[Trimming] Disable assembly scanning for types implementing IVisual

docs/design/FeatureSwitches.md

@@ -5,9 +5,14 @@ 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. |
+| MauiEnableIVisualAssemblyScanning | Microsoft.Maui.RuntimeFeature.IsIVisualAssemblyScanningEnabled | When enabled, MAUI will scan assemblies for types implementing `IVisual` and for `[assembly: Visual(...)]` attributes and register these types. |
 
 ## MauiXamlRuntimeParsingSupport
 
 When this feature is disabled, the following APIs are affected:
 - [`LoadFromXaml` extension methods](https://learn.microsoft.com/en-us/dotnet/maui/xaml/runtime-load) will throw runtime exceptions.
 - [Disabling XAML compilation](https://learn.microsoft.com/en-us/dotnet/maui/xaml/xamlc#disable-xaml-compilation) using `[XamlCompilation(XamlCompilationOptions.Skip)]` on pages and controls or whole assemblies will cause runtime exceptions.
+
+## MauiEnableIVisualAssemblyScanning
+
+When this feature is not enabled, custom and third party `IVisual` types will not be automatically discovered and registerd.

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>
+      <MauiEnableIVisualAssemblyScanning Condition="'$(MauiEnableIVisualAssemblyScanning)' == ''">false</MauiEnableIVisualAssemblyScanning>
     </PropertyGroup>
     <ItemGroup>
       <RuntimeHostConfigurationOption Include="Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported"
                                       Condition="'$(MauiXamlRuntimeParsingSupport)' != ''"
                                       Value="$(MauiXamlRuntimeParsingSupport)"
                                       Trim="true" />
+      <RuntimeHostConfigurationOption Include="Microsoft.Maui.RuntimeFeature.IsIVisualAssemblyScanningEnabled"
+                                      Condition="'$(MauiEnableIVisualAssemblyScanning)' != ''"
+                                      Value="$(MauiEnableIVisualAssemblyScanning)"
+                                      Trim="true" />
     </ItemGroup>
   </Target>
 

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

@@ -26,6 +26,24 @@ public override bool CanConvertTo(ITypeDescriptorContext context, Type destinati
 		void InitMappings()
 		{
 			var mappings = new Dictionary<string, IVisual>(StringComparer.OrdinalIgnoreCase);
+
+			if (RuntimeFeature.IsIVisualAssemblyScanningEnabled)
+			{
+				ScanAllAssemblies(mappings);
+			}
+			else
+			{
+				Register(typeof(VisualMarker.MaterialVisual), mappings);
+				Register(typeof(VisualMarker.DefaultVisual), mappings);
+				Register(typeof(VisualMarker.MatchParentVisual), mappings);
+			}
+
+			_visualTypeMappings = mappings;
+		}
+
+		[RequiresUnreferencedCode("The IVisual types might be removed by trimming and automatic registration via assembly scanning may not work as expected.")]
+		void ScanAllAssemblies(Dictionary<string, IVisual> mappings)
+		{
 			Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();
 
 			// Check for IVisual Types
@@ -45,8 +63,6 @@ void InitMappings()
 			if (Internals.Registrar.ExtraAssemblies != null)
 				foreach (var assembly in Internals.Registrar.ExtraAssemblies)
 					RegisterFromAttributes(assembly, mappings);
-
-			_visualTypeMappings = mappings;
 		}
 
 		static void RegisterFromAttributes(Assembly assembly, Dictionary<string, IVisual> mappings)
@@ -138,6 +154,13 @@ public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo c
 				if (_visualTypeMappings.TryGetValue(strValue, out IVisual returnValue))
 					return returnValue;
 
+				if (!RuntimeFeature.IsIVisualAssemblyScanningEnabled)
+				{
+					Application.Current?.FindMauiContext()?.CreateLogger<IVisual>()?.LogWarning(
+						"Unable to find visual {key}. Automatic discovery of IVisual types is disabled. You can enabled it by setting the $(MauiEnableIVisualAssemblyScanning)=true MSBuild property. " +
+						"Note: automatic registration of IVisual types through assembly scanning is not trimming-compatible and it can lead to slower app startup.", strValue);
+				}
+
 				return VisualMarker.Default;
 			}
 

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_IsIVisualAssemblyScanningEnabled()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsIVisualAssemblyScanningEnabled" value="false" featurevalue="false" />
+      <method signature="System.Boolean get_IsIVisualAssemblyScanningEnabled()" body="stub" feature="Microsoft.Maui.RuntimeFeature.IsIVisualAssemblyScanningEnabled" 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 IsIVisualAssemblyScanningEnabledByDefault = false;
 
 		internal static bool IsXamlRuntimeParsingSupported
 			=> AppContext.TryGetSwitch("Microsoft.Maui.RuntimeFeature.IsXamlRuntimeParsingSupported", out bool isEnabled)
 				? isEnabled
 				: IsXamlRuntimeParsingSupportedByDefault;
+
+		internal static bool IsIVisualAssemblyScanningEnabled =>
+			AppContext.TryGetSwitch("Microsoft.Maui.RuntimeFeature.IsIVisualAssemblyScanningEnabled", out bool isEnabled)
+				? isEnabled
+				: IsIVisualAssemblyScanningEnabledByDefault;
 	}
 }

src/TestUtils/src/Microsoft.Maui.IntegrationTests/AppleTemplateTests.cs

@@ -49,6 +49,7 @@ public void RunOniOS(string id, string config, string framework, string runtimeI
 				buildProps.Add("PublishAotUsingRuntimePack=true"); // TODO: This parameter will become obsolete https://github.com/dotnet/runtime/issues/87060
 				buildProps.Add("_IsPublishing=true"); // using dotnet build with -p:_IsPublishing=true enables targeting simulators
 				buildProps.Add($"RuntimeIdentifier={runtimeIdentifier}");
+				buildProps.Add("IlcTreatWarningsAsErrors=false"); // TODO: Remove this once all warnings are fixed https://github.com/dotnet/maui/issues/19397
 			}
 
 			Assert.IsTrue(DotnetInternal.Build(projectFile, config, framework: $"{framework}-ios", properties: buildProps),

src/TestUtils/src/Microsoft.Maui.IntegrationTests/Utilities/BuildWarningsUtilities.cs

@@ -225,29 +225,6 @@ public static void AssertWarnings(this List<WarningsPerFile> actualWarnings, Lis
 				}
 			},
 			new WarningsPerFile
-			{
-				File = "src/Controls/src/Core/Visuals/VisualTypeConverter.cs",
-				WarningsPerCode = new List<WarningsPerCode>
-				{
-					new WarningsPerCode
-					{
-						Code = "IL2026",
-						Messages = new List<string>
-						{
-							"Microsoft.Maui.Controls.VisualTypeConverter.Register(Assembly,Dictionary`2<String,IVisual>): Using member 'System.Reflection.Assembly.GetExportedTypes()' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.",
-						}
-					},
-					new WarningsPerCode
-					{
-						Code = "IL2062",
-						Messages = new List<string>
-						{
-							"Microsoft.Maui.Controls.VisualTypeConverter.Register(Assembly,Dictionary`2<String,IVisual>): Value passed to parameter '' of method 'Microsoft.Maui.Controls.VisualTypeConverter.Register(Type,Dictionary`2<String,IVisual>)' can not be statically determined and may not meet 'DynamicallyAccessedMembersAttribute' requirements.",
-						}
-					},
-				}
-			},
-			new WarningsPerFile
 			{
 				File = "src/Core/src/Hosting/Internal/MauiFactory.cs",
 				WarningsPerCode = new List<WarningsPerCode>