Extend BindableProperty source gen to handle partial property initializers

src/CommunityToolkit.Maui.SourceGenerators.Internal.UnitTests/BindablePropertyAttributeSourceGeneratorTests/CommonUsageTests.cs

@@ -652,4 +652,74 @@ public partial class {{defaultTestClassName}}
 
 		await VerifySourceGeneratorAsync(source, expectedGenerated);
 	}
+
+
+	[Fact]
+	public async Task GenerateBindableProperty_WithInitializers_GeneratesCorrectCode()
+	{
+		const string source =
+			/* language=C#-test */
+			//lang=csharp
+			$$"""
+			using CommunityToolkit.Maui;
+			using Microsoft.Maui.Controls;
+			using System;
+
+			namespace {{defaultTestNamespace}};
+
+			public partial class {{defaultTestClassName}} : View
+			{
+			    [BindablePropertyAttribute]
+			    public partial string Text { get; set; } = "Initial Value";
+
+			    [BindablePropertyAttribute]
+			    public partial TimeSpan CustomDuration { get; set; } = TimeSpan.FromSeconds(30);
+			}
+			""";
+
+		const string expectedGenerated =
+			/* language=C#-test */
+			//lang=csharp
+			$$"""
+			// <auto-generated>
+			// See: CommunityToolkit.Maui.SourceGenerators.Internal.BindablePropertyAttributeSourceGenerator
+			#pragma warning disable
+			#nullable enable
+			namespace {{defaultTestNamespace}};
+			public partial class {{defaultTestClassName}}
+			{
+			    /// <summary>
+			    /// Backing BindableProperty for the <see cref = "Text"/> property.
+			    /// </summary>
+			    public static readonly global::Microsoft.Maui.Controls.BindableProperty TextProperty = global::Microsoft.Maui.Controls.BindableProperty.Create("Text", typeof(string), typeof({{defaultTestNamespace}}.{{defaultTestClassName}}), null, Microsoft.Maui.Controls.BindingMode.OneWay, null, null, null, null, __createDefaultText);
+			    bool __initializingText = false;
+			    static object __createDefaultText(global::Microsoft.Maui.Controls.BindableObject bindable)
+			    {
+			        (({{defaultTestClassName}})bindable).__initializingText = true;
+			        var defaultValue = (({{defaultTestClassName}})bindable).Text;
+			        (({{defaultTestClassName}})bindable).__initializingText = false;
+			        return defaultValue;
+			    }
+
+			    public partial string Text { get => __initializingText ? field : (string)GetValue(TextProperty); set => SetValue(TextProperty, field = value); }
+
+			    /// <summary>
+			    /// Backing BindableProperty for the <see cref = "CustomDuration"/> property.
+			    /// </summary>
+			    public static readonly global::Microsoft.Maui.Controls.BindableProperty CustomDurationProperty = global::Microsoft.Maui.Controls.BindableProperty.Create("CustomDuration", typeof(System.TimeSpan), typeof({{defaultTestNamespace}}.{{defaultTestClassName}}), null, Microsoft.Maui.Controls.BindingMode.OneWay, null, null, null, null, __createDefaultCustomDuration);
+			    bool __initializingCustomDuration = false;
+			    static object __createDefaultCustomDuration(global::Microsoft.Maui.Controls.BindableObject bindable)
+			    {
+			        (({{defaultTestClassName}})bindable).__initializingCustomDuration = true;
+			        var defaultValue = (({{defaultTestClassName}})bindable).CustomDuration;
+			        (({{defaultTestClassName}})bindable).__initializingCustomDuration = false;
+			        return defaultValue;
+			    }
+
+			    public partial System.TimeSpan CustomDuration { get => __initializingCustomDuration ? field : (System.TimeSpan)GetValue(CustomDurationProperty); set => SetValue(CustomDurationProperty, field = value); }
+			}
+			""";
+
+		await VerifySourceGeneratorAsync(source, expectedGenerated);
+	}
 }

src/CommunityToolkit.Maui.SourceGenerators.Internal.UnitTests/BindablePropertyModelTests.cs

