feat: add BindCommand and BindInteraction source-generated bindings

[glennawatson]

What's Changed

This PR adds compile-time source generation for BindCommand and BindInteraction, completing the core ReactiveUI binding API surface. Both APIs follow the same CallerFilePath/CallerLineNumber dispatch pattern used by the existing WhenChanged, WhenAnyValue, and Bind APIs.

BindCommand

Wire an ICommand on your ViewModel to a control with a single call — the source generator handles event subscription, command property assignment, and parameter binding at compile time.

// Basic — subscribes to the control's default event (e.g. Click) and invokes the command
this.BindCommand(ViewModel, vm => vm.SaveCommand, v => v.SaveButton)
    .DisposeWith(disposables);

// Custom event — bind to a specific event instead of the default
this.BindCommand(ViewModel, vm => vm.SaveCommand, v => v.SaveButton, "MouseDown")
    .DisposeWith(disposables);

// With command parameter from a property expression
this.BindCommand(ViewModel, vm => vm.DeleteCommand, v => v.DeleteButton, vm => vm.SelectedItem)
    .DisposeWith(disposables);

// With command parameter from an observable
this.BindCommand(ViewModel, vm => vm.DeleteCommand, v => v.DeleteButton, SelectedItems.CountChanged)
    .DisposeWith(disposables);

How it works under the hood:

The generator inspects the target control type at compile time and selects the best binding strategy:

1. Command property binding — If the control has a settable ICommand Command property (WPF ButtonBase, MAUI Button, etc.), the generator assigns the command directly and optionally sets CommandParameter. This is the most efficient path.

2. Event + Enabled binding — If the control has a default event (like Click) and a settable bool Enabled property (WinForms Button, etc.), the generator subscribes to the event, invokes ICommand.Execute(), and synchronizes Enabled with ICommand.CanExecute().

3. Event-only binding — If the control has a default event but no Enabled property, the generator subscribes to the event and invokes the command.

4. Runtime fallback — If no event or command property is detected at compile time (custom controls, third-party libraries), the generated code delegates to CommandBinderService, which resolves an ICreatesCommandBinding plugin at runtime. This preserves compatibility with platform-specific controls that the generator can't introspect.

BindInteraction

Type-safe interaction handler registration, generated at compile time:

// Register a handler using a Task-returning delegate
this.BindInteraction(ViewModel, vm => vm.ConfirmDelete, async context =>
{
    var result = await ShowDialog("Are you sure?");
    context.SetOutput(result);
})
.DisposeWith(disposables);

// Register a handler using an Observable-returning delegate
this.BindInteraction(ViewModel, vm => vm.ShowError, context =>
{
    ShowNotification(context.Input);
    context.SetOutput(Unit.Default);
    return Observable.Return(Unit.Default);
})
.DisposeWith(disposables);

The generator resolves the Interaction<TInput, TOutput> property type at compile time and wires the handler with full generic type safety — no runtime type casting.

Runtime Library Additions

• Interaction<TInput, TOutput> — the interaction type with Handle() method and handler registration
• InteractionContext<TInput, TOutput> — carries input to handlers, collects output via SetOutput()
• ICreatesCommandBinding — plugin interface for runtime command binding resolution
• CommandBinderService — resolves the best ICreatesCommandBinding for a given control type
• SerialDisposable — replaces previous subscription on re-binding (used in generated code)

---

Source Generator Internals

Restructured Helpers

Broke apart the monolithic MetadataExtractor into focused, single-responsibility helpers under Helpers/:

Helper	Responsibility
ObservationExtractor	WhenChanged, WhenChanging, WhenAnyValue, WhenAny extraction
WhenAnyObservableExtractor	WhenAnyObservable extraction
BindingExtractor	Bind, BindOneWay, BindTwoWay, OneWayBind extraction
CommandExtractor	BindCommand extraction with control introspection
InteractionExtractor	BindInteraction extraction
TypeDetectionExtractor	Pipeline A type detection (INPC, ReactiveObject, etc.)
ExtractorValidation	Shared guard clauses and validation methods
SyntaxHelpers	Lambda parsing and property path extraction
SymbolHelpers	Type introspection utilities
EventHelpers	Event/delegate type resolution
InvalidOperationExceptionHelper	Defensive guards for impossible null states

New Code Generators

• BindCommandCodeGenerator — generates command binding dispatch with plugin selection
• BindInteractionCodeGenerator — generates interaction handler wiring

Observation generators moved

Moved platform-specific observation generators to Generators/Observation/ subfolder for clearer organization.

Eliminated phantom IL branches

Extracted inline pattern matching from invocation generator predicates into named RoslynHelpers methods, eliminating 18 phantom uncovered branches that were artifacts of the C# compiler's IL generation.

---

How We Tested

• 100% line, branch, and method coverage across all 4 production assemblies (5585 lines, 2108 branches, 677 methods)
• 6981 tests passing across net8.0, net9.0, and net10.0
• Added NSubstitute dependency for mocking Roslyn interfaces (IMethodSymbol, IParameterSymbol, INamedTypeSymbol) in unit tests
• New unit test classes for extracted helpers: ExtractorValidationTests, CommandExtractorHelperTests, SymbolHelpersTests, SyntaxHelpersTests, EventHelpersTests, TypeDetectionExtractorTests, RoslynHelpersTests, AnalyzerHelpersTests
• Snapshot tests for all BindCommand scenarios (basic, custom event, command property, expression/observable parameter, deep paths, Enabled sync, CFP fallback)
• Snapshot tests for all BindInteraction scenarios (task handler, observable handler, deep paths, non-INPC viewmodel, CFP fallback)
• Runtime execution tests verifying generated code compiles and executes correctly

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8bb90f7) to head (c5c0498).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #13      +/-   ##
===========================================
+ Coverage   97.82%   100.00%   +2.17%     
===========================================
  Files         164       118      -46     
  Lines        5556      1949    -3607     
  Branches      995       364     -631     
===========================================
- Hits         5435      1949    -3486     
+ Misses         30         0      -30     
+ Partials       91         0      -91     

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[github-actions]

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