Runic.Navigation 0.8.0-preview.3

Runic.Navigation

Typed, host-neutral navigation for .NET ViewModels. RunicNavigator owns navigation regions with stable entry ids, awaited departure guards, initialize-once and resume-on-return hooks, navigated-to and navigated-from notifications, results from an entry, and owned or borrowed content. IRunicModelContext serializes the ViewModel changes that navigation commits. The types are in the Runic.Navigation namespace.

The package has no build targets, generator, Node or UI host, and it supports trimming and NativeAOT. Its only dependencies are Microsoft.Extensions.DependencyInjection.Abstractions and Microsoft.Extensions.Logging.Abstractions. Runic.Application.Views depends on it and presents regions in web Views.

Install

dotnet add package Runic.Navigation --prerelease

Runic.Application.Views apps already have it through that package.

One engine, idiomatic integrations

RunicNavigator is the navigation implementation for every consumer. Guards, history, outcomes, cancellation and entry ownership have one set of semantics. Presentation hosts and MVVM integrations are independent choices:

An integration can optimize its framework's API without duplicating navigation behavior. Replacing WPF Views with web Views can retain the navigation engine and ViewModels. Keep native UI activation and dispatcher concerns in the presentation integration; retained entry lifetime belongs to navigation.

The Toolkit WPF example and ReactiveUI recipes show each framework's native command style.

Navigator

RunicNavigator owns typed navigation regions. Each region gives its entries stable ids, and it supports awaited departure guards, initialize-once and resume-on-return hooks, and owned or borrowed content. The navigator replaces ReactiveUI's RoutingState; it does not wrap one. See the ReactiveUI adapter for observables and a back command. WPF apps present regions with Runic.Navigation.Wpf.

using Runic.Navigation;

services.AddRunicNavigation(); // one model context and navigator per window scope

var main = navigator.CreateRegion<IMainViewModel>(this, NavigationTarget.Borrow<IMainViewModel>(home));
// The navigator constructs the document from its services, and owns it.
var result = await main.PushAsync<DocumentViewModel>();
if (result is NavigationResult<IMainViewModel>.Committed) { /* main.Current is the document */ }
await main.BackAsync(); // the document retires and is disposed; home resumes

AddRunicNavigation() registers a scoped IRunicModelContext and a scoped RunicNavigator that uses it. Dispose window scopes with DisposeAsync (CreateAsyncScope). The navigator is also IDisposable for containers that only dispose synchronously: Dispose starts the same shutdown and returns without waiting for it. Without DI, construct the navigator with RunicNavigatorOptions (ModelContext, Services, CreateEntryScopes, LoggerFactory, TimeProvider, CloseTimeout), and use the same model context for the window's session.

The root page

Give a region its root page once, where that page exists. When the region's owner has it, pass it as the initial target of CreateRegion, as above. When the root page itself navigates the region, for example a list that pushes its detail page, it depends on the service that creates the region, so that service creates the region empty and the window's ViewModel starts it:

public sealed class AppNavigation // registered per window scope
{
    public AppNavigation(RunicNavigator navigator) => Main = navigator.CreateRegion<IPage>(this);

    public NavigationRegion<IPage> Main { get; }
}

public sealed class ListViewModel(AppNavigation navigation) : IPage { /* pushes into navigation.Main */ }

public sealed class ShellViewModel
{
    public ShellViewModel(AppNavigation navigation, ListViewModel list)
    {
        Main = navigation.Main;
        Main.Start(NavigationTarget.Borrow<IPage>(list)); // or NavigationTarget.Create<ListViewModel>()
    }

    public NavigationRegion<IPage> Main { get; }
}

Start(target) takes the same path as an initial target: it commits synchronously, and the entry's StateChanged and OnNavigatedTo(Reset), then the region's PropertyChanged, run on the caller's thread before it returns. So a window opened through IRunicWindowService shows its root page as soon as OpenWindowAsync returns, with no startup hook. Unlike a navigation request, Start throws: an exception from the target's factory or constructor propagates from the ViewModel's constructor, the window's scope is released, and OpenWindowAsync fails. Start throws InvalidOperationException when the region already has its initial entry (a second Start, or a region created with an initial target) or already received any navigation request; use ResetAsync from then on. Don't start a region with a discarded _ = Main.ResetAsync(...): a failure would only be logged (event 1061), and the window would show an empty region until a later turn commits it.