@@ -29,7 +29,8 @@ public void BindablePropertyName_ReturnsCorrectPropertyName()
 			"null",
 			string.Empty,
 			true, // IsReadOnlyBindableProperty
-			string.Empty // SetterAccessibility
+			string.Empty, // SetterAccessibility
+			false
 		);
 
 		// Act
@@ -56,6 +57,7 @@ public void BindablePropertyModel_WithAllParameters_StoresCorrectValues()
 		const string coerceValueMethodName = "CoerceValue";
 		const string defaultValueCreatorMethodName = "CreateDefaultValue";
 		const string newKeywordText = "new ";
+		const bool hasInitializer = false;
 
 		// Act
 		var model = new BindablePropertyModel(
@@ -71,7 +73,8 @@ public void BindablePropertyModel_WithAllParameters_StoresCorrectValues()
 			defaultValueCreatorMethodName,
 			newKeywordText,
 			true, // IsReadOnlyBindableProperty
-			string.Empty // SetterAccessibility
+			string.Empty, // SetterAccessibility
+			hasInitializer
 		);
 
 		// Assert
@@ -86,6 +89,7 @@ public void BindablePropertyModel_WithAllParameters_StoresCorrectValues()
 		Assert.Equal(coerceValueMethodName, model.CoerceValueMethodName);
 		Assert.Equal(defaultValueCreatorMethodName, model.DefaultValueCreatorMethodName);
 		Assert.Equal(newKeywordText, model.NewKeywordText);
+		Assert.Equal(hasInitializer, model.HasInitializer);
 		Assert.Equal("TestPropertyProperty", model.BindablePropertyName);
 	}
 
@@ -128,7 +132,8 @@ public void SemanticValues_WithClassInfoAndProperties_StoresCorrectValues()
 			"null",
 			string.Empty,
 			true, // IsReadOnlyBindableProperty
-			string.Empty // SetterAccessibilityText
+			string.Empty, // SetterAccessibilityText
+			false
 		);
 
 		var bindableProperties = new[] { bindableProperty }.ToImmutableArray();
@@ -156,4 +161,56 @@ static Compilation CreateCompilation(string source)
 			references,
 			new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));
 	}
+
+	[Fact]
+	public void BindablePropertyModel_WithInitializer_GeneratesDefaultValueCreator()
+	{
+		// Arrange
+		var compilation = CreateCompilation("public class TestClass { public string TestProperty { get; set; } = \"Hello\"; }");
+		var typeSymbol = compilation.GetTypeByMetadataName("TestClass")!;
+		var propertySymbol = typeSymbol.GetMembers("TestProperty").OfType<IPropertySymbol>().First();
+
+		const string propertyName = "TestProperty";
+		const string defaultValue = "\"Hello\"";
+		const string defaultBindingMode = "Microsoft.Maui.Controls.BindingMode.TwoWay";
+		const string validateValueMethodName = "ValidateValue";
+		const string propertyChangedMethodName = "OnPropertyChanged";
+		const string propertyChangingMethodName = "OnPropertyChanging";
+		const string coerceValueMethodName = "CoerceValue";
+		const string defaultValueCreatorMethodName = "__createDefaultTestProperty";
+		const string newKeywordText = "new ";
+		const bool hasInitializer = true;
+
+		// Act
+		var model = new BindablePropertyModel(
+			propertyName,
+			propertySymbol.Type,
+			typeSymbol,
+			defaultValue,
+			defaultBindingMode,
+			validateValueMethodName,
+			propertyChangedMethodName,
+			propertyChangingMethodName,
+			coerceValueMethodName,
+			defaultValueCreatorMethodName,
+			newKeywordText,
+			true, // IsReadOnlyBindableProperty
+			string.Empty, // SetterAccessibility
+			hasInitializer);
+
+		// Assert
+		Assert.Equal(propertyName, model.PropertyName);
+		Assert.Equal(propertySymbol.Type, model.ReturnType);
+		Assert.Equal(typeSymbol, model.DeclaringType);
+		Assert.Equal(defaultValue, model.DefaultValue);
+		Assert.Equal(defaultBindingMode, model.DefaultBindingMode);
+		Assert.Equal(validateValueMethodName, model.ValidateValueMethodName);
+		Assert.Equal(propertyChangedMethodName, model.PropertyChangedMethodName);
+		Assert.Equal(propertyChangingMethodName, model.PropertyChangingMethodName);
+		Assert.Equal(coerceValueMethodName, model.CoerceValueMethodName);
+		Assert.Equal(defaultValueCreatorMethodName, model.DefaultValueCreatorMethodName);
+		Assert.Equal(newKeywordText, model.NewKeywordText);
+		Assert.Equal(hasInitializer, model.HasInitializer);
+		Assert.Equal("TestPropertyProperty", model.BindablePropertyName);
+	}
 }

