[XSG][BindingSourceGen] Add support for CommunityToolkit.Mvvm ObservablePropertyAttribute

[Copilot]

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It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description of Change

Binding source generators now support properties generated by CommunityToolkit.Mvvm's [ObservableProperty] attribute. Previously, bindings failed because MAUI generators cannot see properties generated by other source generators.

Implementation (following PR #32954 pattern for RelayCommand):

• BindingSourceGen/ITypeSymbolExtensions.cs: Added TryGetObservablePropertyType() to detect fields with [ObservableProperty] and infer property types. Supports all three documented field naming patterns: camelCase, _camelCase, and m_camelCase. Includes validation for empty property names.

• BindingSourceGen/PathParser.cs: Enhanced TryHandleSpecialCases() to handle ObservableProperty patterns as fallback when symbol resolution fails.

• BindingSourceGen/BindingSourceGenerator.cs: Updated GetLambdaReturnType() to infer types from ObservableProperty fields, enabling C# lambda bindings.

• SourceGen/ITypeSymbolExtensions.cs: Updated TryGetProperty() to check ObservableProperty-inferred properties, enabling XAML string-based bindings.

• Tests: Added comprehensive unit tests in BindingSourceGen.UnitTests (8 tests) to validate ObservableProperty binding support across all field naming patterns and scenarios. Tests verify the generated binding code by checking:

  • Handler patterns: new(static source => source, "PropertyName") - validates property path in generated code
  • Setter expressions: source.PropertyName = value; - validates property assignment

Field Naming Pattern Support:

All three CommunityToolkit.Mvvm documented naming conventions are supported:

• name → Name
• _name → Name
• m_name → Name

Works for both XAML and C# bindings:

public class MyViewModel
{
    [ObservableProperty]
    private string? name;  // Generates Name property
    
    [ObservableProperty]
    private ObservableCollection<Tag> _tags = new();  // Generates Tags property
    
    [ObservableProperty]
    private int m_count;  // Generates Count property
}

<Label Text="{Binding Name}" />
<ListView ItemsSource="{Binding Tags}" />

label.SetBinding(Label.TextProperty, static (MyViewModel vm) => vm.Name);
listView.SetBinding(ListView.ItemsSourceProperty, static (MyViewModel vm) => vm.Tags);

Issues Fixed

Fixes #32597

Original prompt

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This section details on the original issue you should resolve

<issue_title>[XSG][BindingSourceGen] Add support for CommunityToolkit.Mvvm ObservablePropertyAttribute source generator</issue_title>
<issue_description>Similar to #32597
Related to #32953

The CommunityToolkit.Mvvm package contains a source generator which generates bindable property bolierplate code based on the [ObservableProperty] attribute: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/observableproperty

Scenarios:

// applied to a field
[ObservableProperty]
private string? name;

// generated code:
/// <inheritdoc cref="_tags"/>
[global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator", "8.3.0.0")]
[global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
public global::System.Collections.ObjectModel.ObservableCollection<global::TestSampleContentBindings.Models.Tag> Tags
{
    get => _tags;
    [global::System.Diagnostics.CodeAnalysis.MemberNotNull("_tags")]
    set
    {
        if (!global::System.Collections.Generic.EqualityComparer<global::System.Collections.ObjectModel.ObservableCollection<global::TestSampleContentBindings.Models.Tag>>.Default.Equals(_tags, value))
        {
            OnTagsChanging(value);
            OnTagsChanging(default, value);
            OnPropertyChanging(global::CommunityToolkit.Mvvm.ComponentModel.__Internals.__KnownINotifyPropertyChangingArgs.Tags);
            _tags = value;
            OnTagsChanged(value);
            OnTagsChanged(default, value);
            OnPropertyChanged(global::CommunityToolkit.Mvvm.ComponentModel.__Internals.__KnownINotifyPropertyChangedArgs.Tags);
        }
    }
}

Our source generators cannot see properties generated by other source generators, so the binding compilation will fail.

Our sample content template uses this source generator and we are currently failing over to reflection-based bindings.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [XSG][BindingSourceGen] Add support for CommunityToolkit.Mvvm ObservablePropertyAttribute source generator #32955

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[simonrozsival]

/azp run

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Azure Pipelines could not run because the pipeline triggers exclude this branch/path.

[simonrozsival]

/azp run

[azure-pipelines]

Azure Pipelines could not run because the pipeline triggers exclude this branch/path.

[simonrozsival]

@copilot please add also a test case in Xaml.UnitTest

[Copilot]

@copilot please add also a test case in Xaml.UnitTest

Added XAML unit tests in commit 5a2eadd. The tests cover bindings to ObservableProperty-generated properties from camelCase fields (name → Name), underscore-prefixed fields (_title → Title), and collections (_tags → Tags). Tests validate all three XamlInflator modes (Runtime, XamlC, SourceGen).

[simonrozsival]

@copilot please address these comments

[Copilot]

@copilot please address these comments

All code review feedback addressed in commit bf6d334:

• Added empty propertyName check
• Added m_ prefix support
• Simplified GetAllFields method
• Improved test assertions to check getter/setter expressions
• Added test for m_ prefix

XAML unit tests remain reverted as they don't test the actual code without CommunityToolkit.Mvvm source generator.

[simonrozsival]

@copilot address these comments

[Copilot]

@copilot address these comments

Fixed test assertions to check handler patterns instead of method signatures in commit 80bcfb1. All tests passing.