InteractionBinder: immediately unregister interaction handlers when ViewModel becomes null

[glennawatson]

What’s fixed

When a view’s ViewModel is set to null, any previously bound Interaction<TInput,TOutput> handlers are now unregistered immediately. This restores the expected behavior where calling Interaction.Handle(...) after clearing the ViewModel throws an UnhandledInteractionException<,>.

Symptoms

• After view.ViewModel = null;, invoking vm.Interaction1.Handle("123") still hits the old handler instead of throwing.

• NUnit tests like:

  Assert.That(async () => await vm.Interaction1.Handle("123").ToTask(),
              Throws.TypeOf<UnhandledInteractionException<string,bool>>());

  fail with “But was: no exception thrown”.

Before vs After

Before

• InteractionBinderImplementation only reacted to non-null ViewModel transitions.
• If ViewModel became null, the binding stream didn’t emit a value to trigger disposal, so the previously registered handler could remain attached.

After

• We explicitly merge a “VM became null” stream into the binding sequence:

  • When ViewModel is null, we emit a default(IInteraction<,>) value.
  • The binder disposes the current handler (SerialDisposable) on that emission.

• Result: as soon as the ViewModel is cleared, handlers are unregistered and subsequent Handle(...) calls correctly surface UnhandledInteractionException<,>.

Technical summary

• Added:

  var vmNulls = view.WhenAnyValue(x => x.ViewModel)
                    .Where(x => x is null)
                    .Select(_ => default(IInteraction<TInput, TOutput>));
  var source = Reflection.ViewModelWhenAnyValue(viewModel, view, vmExpression)
      .Cast<IInteraction<TInput, TOutput>?>()
      .Merge(vmNulls);

• On each emission we set interactionDisposable.Disposable to either the new handler registration or Disposable.Empty when null is seen, which disposes the prior registration.

• Implemented in both overloads (Task and IObservable<TDontCare> handlers).

Impact

• Restores the contract many apps rely on: no handler after ViewModel = null.
• Aligns runtime behavior with test expectations in NUnit (no timing hacks/sleeps needed).
• No API changes; behavior is more predictable and correct.

How to verify

1. Bind an interaction handler.

2. Set ViewModel to null.

3. Call Interaction.Handle(...).

   • Expected: UnhandledInteractionException<,> is thrown.

4. Run the test suite. Previously failing tests like:

   • UnregisterTaskHandlerWhenViewModelIsSetToNull
   • UnregisterObservableHandlerWhenViewModelIsSetToNull
     should pass.

Compatibility & risk

• Low risk: only affects the unbinding path when ViewModel to null.
• If any app previously depended on handlers remaining active after clearing the ViewModel (unlikely and contrary to docs/intent), behavior will now be corrected.

[Copilot]

Pull Request Overview

Fixes immediate unregistration of Interaction<TInput,TOutput> handlers when a view’s ViewModel becomes null so subsequent Handle(...) calls correctly throw UnhandledInteractionException<,>.

• Emit a null interaction when ViewModel transitions to null and merge it into the binding stream.
• Replace WhereNotNull() with logic that disposes the current registration on null emissions via SerialDisposable.
• Adjust test parallelization attributes and csproj condition for WinUI.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/ReactiveUI/Bindings/Interaction/InteractionBinderImplementation.cs	Merge a VM-null stream into binding source and dispose handler on null to unregister immediately.
src/ReactiveUI.WinUI/ReactiveUI.WinUI.csproj	Fix ItemGroup condition to only include CsWinRT on Windows targets.
src/ReactiveUI.Builder.Tests/ReactiveUIBuilderCoreTests.cs	Mark test fixture NonParallelizable.
src/ReactiveUI.Builder.Tests/ReactiveUIBuilderBlockingTests.cs	Mark test fixture NonParallelizable.
src/ReactiveUI.Builder.Tests/AssemblyInfo.Parallel.cs	Reduce assembly test parallelism to 1.
src/ReactiveUI.Builder.Maui.Tests/AssemblyInfo.Parallel.cs	Reduce assembly test parallelism to 1.

Comments suppressed due to low confidence (1)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.16%. Comparing base (d75f999) to head (6f5398f).
⚠️ Report is 273 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4140       +/-   ##
===========================================
- Coverage   59.03%   37.16%   -21.88%     
===========================================
  Files         160      138       -22     
  Lines        5847     5686      -161     
  Branches     1031      852      -179     
===========================================
- Hits         3452     2113     -1339     
- Misses       2007     3357     +1350     
+ Partials      388      216      -172     

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