src/CommunityToolkit.Maui.SourceGenerators.Internal/Generators/BindablePropertyAttributeSourceGenerator.cs

@@ -183,6 +183,12 @@ static string GenerateSource(SemanticValues value)
 				GenerateBindableProperty(sb, in info);
 			}
 
+			if (info.HasInitializer)
+			{
+				GenerateInitializingProperty(sb, in info);
+				GenerateDefaultValueMethod(sb, in info, classNameWithGenerics);
+			}
+
 			GenerateProperty(sb, in info);
 		}
 
@@ -236,7 +242,7 @@ static void GenerateReadOnlyBindableProperty(StringBuilder sb, in BindableProper
 			.Append(", ")
 			.Append(info.CoerceValueMethodName)
 			.Append(", ")
-			.Append(info.DefaultValueCreatorMethodName)
+			.Append(GetDefaulteValueCreatorMethod(in info))
 			.Append(");\n");
 
 		// Generate public BindableProperty from the key
@@ -292,7 +298,7 @@ static void GenerateBindableProperty(StringBuilder sb, in BindablePropertyModel
 			.Append(", ")
 			.Append(info.CoerceValueMethodName)
 			.Append(", ")
-			.Append(info.DefaultValueCreatorMethodName)
+			.Append(GetDefaulteValueCreatorMethod(in info))
 			.Append(");\n");
 
 		sb.Append('\n');
@@ -310,18 +316,42 @@ static void GenerateProperty(StringBuilder sb, in BindablePropertyModel info)
 			.Append(formattedReturnType)
 			.Append(' ')
 			.Append(sanitizedPropertyName)
-			.Append("\n{\nget => (")
-			.Append(formattedReturnType)
-			.Append(")GetValue(")
-			.Append(info.BindablePropertyName)
-			.Append(");\n");
+			.Append("\n{\nget => ");
+
+		if (info.HasInitializer)
+		{
+			sb.Append("__initializing")
+				.Append(info.PropertyName)
+				.Append(" ? field : ")
+				.Append("(")
+				.Append(formattedReturnType)
+				.Append(")GetValue(")
+				.Append(info.BindablePropertyName)
+				.Append(");\n");
+		}
+		else
+		{
+			sb.Append("(")
+				.Append(formattedReturnType)
+				.Append(")GetValue(")
+				.Append(info.BindablePropertyName)
+				.Append(");\n");
+		}
 
 		if (info.SetterAccessibility is not null)
 		{
 			sb.Append(info.SetterAccessibility)
 				.Append("set => SetValue(")
-				.Append(info.IsReadOnlyBindableProperty ? info.BindablePropertyKeyName : info.BindablePropertyName)
-				.Append(", value);\n");
+				.Append(info.IsReadOnlyBindableProperty ? info.BindablePropertyKeyName : info.BindablePropertyName);
+
+			if (info.HasInitializer)
+			{
+				sb.Append(", field = value);\n");
+			}
+			else
+			{
+				sb.Append(", value);\n");
+			}
 		}
 		// else Do not create a Setter because the property is read-only
 
