[glennawatson]

Summary

RoutingState reports navigation through plain IObservable<T> streams of values, and routing no longer depends on DynamicData.

• NavigationStackChanged replaces NavigationChanges. It emits a read-only snapshot of the whole navigation stack after each change, oldest view model first, instead of a change set.
• CanNavigateBack is new. It emits whether the stack holds a view model to go back to, when you subscribe and whenever the answer changes. NavigateBack uses it to decide when it can execute.
• CurrentViewModel is typed IObservable<IRoutableViewModel?>. It already emitted null while the stack was empty; the type now says so.
• Every routing consumer follows the new streams. WhenNavigatedTo, WhenNavigatedToObservable, WhenNavigatingFromObservable, the MAUI RoutedViewHost stack sync and the UIKit RoutedViewHost no longer use change sets or CollectionChanged events.
• The ReactiveUI.Routing and ReactiveUI.Routing.Reactive packages are removed, with their tests, the change-set count benchmark and the DynamicData package version.
• The routing handbook examples and the README routing paragraph describe the new streams.

Why

Routing exposed a generic change-set type that consumers had to decode, and the DynamicData interop lived in separate packages that pulled System.Reactive back in.

• Views only ever needed the current view model, the stack, and whether they could go back. Each is now a stream of exactly that value.
• Removing DynamicData from routing in both flavours finishes the split that issue Adding ReactiveUI.Routing to a lean ReactiveUI app makes every observable.Subscribe(Action<T>) fail with CS0121 #4478 asked for.

Closes #4478

Breaking changes

Routing members and packages changed; each has a direct replacement.

• RoutingState.NavigationChanges is removed. Subscribe to RoutingState.NavigationStackChanged, which emits after each change only; read NavigationStack for the stack as it is when you subscribe.
• RoutingState.CurrentViewModel is now IObservable<IRoutableViewModel?>. Handle the null it emits for an empty stack.
• Code that counted stack changes to drive a back button can subscribe to RoutingState.CanNavigateBack.
• The ReactiveUI.Routing and ReactiveUI.Routing.Reactive packages are gone. NavigationChanged() becomes NavigationStackChanged; CountChanged() and HasCountChanged() become WhenCountChanged() and CountHasChanged() on ToReactiveChangeSet(); ActOnEveryObject on a DynamicData change set becomes ActOnEveryObject on the collection; ToDynamicDataChangeSet() has no replacement, so call DynamicData's ToObservableChangeSet() in your own code.

How this was verified

New tests cover every new and changed RoutingState member, the focus mixins' seeding, error and completion paths, and the MAUI stack resync on clear, in both the lean and System.Reactive flavours; a lean console app subscribes to a signal with a lambda and to all three routing streams without loading System.Reactive or DynamicData.

• The UIKit RoutedViewHost compiles only for Apple targets and has no automated test here.

Notes for the reviewer

Start with RoutingState and RoutableViewModelMixins; the project, package and benchmark removals are mechanical.

• CurrentViewModel and CanNavigateBack derive from the NavigationStackChanged property at subscribe time, so a subclass that replaces that stream drives all three.
• The focus mixins keep their previous rules: they react to changes in stack size, and they complete when the watched view model leaves the stack. With a snapshot, a view model that appears twice completes only when its last copy leaves.
• The UIKit host now tells a push from a reset by comparing stack sizes: a larger stack pushes the view for the new top, and an empty stack pops to the root.
• Core's general collection change-set APIs are unchanged.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.63%. Comparing base (76926d8) to head (ebf4646).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4497      +/-   ##
==========================================
+ Coverage   93.42%   93.63%   +0.20%     
==========================================
  Files         168      163       -5     
  Lines        6346     6268      -78     
  Branches      733      728       -5     
==========================================
- Hits         5929     5869      -60     
+ Misses        416      398      -18     
  Partials        1        1              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.