With CreateEntryScopes, a Create target resolves scoped services from its own entry scope, so Create<ListViewModel>() would receive a second AppNavigation. Start such a region with Borrow, as above.

Operations and results

PushAsync, ReplaceAsync and ResetAsync take a target, or a ViewModel type that the navigator constructs: PushAsync<DocumentViewModel>(), or PushAsync<DocumentViewModel, NoteId>(id) for a ViewModel initialized with input. PushForResult<ConfirmViewModel, bool>() does the same for a result. See Container-resolved targets.

• Every operation returns a NavigationResult<TContent>. The outcomes are returned, never thrown:

  • Committed, with the new current entry and the retired entries.
  • Rejected, with a Reason: Guard, Cancelled, Closed, Reentrant, NoHistory, NotCurrent or EntryNotFound.
  • Failed, with the Phase (Guarding, Preparing or Committing) and the exception (Error).
  • Superseded.

  Operations throw only for argument and ownership errors.

  IsCommitted tells code after the request whether to continue, without a pattern match on the content type. ThrowIfFailed() rethrows a Failed outcome's Error with its original stack trace and returns the result otherwise, so a caller that treats failures as bugs can write if ((await main.PushAsync<EditorViewModel>()).ThrowIfFailed().IsCommitted) { ... }.

• result.Outcome is a NavigationOutcome: the Kind (Committed, Rejected, Superseded or Failed) and, for a rejection, its Rejection reason. It holds no .NET content or exception, so a ViewModel can return it to a Runic View, which cannot receive a NavigationResult<TContent>.

• NavigationRequestOptions.ExpectedCurrent makes a request conditional. It is rejected as NotCurrent if that entry is not current at admission or at commit. Use it, or NavigationEntryContext.BackAsync, for work that completes late, such as a double click or a dialog that answers after the user moved on.

• A region exposes Current, CurrentEntry, History, CanGoBack and IsTransitioning. It raises PropertyChanged for them inside a model turn, the commit turn for a navigation.

Entry lifecycle

An entry starts Pending and becomes Active when its push commits. It then moves between Active and Retained as pushes and Backs commit, and ends Retired. Every state can retire:

Pending ──commit──▶ Active ◀──Push / Back(To)──▶ Retained
   │                  │                              │
   │ not committed    │ Back, Replace,               │ BackTo, Reset,
   │                  │ Reset, Clear                 │ ClearHistory, Clear
   ▼                  ▼                              ▼
                    Retired

A closing parent region or navigator disposal also retires entries in any state.

The entry's context. Each entry has a NavigationEntryContext: its Id, State, Services, Region, Retirement token, and the operations it performs on itself (BackAsync, CompleteAsync, DismissAsync). Content that the navigator constructs receives it as a constructor parameter, so it holds its entry from the start:

public sealed class ConfirmViewModel(NavigationEntryContext entry) : ObservableObject
{
    public Task Confirm() => entry.CompleteAsync(true).AsTask();
    public Task Cancel() => entry.DismissAsync().AsTask();
}

await dialog.PushForResult<ConfirmViewModel, bool>().Completion; // the navigator passes the dialog its own entry

• Create<T>(), Create<T, TInput>(input) and the region methods that wrap them (PushAsync<T>(), ReplaceAsync<T>(), ResetAsync<T>(), PushForResult<T, TResult>()) pass the new entry's context to a constructor parameter of type NavigationEntryContext. A factory target takes it as its second argument: NavigationTarget.Create<IDialogViewModel>((services, entry) => new ConfirmViewModel(entry)). Initial targets of CreateRegion receive it the same way.
• The content is constructed while its entry is still Pending, during the preparation; the entry becomes Active when the transition commits. Keep the context for commands rather than navigating from the constructor.
• Only that content receives the context. A service resolved from the container can't: with AddRunicNavigation(), resolving NavigationEntryContext from the container throws an InvalidOperationException that says where it is available. Pass the context on from the content to the objects that need it.
• Content created with Own or Borrow exists before its entry, so it receives the context in INavigationInitialize.InitializeAsync.

