feat: add AOT-safe ViewLocator with source-generated view dispatch

[glennawatson]

Summary

This PR ports ReactiveUI's ViewLocator system into ReactiveUI.Binding, replacing the reflection-based MakeGenericType approach with a fully AOT-safe, source-generated alternative.

In ReactiveUI, DefaultViewLocator.ResolveView(object) uses typeof(IViewFor<>).MakeGenericType(vmType) to resolve views at runtime. This breaks under AOT compilation and trimming — a problem for RoutedViewHost and ViewModelViewHost across all platforms (WPF, MAUI, WinForms, Apple/iOS) where the view model type is only known at runtime (e.g., navigation stacks with IRoutableViewModel).

The source generator now scans for all IViewFor<T> implementations at compile time and generates a type-switch dispatch function. At runtime, DefaultViewLocator.ResolveView(object) calls this generated dispatch first — no reflection, no MakeGenericType, fully AOT and trimming safe. Views are resolved through a three-tier strategy:

1. Generated dispatch — the compile-time type-switch tries the service locator for each known IViewFor<T>, then falls back to direct construction if the view has an accessible parameterless constructor.
2. Explicit mappings — runtime Map<TVM, TView>() registrations via the fluent ConfigureViewLocator builder API.
3. Service locator fallback — the existing Splat-based resolution as a last resort.

The builder integration follows the same pattern as the rest of ReactiveUI.Binding — ConfigureViewLocator is available on both IReactiveUIBindingBuilder and as an IAppBuilder extension via BuilderMixins, supporting fluent configuration like:

builder
    .WithCoreServices()
    .ConfigureViewLocator(mappings => mappings.Map<MyViewModel, MyView>())
    .BuildApp();

Tests

• Add 7 snapshot tests for ViewLocatorDispatchGenerator covering single/multiple views, abstract exclusion, private/missing constructors, deduplication, and non-IViewFor classes.
• Add 31 runtime tests for DefaultViewLocator, ViewLocator, ViewMappingBuilder, ViewLocatorNotFoundException, and builder integration.
• Add TestExecutor infrastructure following ReactiveUI's executor pattern for clean test isolation.

[Copilot]

Pull request overview

This PR adds an AOT-safe ViewLocator system to ReactiveUI.Binding, replacing the reflection-based MakeGenericType approach with a source-generated type-switch dispatch. The implementation follows a three-tier resolution strategy: compile-time generated dispatch, explicit runtime Map<TVM, TView>() mappings, and Splat service locator fallback.

Changes:

• Adds DefaultViewLocator, ViewLocator, IViewLocator, ViewMappingBuilder, and ViewLocatorNotFoundException to the runtime library, with builder integration via ConfigureViewLocator on both IReactiveUIBindingBuilder and IAppBuilder extension mixins.
• Adds a new source generator pipeline (Pipeline C) in ViewLocatorDispatchGenerator that scans IViewFor<T> implementations at compile time and generates a type-switch dispatch method in ViewDispatch.g.cs.
• Adds comprehensive tests: 7 snapshot tests for the generator and 31+ runtime tests for DefaultViewLocator, ViewLocator, ViewMappingBuilder, exception handling, and builder integration, using the project's TestExecutor infrastructure.

Reviewed changes

Copilot reviewed 43 out of 43 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/ReactiveUI.Binding/View/DefaultViewLocator.cs	Core three-tier view resolution implementation
src/ReactiveUI.Binding/View/ViewLocator.cs	Static accessor for the current IViewLocator
src/ReactiveUI.Binding/View/ViewLocatorNotFoundException.cs	Custom exception for missing IViewLocator
src/ReactiveUI.Binding/View/ViewMappingBuilder.cs	Fluent builder for registering view-to-VM mappings
src/ReactiveUI.Binding/Interfaces/IViewLocator.cs	IViewLocator interface with generic and non-generic overloads
src/ReactiveUI.Binding/Builder/IReactiveUIBindingBuilder.cs	Adds ConfigureViewLocator to the builder interface
src/ReactiveUI.Binding/Builder/ReactiveUIBindingBuilder.cs	Implements ConfigureViewLocator and registers default IViewLocator in WithCoreServices
src/ReactiveUI.Binding/Mixins/BuilderMixins.cs	Adds ConfigureViewLocator extension for IAppBuilder
src/ReactiveUI.Binding.SourceGenerators/Generators/ViewLocatorDispatchGenerator.cs	Source generator that produces ViewDispatch.g.cs
src/ReactiveUI.Binding.SourceGenerators/Helpers/ViewRegistrationExtractor.cs	Extracts IViewFor implementations from syntax
src/ReactiveUI.Binding.SourceGenerators/Models/ViewRegistrationInfo.cs	POCO model for detected view registrations
src/ReactiveUI.Binding.SourceGenerators/Constants.cs	Adds IViewForGenericMetadataName constant
src/ReactiveUI.Binding.SourceGenerators/BindingGenerator.cs	Registers Pipeline C in the generator
src/tests/.../ViewLocatorDispatchGeneratorTests.cs	7 snapshot tests for the generator
src/tests/.../DefaultViewLocatorTests.cs	Runtime tests for DefaultViewLocator
src/tests/.../ViewLocatorTests.cs	Tests for ViewLocator static accessor
src/tests/.../ViewLocatorNotFoundExceptionTests.cs	Tests for exception constructors
src/tests/.../ViewMappingBuilderTests.cs	Tests for ViewMappingBuilder
src/tests/.../Builder/ReactiveUIBindingBuilderTests.cs	Tests for ConfigureViewLocator builder integration
src/tests/.../Builder/BuilderMixinsTests.cs	Tests for ConfigureViewLocator mixin
src/tests/.../TestExecutors/*	Test executor infrastructure for builder state isolation
src/tests/.../Helpers/TestHelper.cs	Adds VDG abbreviation for snapshot tests
src/tests/.../*.verified.cs	Verified snapshot files for generator output

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0281c27) to head (e0995ea).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #15    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files          121       125     +4     
  Lines         2014      2116   +102     
  Branches       375       395    +20     
==========================================
+ Hits          2014      2116   +102     

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