@@ -333,6 +363,7 @@ static SemanticValues SemanticTransform(GeneratorAttributeSyntaxContext context,
 		var propertyDeclarationSyntax = Unsafe.As<PropertyDeclarationSyntax>(context.TargetNode);
 		var semanticModel = context.SemanticModel;
 		var propertySymbol = (IPropertySymbol?)ModelExtensions.GetDeclaredSymbol(semanticModel, propertyDeclarationSyntax, cancellationToken);
+		var hasInitializer = propertyDeclarationSyntax.Initializer is not null;
 
 		if (propertySymbol is null)
 		{
@@ -359,7 +390,7 @@ static SemanticValues SemanticTransform(GeneratorAttributeSyntaxContext context,
 		var (isReadOnlyBindableProperty, setterAccessibility) = GetPropertyAccessibility(propertySymbol, propertyDeclarationSyntax);
 
 		var attributeData = context.Attributes[0];
-		bindablePropertyModels[0] = CreateBindablePropertyModel(attributeData, propertySymbol.ContainingType, propertySymbol.Name, returnType, doesContainNewKeyword, isReadOnlyBindableProperty, setterAccessibility);
+		bindablePropertyModels[0] = CreateBindablePropertyModel(attributeData, propertySymbol.ContainingType, propertySymbol.Name, returnType, doesContainNewKeyword, isReadOnlyBindableProperty, setterAccessibility, hasInitializer);
 
 		return new(propertyInfo, ImmutableArray.Create(bindablePropertyModels));
 	}
@@ -456,7 +487,7 @@ static string GetGenericTypeParameters(INamedTypeSymbol typeSymbol)
 		return sb.ToString();
 	}
 
-	static BindablePropertyModel CreateBindablePropertyModel(in AttributeData attributeData, in INamedTypeSymbol declaringType, in string propertyName, in ITypeSymbol returnType, in bool doesContainNewKeyword, in bool isReadOnly, in string? setterAccessibility)
+	static BindablePropertyModel CreateBindablePropertyModel(in AttributeData attributeData, in INamedTypeSymbol declaringType, in string propertyName, in ITypeSymbol returnType, in bool doesContainNewKeyword, in bool isReadOnly, in string? setterAccessibility, in bool hasInitializer)
 	{
 		if (attributeData.AttributeClass is null)
 		{
@@ -472,7 +503,7 @@ static BindablePropertyModel CreateBindablePropertyModel(in AttributeData attrib
 		var validateValueMethodName = attributeData.GetNamedMethodGroupArgumentsAttributeValueByNameAsString(nameof(BindablePropertyModel.ValidateValueMethodName));
 		var newKeywordText = doesContainNewKeyword ? "new " : string.Empty;
 
-		return new BindablePropertyModel(propertyName, returnType, declaringType, defaultValue, defaultBindingMode, validateValueMethodName, propertyChangedMethodName, propertyChangingMethodName, coerceValueMethodName, defaultValueCreatorMethodName, newKeywordText, isReadOnly, setterAccessibility);
+		return new BindablePropertyModel(propertyName, returnType, declaringType, defaultValue, defaultBindingMode, validateValueMethodName, propertyChangedMethodName, propertyChangingMethodName, coerceValueMethodName, defaultValueCreatorMethodName, newKeywordText, isReadOnly, setterAccessibility, hasInitializer);
 	}
 