Navigated to and from. Content that implements INavigationAware is told when its entry becomes current and when it stops being current:

NavigationEntryContext.StateChanged reports every state change, for content of any kind and for the objects it passes the context to. Its arguments carry the previous state, the new state and the operation. A retained entry that retires reports Retained to Retired without another OnNavigatedFrom, and a pending entry whose transition didn't commit reports Pending to Retired.

• Both are synchronous and run in the commit turn: after the new navigation state is applied and after the departure's OnCommitted actions, before the regions raise PropertyChanged. The departing entry is notified first, then entries that retire without having been current, then the arriving entry. For each entry, StateChanged runs before the INavigationAware call. So OnNavigatedTo and OnNavigatedFrom alternate for an entry, starting with OnNavigatedTo, and State is already the new state in both.
• A transition that is rejected, superseded, cancelled or fails never calls OnNavigatedTo or OnNavigatedFrom, and it doesn't change the state of the current entry. Its pending entry reports Pending to Retired in a later turn, if it has StateChanged handlers.
• An initial target reports Pending to Active and OnNavigatedTo(Reset) inline in CreateRegion or NavigationRegion<T>.Start, so it doesn't run in a model turn unless that call was made in one.
• Each handler is called separately. One that throws is logged (event 1075), and the other handlers still run; the change stands. Handlers must not pump or await navigation; start navigation after the turn instead.
• INavigationInitialize and INavigationResume remain the asynchronous preparation hooks: they run before the commit, outside turns, and can fail the transition. OnNavigatedTo runs after ResumeAsync, once the resume commits.

Retirement runs once per entry, whichever path reaches it first: commit cleanup, a closing parent, a failed preparation or navigator disposal. It runs these steps in order, and each step runs even if an earlier one failed:

1. NavigationEntryContext.Retirement is cancelled.
2. The child regions that the entry's content owns close. Their in-flight transitions end as Rejected(Closed), and their entries retire depth-first.
3. Owned content only: its presentations are forgotten.
4. Owned content only: the content is disposed, outside model turns. Then the entry's service scope, if the navigator created one, is disposed.
5. Owned content only: its model-context lease is released.

A failed step is logged as event 1064.

Transitions

• A BackAsync() that arrives while the region's latest request is another Back that hasn't committed, and neither the region nor the child regions the departing entry owns have changed since that Back was admitted, joins it instead of superseding it: interested callers get the same result, the departure guard runs once, and a confirm dialog stays open (event 1074). Back from code while the guard asks therefore pops once and asks once. A Back that starts after the first one committed is a new request and pops again; that is stack semantics. Token-bearing Backs join too: cancelling one caller before commit returns Rejected(Cancelled) for that caller immediately. The shared Back is cancelled only when every caller has cancelled; a caller without a cancellable token keeps it alive. Cancellation after commit does not change the outcome. A return from a result entry (CompleteAsync, DismissAsync) is never joined.
• IsTransitioning is true from admission, synchronously. A command that reads it in CanExecute, such as NavigationHost's BrowseBack in Runic.Navigation.Wpf, is disabled before the Back call returns, so a double click pops once.
• A dialog answers its guard with CompleteAsync or DismissAsync. Code run from the confirm dialog that awaits BackAsync() on the guarded region before answering joins the Back that waits for that answer, so it doesn't complete until the dialog closes another way.
• A request is superseded only before it commits, never after. A cancelled token gives Rejected(Cancelled) and navigator disposal gives Rejected(Closed), in any phase before the commit. So does the close of the request's region: a parent's Back that retires a child region supersedes the child requests pending at its admission, and a child request admitted after it is Rejected(Closed) when the parent commits, never Superseded, whichever phase it is in.
• Hook tokens are cancelled when the request is superseded, cancelled or closed. A superseded request that is still running 5 seconds later is logged once (event 1067). The next request waits for it.
• Admission inside a model turn does not block. The transition continues on the thread pool, so no hook runs in the caller's turn. Don't block on its result in that turn.
• Admission is per region, so a guard in one region may await a request in a sibling region, as the confirm dialog below does. Parent and child regions coordinate through the plan: a child commit during the parent's guard supersedes the parent's request.
• One entry's guard never runs twice at the same time. It can still run again for the same departure: a parent transition that retires a child region asks the child's guards even when a child transition already asked them. Write guards to be repeatable. A LeaveConfirmation keeps its yes while the transition it allowed is unsettled and passes it to the transition that supersedes it, so the rerun doesn't ask again. When a change in another region, rather than a later request, superseded the transition, the yes ends, and the user may be asked twice.

