feat: FauxGradientBorderPresenter

src/Uno.UI/Controls/FauxGradientBorderPresenter.cs

@@ -0,0 +1,152 @@
+#nullable enable
+
+using System.Collections.Generic;
+using System.Linq;
+using Uno.Foundation.Logging;
+using Uno.UI.Xaml.Media;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media;
+
+namespace Uno.UI.Controls;
+
+/// <summary>
+/// This presenter provides a way to display "fake" LinearGradientBrush on element border for
+/// cases which are unsupported by WebAssembly, iOS and macOS.
+/// </summary>
+/// <remarks>
+/// WASM - the presenter will be used in case the element has CornerRadius applied.
+/// iOS and macOS - the presenter will be used in case the element has LinearGradientBrush with a transform applied.
+/// All other cases - the presenter is not visible.
+/// </remarks>
+public partial class FauxGradientBorderPresenter : ContentPresenter
+{
+#if __WASM__ || __IOS__ || __MACOS__
+	private readonly Border? _displayBorder;
+#endif
+
+	public FauxGradientBorderPresenter()
+	{
+#if __WASM__ || __IOS__ || __MACOS__
+		HorizontalContentAlignment = HorizontalAlignment.Stretch;
+		VerticalContentAlignment = VerticalAlignment.Stretch;
+		Content = _displayBorder = new Border();
+#else
+		Visibility = Visibility.Collapsed;
+#endif
+	}
+
+	/// <summary>
+	/// Gets or sets the border brush that is supposed to be displayed.
+	/// </summary>
+	public Brush RequestedBorderBrush
+	{
+		get => (Brush)GetValue(RequestedBorderBrushProperty);
+		set => SetValue(RequestedBorderBrushProperty, value);
+	}
+
+	/// <summary>
+	/// Identifies the RequestedBorderBrush dependency property.
+	/// </summary>
+	public static DependencyProperty RequestedBorderBrushProperty { get; } =
+		DependencyProperty.Register(
+			nameof(RequestedBorderBrush),
+			typeof(Brush),
+			typeof(FauxGradientBorderPresenter),
+			new FrameworkPropertyMetadata(null, propertyChangedCallback: (s, args) => ((FauxGradientBorderPresenter)s)?.OnBorderChanged()));
+
+	/// <summary>
+	/// Gets or sets the thickness of the border that is supposed to be displayed.
+	/// </summary>
+	public Thickness RequestedBorderThickness
+	{
+		get => (Thickness)GetValue(RequestedBorderThicknessProperty);
+		set => SetValue(RequestedBorderThicknessProperty, value);
+	}
+
+	/// <summary>
+	/// Identifies the RequestedBorderThickness dependency property.
+	/// </summary>
+	public static DependencyProperty RequestedBorderThicknessProperty { get; } =
+		DependencyProperty.Register(
+			nameof(RequestedBorderThickness),
+			typeof(Thickness),
+			typeof(FauxGradientBorderPresenter),
+			new FrameworkPropertyMetadata(default(Thickness), propertyChangedCallback: (s, args) => ((FauxGradientBorderPresenter)s)?.OnBorderChanged()));
+
+	public CornerRadius RequestedCornerRadius
+	{
+		get => (CornerRadius)GetValue(RequestedCornerRadiusProperty);
+		set => SetValue(RequestedCornerRadiusProperty, value);
+	}
+
+	public static DependencyProperty RequestedCornerRadiusProperty { get; } =
+		DependencyProperty.Register(
+			nameof(RequestedCornerRadius),
+			typeof(CornerRadius),
+			typeof(FauxGradientBorderPresenter),
+			new FrameworkPropertyMetadata(CornerRadius.None, propertyChangedCallback: (s, args) => ((FauxGradientBorderPresenter)s)?.OnBorderChanged()));
+
+	private void OnBorderChanged()
+	{
+#if __WASM__ || __IOS__ || __MACOS__
+		if (_displayBorder == null)
+		{
+			return;
+		}
+
+		var requestedThickness = RequestedBorderThickness;
+		var requestedBorderBrush = RequestedBorderBrush;
+		var requestedCornerRadius = RequestedCornerRadius;
+
+		if (requestedBorderBrush is not LinearGradientBrush gradientBrush ||
+			!gradientBrush.CanApplySolidColorRendering())
+		{
+			_displayBorder.Visibility = Visibility.Collapsed;
+			return;
+		}
+
+#if __WASM__
+		if (requestedCornerRadius == CornerRadius.None)
+		{
+			// WASM can render linear gradient border unless corner radius is set.
+			_displayBorder.Visibility = Visibility.Collapsed;
+			return;
+		}
+#endif
+
+#if __IOS__ || __MACOS__
+		if (gradientBrush.RelativeTransform == null)
+		{
+			// iOS can render linear gradient border unless relative transform is used.
+			_displayBorder.Visibility = Visibility.Collapsed;
+			return;
+		}
+#endif
+
+		requestedThickness.Left = 0;
+		requestedThickness.Right = 0;
+		var minorStopAlignment = gradientBrush.GetMinorStopAlignment();
+		if (minorStopAlignment == VerticalAlignment.Top)
+		{
+			requestedThickness.Bottom = 0;
+		}
+		else
+		{
+			requestedThickness.Top = 0;
+		}
+
+		if (requestedThickness == Thickness.Empty)
+		{
+			_displayBorder.Visibility = Visibility.Collapsed;
+			return;
+		}
+
+		_displayBorder.Visibility = Visibility.Visible;
+
+		_displayBorder.CornerRadius = requestedCornerRadius;
+		_displayBorder.BorderThickness = requestedThickness;
+		_displayBorder.BorderBrush = gradientBrush.FauxOverlayBrush;
+#endif
+	}
+}