 	[MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -529,4 +560,72 @@ static string GetFormattedReturnType(ITypeSymbol typeSymbol)
 			return typeSymbol.ToDisplayString(SymbolDisplayFormat.CSharpErrorMessageFormat);
 		}
 	}
+
+	/// <summary>
+	/// Determines the name of the method used to create the default value for the specified bindable property model.
+	/// </summary>
+	/// <param name="info">The bindable property model for which to retrieve the default value creator method name.</param>
+	/// <returns>A string containing the name of the method that creates the default value for the property.
+	/// If the property has an initializer, the returned name is of the generated default value method;
+	/// otherwise, the default value creator method name from the model is returned.</returns>
+	static string GetDefaulteValueCreatorMethod(in BindablePropertyModel info)
+	{
+		if (info.HasInitializer)
+		{
+			return "__createDefault" + info.PropertyName;
+		}
+
+		return info.DefaultValueCreatorMethodName;
+	}
+
+	/// <summary>
+	/// Generates the boolean initialization flag used by bindable properties with initializers to indicate that the getter
+	/// should return the backing field while the generated default value method is executing.
+	/// </summary>
+	/// <param name="sb">The StringBuilder instance to which the initialization field declaration will be appended.</param>
+	/// <param name="info">The model containing metadata about the property for which the initialization field is generated.</param>
+	static void GenerateInitializingProperty(StringBuilder sb, in BindablePropertyModel info)
+	{
+		sb.Append("bool ")
+			.Append("__initializing")
+			.Append(info.PropertyName)
+			.Append(" = false;\n");
+	}
+
+	/// <summary>
+	/// Generates the default value creator static method used by BindableProperty instances with initializers.
+	/// This method temporarily switches the property's getter to return its backing field while the default value is being computed,
+	/// ensuring that the initializer-provided value is captured and returned as the BindableProperty's default.
+	/// </summary>
+	/// <param name="sb">The StringBuilder instance to which the initialization field declaration will be appended.</param>
+	/// <param name="info">The model containing metadata for the property that requires a default value creator.</param>
+	/// <param name="classNameWithGenerics">The declaring class name including generic type parameters, if any.</param>
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	static void GenerateDefaultValueMethod(StringBuilder sb, in BindablePropertyModel info, string classNameWithGenerics)
+	{
+		var sanitizedPropertyName = IsDotnetKeyword(info.PropertyName) ? string.Concat("@", info.PropertyName) : info.PropertyName;
+
+		sb.Append("static object ")
+			.Append(GetDefaulteValueCreatorMethod(in info))
+			.Append("(global::Microsoft.Maui.Controls.BindableObject bindable)\n")
+			.Append("{\n")
+			.Append("((")
+			.Append(classNameWithGenerics)
+			.Append(")bindable).__initializing")
+			.Append(info.PropertyName)
+			.Append(" = true;\n")
+			.Append("var defaultValue = ")
+			.Append("((")
+			.Append(classNameWithGenerics)
+			.Append(")bindable).")
+			.Append(sanitizedPropertyName)
+			.Append(";\n")
+			.Append("((")
+			.Append(classNameWithGenerics)
+			.Append(")bindable).__initializing")
+			.Append(info.PropertyName)
+			.Append(" = false;\n")
+			.Append("return defaultValue;\n")
+			.Append("}\n");
+	}
 }

src/CommunityToolkit.Maui.SourceGenerators.Internal/Models/Records.cs

@@ -3,7 +3,7 @@
 
 namespace CommunityToolkit.Maui.SourceGenerators.Internal.Models;
 
-record BindablePropertyModel(string PropertyName, ITypeSymbol ReturnType, ITypeSymbol DeclaringType, string DefaultValue, string DefaultBindingMode, string ValidateValueMethodName, string PropertyChangedMethodName, string PropertyChangingMethodName, string CoerceValueMethodName, string DefaultValueCreatorMethodName, string NewKeywordText, bool IsReadOnlyBindableProperty, string? SetterAccessibility)
+record BindablePropertyModel(string PropertyName, ITypeSymbol ReturnType, ITypeSymbol DeclaringType, string DefaultValue, string DefaultBindingMode, string ValidateValueMethodName, string PropertyChangedMethodName, string PropertyChangingMethodName, string CoerceValueMethodName, string DefaultValueCreatorMethodName, string NewKeywordText, bool IsReadOnlyBindableProperty, string? SetterAccessibility, bool HasInitializer)
 {
 	public string BindablePropertyName => $"{PropertyName}Property";
 	public string BindablePropertyKeyName => $"{char.ToLower(PropertyName[0])}{PropertyName[1..]}PropertyKey";