IView.Width)] = HasDefaultWidth,
+			// [nameof(IView.Height)] = HasDefaultHeight,
+
+			[nameof(IView.MinimumWidth)] = HasDefaultMinimumWidth,
+			[nameof(IView.MaximumWidth)] = HasDefaultMaximumWidth,
+
+			[nameof(IView.MinimumHeight)] = HasDefaultMinimumHeight,
+			[nameof(IView.MaximumHeight)] = HasDefaultMaximumHeight,
+
+			[nameof(IView.Scale)] = HasDefaultScale,
+			[nameof(IView.ScaleX)] = HasDefaultScaleX,
+			[nameof(IView.ScaleY)] = HasDefaultScaleY,
+
+			[nameof(IView.Rotation)] = HasDefaultRotation,
+			[nameof(IView.RotationX)] = HasDefaultRotationX,
+			[nameof(IView.RotationY)] = HasDefaultRotationY,
+
+			[nameof(IView.AnchorX)] = HasDefaultAnchorX,
+			[nameof(IView.AnchorY)] = HasDefaultAnchorY,
+
+			[nameof(IView.Opacity)] = HasDefaultOpacity,
+			[nameof(IView.TranslationX)] = HasDefaultTranslationX,
+			[nameof(IView.TranslationY)] = HasDefaultTranslationY,
+
+			[nameof(IContextFlyoutElement.ContextFlyout)] = HasDefaultContextFlyout,
+
+			// Input transparency must always be mapped to make views user-interactable and allow gesture recognizers to work properly
+			// [nameof(IView.InputTransparent)] = HasDefaultInputTransparent,
+
+			[nameof(IToolbarElement.Toolbar)] = HasDefaultToolbar,
+			[nameof(IToolTipElement.ToolTip)] = HasDefaultToolTip,
+		};
+
 		/// <summary>
 		/// A dictionary that maps the virtual view properties to their platform view counterparts.
 		/// </summary>
@@ -31,6 +79,9 @@ public abstract partial class ViewHandler : ElementHandler, IViewHandler
 			new PropertyMapper<IView, IViewHandler>(ElementHandler.ElementMapper)
 #endif
 			{
+				// This property is a special one and needs to be set before other properties.
+				[nameof(IViewHandler.ContainerView)] = MapContainerView,
+
 				[nameof(IView.AutomationId)] = MapAutomationId,
 				[nameof(IView.Clip)] = MapClip,
 				[nameof(IView.Shadow)] = MapShadow,
@@ -56,7 +107,6 @@ public abstract partial class ViewHandler : ElementHandler, IViewHandler
 				[nameof(IView.RotationY)] = MapRotationY,
 				[nameof(IView.AnchorX)] = MapAnchorX,
 				[nameof(IView.AnchorY)] = MapAnchorY,
-				[nameof(IViewHandler.ContainerView)] = MapContainerView,
 #pragma warning disable CS0618 // Type or member is obsolete
 				[nameof(IBorder.Border)] = MapBorderView,
 #pragma warning restore CS0618 // Type or member is obsolete
@@ -100,6 +150,7 @@ public abstract partial class ViewHandler : ElementHandler, IViewHandler
 		protected ViewHandler(IPropertyMapper mapper, CommandMapper? commandMapper = null)
 			: base(mapper, commandMapper ?? ViewCommandMapper)
 		{
+			PlatformPropertyDefaults = ViewPropertyDefaults;
 		}
 
 		/// <summary>
@@ -535,5 +586,79 @@ public static void MapToolTip(IViewHandler handler, IView view)
 				handler.ToPlatform().UpdateToolTip(tooltipContainer.ToolTip);
 #endif
 		}
+
+		internal static bool HasDefaultAutomationId(IView view)
+			=> string.IsNullOrEmpty(view.AutomationId);
+
+		internal static bool HasDefaultClip(IView view)
+			=> view.Clip is null;
+
+		internal static bool HasDefaultPadding(IView view)
+			=> view is IPadding padding && (padding.Padding.IsEmpty || padding.Padding.IsNaN);
+
+		internal static bool HasDefaultShadow(IView view)
+			=> view.Shadow is null;
+
+		internal static bool HasDefaultVisibility(IView view)
+			=> view.Visibility == Visibility.Visible;
+
+		internal static bool HasDefaultFlowDirection(IView view)
+			=> view.FlowDirection == FlowDirection.MatchParent;
+
+		internal static bool HasDefaultMinimumWidth(IView view)
+			=> double.IsNaN(view.MinimumWidth);
+
+		internal static bool HasDefaultMaximumWidth(IView view)
+			=> double.IsNaN(view.MaximumWidth);
+
+		internal static bool HasDefaultMinimumHeight(IView view)
+			=> double.IsNaN(view.MinimumHeight);
+
+		internal static bool HasDefaultMaximumHeight(IView view)
+			=> double.IsNaN(view.MaximumHeight);
+
+		internal static bool HasDefaultScale(IView view)
+			=> view.Scale == 1;
+
+		internal static bool HasDefaultScaleX(IView view)
+			=> view.Scale == 1 && view.ScaleX == 1;
+
+		internal static bool HasDefaultScaleY(IView view)
+			=> view.Scale == 1 && view.ScaleY == 1;
+
+		internal static bool HasDefaultRotation(IView view)
+			=> view.Rotation % 360 == 0;
+
+		internal static bool HasDefaultRotationX(IView view)
+			=> view.RotationX % 360 == 0;
+
+		internal static bool HasDefaultRotationY(IView view)
+			=> view.RotationY % 360 == 0;
+
+		internal static bool HasDefaultAnchorX(IView view)
+			=> view.AnchorX == 0.5;
+
+		internal static bool HasDefaultAnchorY(IView view)
+			=> view.AnchorY == 0.5;
+
+		internal static bool HasDefaultOpacity(IView view)
+			=> view.Opacity == 1;
+
+		internal static bool HasDefaultTranslationX(IView view)
+			=> view.TranslationX == 0;
+
+		internal static bool HasDefaultTranslationY(IView view)
+			=> view.TranslationY == 0;
+
+		internal static bool HasDefaultContextFlyout(IView view)
+			=> view is IContextFlyoutElement { ContextFlyout: null };
+		internal static bool HasDefaultInputTransparent(IView view)
+			=> !view.InputTransparent;
+
+		internal static bool HasDefaultToolbar(IView view)
+			=> view is IToolbarElement { Toolbar: null };
+
+		internal static bool HasDefaultToolTip(IView view)
+			=> view is IToolTipElement { ToolTip: null };
 	}
 }