[Extensibility] XAML: Expose GTK, macOS, and WPF in OnPlatform

src/Controls/src/Xaml/MarkupExtensions/OnPlatformExtension.cs

@@ -10,6 +10,32 @@ namespace Microsoft.Maui.Controls.Xaml
 	/// <summary>
 	/// Provides a XAML markup extension that returns different values depending on the platform the app is running on.
 	/// </summary>
+	/// <remarks>
+	/// The value is resolved by matching the <see cref="Microsoft.Maui.Devices.DeviceInfo.Platform"/> identifier
+	/// (the <see cref="Microsoft.Maui.Devices.DevicePlatform"/> struct's <c>ToString()</c> value)
+	/// against the per-platform values, so a custom backend whose platform string matches one of the named
+	/// properties below (for example <c>GTK</c>) resolves correctly at runtime. The match is
+	/// <em>case-sensitive</em> (ordinal, per <see cref="Microsoft.Maui.Devices.DevicePlatform"/> equality), so the
+	/// identifier must match the property name's casing exactly — a backend reporting
+	/// <c>DevicePlatform.Create("gtk")</c> (lowercase) does not match the <c>GTK</c> property and falls through
+	/// to <see cref="Default"/>. Note that the compile-time
+	/// optimization (<c>SimplifyOnPlatformVisitor</c>) currently recognizes only the
+	/// <c>-android</c>/<c>-ios</c>/<c>-macos</c>/<c>-maccatalyst</c> target frameworks; every other target
+	/// framework (including <c>-windows</c> and <c>-tizen</c>) and any custom backend falls back to runtime
+	/// resolution.
+	/// <para>
+	/// Because a XAML markup extension maps each named argument to a CLR property, the inline
+	/// <c>{OnPlatform iOS=…, Android=…}</c> form can only express the platforms exposed as properties here.
+	/// To target an <em>arbitrary</em> custom platform (e.g. a backend reporting
+	/// <c>DevicePlatform.Create("Web")</c>), use the element form, which accepts any platform string:
+	/// <code>
+	/// &lt;OnPlatform x:TypeArguments="x:String"&gt;
+	///     &lt;On Platform="Web" Value="…" /&gt;
+	///     &lt;On Platform="iOS, Android" Value="…" /&gt;
+	/// &lt;/OnPlatform&gt;
+	/// </code>
+	/// </para>
+	/// </remarks>
 	[ContentProperty(nameof(Default))]
 	[RequireService(
 		[typeof(IProvideValueTarget),
@@ -31,14 +57,21 @@ public class OnPlatformExtension : IMarkupExtension
 		/// </summary>
 		public object Android { get; set; } = s_notset;
 
-		internal object GTK { get; set; } = s_notset;
+		/// <summary>
+		/// Gets or sets the value to use on GTK.
+		/// </summary>
+		public object GTK { get; set; } = s_notset;
 
 		/// <summary>
 		/// Gets or sets the value to use on iOS.
 		/// </summary>
 		public object iOS { get; set; } = s_notset;
 
-		internal object macOS { get; set; } = s_notset;
+		/// <summary>
+		/// Gets or sets the value to use on macOS.
+		/// </summary>
+		/// <remarks>Note, this is different than <see cref="MacCatalyst"/>.</remarks>
+		public object macOS { get; set; } = s_notset;
 
 		/// <summary>
 		/// Gets or sets the value to use on Mac Catalyst.
@@ -56,7 +89,10 @@ public class OnPlatformExtension : IMarkupExtension
 		[Obsolete("Use WinUI instead.")]
 		public object UWP { get; set; } = s_notset;
 
-		internal object WPF { get; set; } = s_notset;
+		/// <summary>
+		/// Gets or sets the value to use on WPF.
+		/// </summary>
+		public object WPF { get; set; } = s_notset;
 
 		/// <summary>
 		/// Gets or sets the value to use on Windows (WinUI).

src/Controls/src/Xaml/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/src/Xaml/PublicAPI/net-ios/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/src/Xaml/PublicAPI/net-maccatalyst/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/src/Xaml/PublicAPI/net-tizen/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/src/Xaml/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/src/Xaml/PublicAPI/net/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 Microsoft.Maui.Controls.Xaml.XamlIncrementalHotReloadHandler
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?

