Feature switch for IVisual scanning

docs/design/FeatureSwitches.md

@@ -5,6 +5,7 @@ Certain features of MAUI can be enabled or disabled using feature switches. The
 | MSBuild Property Name | AppContext Setting | Description |
 |-|-|-|
 | MauiXamlRuntimeParsingSupport | Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported | When disabled, all XAML loading at runtime will throw an exception. This will affect usage of APIs such as the `LoadFromXaml` extension method. This feature can be safely turned off when all XAML resources are compiled using XamlC (see [XAML compilation](https://learn.microsoft.com/en-us/dotnet/maui/xaml/xamlc)). This feature is enabled by default for all configurations except for NativeAOT. |
+| MauiIVisualScanningSupport | Microsoft.Maui.RuntimeFeature.IsIVisualScanningSupported | When disabled, only visuals manually registered through `[assembly: Visual(...)]` will be available at runtime. |
 
 ## MauiXamlRuntimeParsingSupport
 

src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.targets

@@ -209,12 +209,17 @@
   <Target Name="_MauiPrepareForILLink" BeforeTargets="PrepareForILLink">
     <PropertyGroup>
       <MauiXamlRuntimeParsingSupport Condition="'$(MauiXamlRuntimeParsingSupport)' == '' and '$(PublishAot)' == 'true'">false</MauiXamlRuntimeParsingSupport>
+      <MauiIVisualScanningSupport Condition="'$(MauiIVisualScanningSupport)' == '' and '$(PublishAot)' == 'true'">false</MauiIVisualScanningSupport>
     </PropertyGroup>
     <ItemGroup>
       <RuntimeHostConfigurationOption Include="Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported"
                                       Condition="'$(MauiXamlRuntimeParsingSupport)' != ''"
                                       Value="$(MauiXamlRuntimeParsingSupport)"
                                       Trim="true" />
+      <RuntimeHostConfigurationOption Include="Microsoft.Maui.RuntimeFeature.IsIVisualScanningSupported"
+                                      Condition="'$(MauiIVisualScanningSupport)' != ''"
+                                      Value="$(MauiIVisualScanningSupport)"
+                                      Trim="true" />
     </ItemGroup>
   </Target>
 

src/Controls/src/Core/Visuals/VisualTypeConverter.cs

@@ -28,14 +28,11 @@ void InitMappings()
 			var mappings = new Dictionary<string, IVisual>(StringComparer.OrdinalIgnoreCase);
 			Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();
 
-			// Check for IVisual Types
-			foreach (var assembly in assemblies)
-				Register(assembly, mappings);
-
-			if (Internals.Registrar.ExtraAssemblies != null)
-				foreach (var assembly in Internals.Registrar.ExtraAssemblies)
-					Register(assembly, mappings);
-
+			if (RuntimeFeature.IsIVisualScanningSupported)
+			{
+				// Check for IVisual Types
+				ScanAssemblies(assemblies, mappings);
+			}
 
 			// Check for visual assembly attributes	after scanning for IVisual Types
 			// this will let users replace the default visual names if they want to
@@ -49,6 +46,19 @@ void InitMappings()
 			_visualTypeMappings = mappings;
 		}
 
+		[RequiresUnreferencedCode("The IVisual types might be removed by trimming. "
+			+ "Use the [assembly: Visual(\"key\", typeof(MyIVisual))] attribute to explicitly register the IVisual types instead.",
+			Url = "TODO")]
+		static void ScanAssemblies(Assembly[] assemblies, Dictionary<string, IVisual> mappings)
+		{
+			foreach (var assembly in assemblies)
+				Register(assembly, mappings);
+
+			if (Internals.Registrar.ExtraAssemblies != null)
+				foreach (var assembly in Internals.Registrar.ExtraAssemblies)
+					Register(assembly, mappings);
+		}
+
 		static void RegisterFromAttributes(Assembly assembly, Dictionary<string, IVisual> mappings)
 		{
 			object[] attributes = assembly.GetCustomAttributesSafe(typeof(VisualAttribute));

src/Core/src/ILLink.Substitutions.xml

@@ -3,6 +3,8 @@
     <type fullname="Microsoft.Maui.RuntimeFeature">
       <method signature="System.Boolean get_IsXamlRuntimeParsingSupported()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported" value="false" featurevalue="false" />
       <method signature="System.Boolean get_IsXamlRuntimeParsingSupported()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported" value="true" featurevalue="true" />
+      <method signature="System.Boolean get_IsIVisualScanningSupported()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsIVisualScanningSupported" value="false" featurevalue="false" />
+      <method signature="System.Boolean get_IsIVisualScanningSupported()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsIVisualScanningSupported" value="true" featurevalue="true" />
     </type>
   </assembly>
 </linker>

src/Core/src/RuntimeFeature.cs

@@ -15,10 +15,16 @@ namespace Microsoft.Maui
 	internal static class RuntimeFeature
 	{
 		private const bool IsXamlRuntimeParsingSupportedByDefault = true;
+		private const bool IsIVisualScanningSupportedByDefault = true;
 
 		internal static bool IsXamlRuntimeParsingSupported
 			=> AppContext.TryGetSwitch("Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported", out bool isEnabled)
 				? isEnabled
 				: IsXamlRuntimeParsingSupportedByDefault;
+
+		internal static bool IsIVisualScanningSupported =>
+			AppContext.TryGetSwitch("Microsoft.Maui.RuntimeFeature.IsIVisualScanningSupported", out bool isSupported)
+				? isSupported
+				: IsIVisualScanningSupportedByDefault;
 	}
 }