fix(hottesting): Create takes the services a view model reads while it is built - #3285

GitHub Issue (If applicable): closes #3282

PR Type

• Bugfix

What is the current behavior?

{Vm}Mock.Create null-injects every constructor parameter except a messenger. A model that reads a service while its view model is built throws NullReferenceException in Create, before any mock is applied:

• a feed written as a method group, ListFeed.Async(Service.GetItems): the generated view-model constructor builds every feed, and binding the method group reads Service;
• a constructor that dereferences a service.

What is the new behavior?

• Create takes such a parameter from its caller: Create(service) and Create(service, mock). This is spec 013's R1, which was planned but not built.
• The analysis counts a parameter read outside a lambda in a feed body as eager, as it already did in the constructor body:
  • this includes a feed set in a property initializer;
  • a receiver that is the member a parameter feeds (a positional record's property, a field assigned in the constructor, an expression-bodied one included) counts as that parameter;
  • this., parentheses, ! and casts are seen through.
• The mocking generator reads [CtorDependency(Eager = true)], hand-declared ones included, and on the source path also runs the analysis.
• A model with no such parameter keeps Create() and Create(mock), emitted exactly as before.
• A messenger keeps its stopgap and still gets a fresh WeakReferenceMessenger.

Reviewer's guide

• Start here: FeedDependencyAnalysis.FindEagerCtorParameters and DereferencedName, then FeedsMockGenerator.Generate and IsTakenFromCaller.
• MVUX generator: one line. Through the shared analysis it now also emits [CtorDependency(Eager = true)] for a parameter a feed body reads eagerly.
• Tests that prove it: the first commit, 62e716da0, adds tests only.
  • Given_GeneratedMock.When_FeedIsAMethodGroup_Then_CreateTakesTheServiceAndInputFlows (fixture ShelfModel): on main the test project does not compile, because Create(service, mock) does not exist.
  • Given_FeedsMockGenerator: a method-group feed, an eager [CtorDependency], and parameters named mock, vm and event fail on main. A lambda feed, whose service stays null-injected, passes on both.
  • When_ModelCarriesNoMetadata_Then_MockIsGeneratedFromSource expects a different call: its feed, Service.Items, reads the service while the view model is built.
• Review fixes: 424599cd2, with its tests. These are the other spellings (this., !, a cast, a property initializer), a field assigned in an expression-bodied constructor, and a hand-declared attribute on the source path.
• Docs commits: 963277482 and 4f860ed59: testing.md, rules.md and spec 013 (spec §8, architecture, implementation, history v20).
• Unrelated, one line: 40e7e12aa replaces a hard tab in spec 026 that came from main (fix(reactive): MVUX hot reload of a command's parameters crashes or loses list updates #3296), so the docs validation passes.

PR Checklist

• Tested code with current supported SDKs: unit tests only (Uno.HotTesting.Reactive.Tests 51 pass; Uno.Extensions.Reactive.Tests 1654 pass, 19 skipped; Uno.Extensions.Core.Tests 61 pass). UI and runtime tests were not run locally.
• Docs have been added/updated.
• Unit Tests for the changes have been added: 8 new (11 cases), 1 updated.
• Contains NO breaking changes: see below.
• Associated with an issue.

Breaking change: a model whose view model reads a service while it is built loses Create() and Create(mock), which threw anyway, and gets Create(service) and Create(service, mock) instead. Callers pass a service, often a fake. The mocked feeds never call it. Because the change is breaking, it should not be backported to release/stable/8.0.

Other information

• On the metadata path (a test project referencing the app), the change reaches a model once the app is rebuilt with this version of the MVUX generator.
• Not walked: constructor initializers (: this(...)) and the initializers of other members. A hand-declared [CtorDependency(Eager = true)] covers them.
• Spec 013's history entry is numbered v20, as in fix(hottesting): mock a list with a selection #3247 and fix(hottesting): a feed that reads a service is a required input, whatever else it reads #3287. Whichever merges later renumbers.
• Not done here: turning the messenger stopgap into an optional IMessenger argument.

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