src/Controls/src/Xaml/PublicAPI/netstandard/PublicAPI.Unshipped.txt

@@ -1,6 +1,12 @@
 #nullable enable
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.get -> System.Globalization.CultureInfo
 ~Microsoft.Maui.Controls.Xaml.BindingExtension.ConverterCulture.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.GTK.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.WPF.set -> void
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.get -> object
+~Microsoft.Maui.Controls.Xaml.OnPlatformExtension.macOS.set -> void
 Microsoft.Maui.Controls.Xaml.XamlComponentRegistry
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.FindStaticResource(object! element, string! key) -> object?
 static Microsoft.Maui.Controls.Xaml.XamlComponentRegistry.GetComponent(object! page, string! nodeId) -> object?

src/Controls/tests/SourceGen.UnitTests/InitializeComponent/SimplifyOnPlatform.cs

@@ -135,6 +135,47 @@ private partial void InitializeComponent()
 		Assert.Equal(expected, generated, ignoreLineEndingDifferences: true);
 	}
 
+	[Fact]
+	public void NonSimplifiedOnPlatformMarkupExtensionUsesPublicCustomPlatformProperties()
+	{
+		var xaml =
+"""
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage
+	xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+	x:Class="Test.TestPage">
+	<Label Text="{OnPlatform Default=default, GTK=gtk, macOS=macos, WPF=wpf}" />
+</ContentPage>
+""";
+
+		var code =
+"""
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Test;
+
+[XamlProcessing(XamlInflator.SourceGen)]
+public partial class TestPage : ContentPage
+{
+	public TestPage()
+	{
+		InitializeComponent();
+	}
+}
+""";
+
+		var (result, generated) = RunGenerator(xaml, code, targetFramework: "netstandard2.0");
+		Assert.False(result.Diagnostics.Any());
+
+		Assert.Contains("global::Microsoft.Maui.Controls.Xaml.OnPlatformExtension", generated, StringComparison.Ordinal);
+		Assert.Matches(@"GTK\s*=\s*""gtk""", generated);
+		Assert.Matches(@"macOS\s*=\s*""macos""", generated);
+		Assert.Matches(@"WPF\s*=\s*""wpf""", generated);
+		Assert.Contains("ProvideValue(xamlServiceProvider)", generated, StringComparison.Ordinal);
+	}
+
 	[Fact]
 	public void SimplifyOnPlatformElement()
 	{

src/Controls/tests/Xaml.UnitTests/MarkupExpressionParserTests.cs

@@ -288,6 +288,11 @@ public void BindingWithStaticConverter()
 		[InlineData("{OnPlatform Android=20, UWP=25}", "WinUI", 25)]
 		[InlineData("{OnPlatform Android=20, WinUI=25, UWP=20}", "WinUI", 25)]
 		[InlineData("{OnPlatform Android=20, UWP=25}", "UWP", 25)]
+		// GTK, macOS and WPF are now public, so custom/community backends can express
+		// platform-conditional values for them through the {OnPlatform} markup extension.
+		[InlineData("{OnPlatform Android=20, GTK=25}", "GTK", 25)]
+		[InlineData("{OnPlatform iOS=20, macOS=25}", "macOS", 25)]
+		[InlineData("{OnPlatform Android=20, WPF=25}", "WPF", 25)]
 		[InlineData("{OnPlatform 20}", "Android", 20)]
 		[InlineData("{OnPlatform 20}", "iOS", 20)]
 		[InlineData("{OnPlatform 20}", "Tizen", 20)]

src/Controls/tests/Xaml.UnitTests/OnPlatformTests.cs

@@ -137,6 +137,24 @@ public void ChecksPreferWinUI()
 			image = new Image().LoadFromXaml(xaml);
 			Assert.Equal("Images/icon_twitter_preferred.png", (image.Source as FileImageSource).File);
 		}
+
+		[Theory]
+		[InlineData("GTK", "gtk")]
+		[InlineData("macOS", "macos")]
+		[InlineData("WPF", "wpf")]
+		// Issue 35695: custom/community backends can express platform-conditional
+		// values through the OnPlatform markup-extension form.
+		public void MarkupExtensionResolvesCustomPlatform(string platform, string expected)
+		{
+			var xaml = @"
+			<Label xmlns=""http://schemas.microsoft.com/dotnet/2021/maui""
+				xmlns:x=""http://schemas.microsoft.com/winfx/2009/xaml""
+				Text=""{OnPlatform Default=default, GTK=gtk, macOS=macos, WPF=wpf}"" />";
+
+			mockDeviceInfo.Platform = DevicePlatform.Create(platform);
+			var label = new Label().LoadFromXaml(xaml);
+			Assert.Equal(expected, label.Text);
+		}
 	}
 
 	[Collection("Xaml Inflation")]