Ownership

• Content resolved from a container (a window scope, the root provider or Splat) belongs to that container, so Borrow it. Use Own and Create only for content constructed with new or by the navigator. A factory receives the entry's IServiceProvider to resolve constructor dependencies. Those dependencies stay container-owned. Without entry scopes, don't resolve disposable transients for it: the provider keeps them until it is disposed.
• An instance can be owned once. Own of an instance the navigator has already owned, whether live or retired, throws InvalidOperationException. A Create factory that returns one gives Failed(Preparing).
• Present owned content only through its region's slot. Retirement forgets every presentation of the instance.
• Borrowed selection hosts use caller/container-owned models. Do not re-borrow content from a navigator-owned entry into another entry; retirement of its owning entry still disposes that content.
• Owned content is disposed outside model turns, on the model's thread when the context schedules hooks (see Threading). A Dispose that touches shared model state uses ModelContext.InvokeAsync, and it must not await navigation on the same navigator.
• A region's owner can be any object. The regions an entry's content owns are that entry's child regions. Those of owned content close when the entry retires. NavigationRegionOptions.WhileParentRetained decides what they do while the entry is retained: Keep (the default), ResetToRoot or Clear. Those of borrowed content keep their entries when the entry retires, because the content lives on. Regions owned by the root, or by an object that is no entry's content, are nobody's child regions.
• An initial target (CreateRegion(owner, initial) or region.Start(target)) becomes current without a transition. Its content must not implement INavigationInitialize or INavigationInitialize<TInput>, and Create(factory, input) and Create<T, TInput>(input) cannot be initial targets.

Which departures ask a child region

A departure asks the guards in the departing entry's child regions, and in theirs, deepest first and before the entry's own guard:

• A guard that rejects keeps the whole departure: the entry stays current, so a tab switch leaves the tab selected, and no region changes.
• A commit in an asked child region while the departure is guarding supersedes the departure, as for owned children.
• A tab that only refers to a region of the root asks nothing when it is left. Create a tab's page regions with the tab's model as their owner: navigator.CreateRegion<IPage>(this, ...) in the tab's constructor.
• A window's guarded close (GuardClose, GuardWindowClose, see "Window close and disposal") clears the regions it guards, so every entry in them and in their owned child regions is asked as Retire, whichever tab is selected.
• LeaveConfirmation allows a Retain departure without asking, because a page kept in its region keeps its draft. Pass askOnRetain: true to ask before the tab is left too; a confirmed switch then runs discard.

Container-resolved targets

NavigationTarget.Create<T>() and Create<T, TInput>(input) let the navigator construct the ViewModel with ActivatorUtilities.CreateInstance from the entry's service provider. The constructor's parameters come from the container; the ViewModel itself is never resolved, so the navigator owns it. The region methods PushAsync<T>(), PushAsync<T, TInput>(input), the same forms of ReplaceAsync and ResetAsync, and PushForResult<T, TResult>() wrap these targets. T must be a TContent, checked at compile time.

await main.PushAsync<DocumentViewModel>();                      // DocumentViewModel(EditorViewModel editor, ...)
await main.PushAsync<NoteViewModel, NoteId>(id);                // NoteViewModel : INavigationInitialize<NoteId>
var confirm = dialog.PushForResult<ConfirmViewModel, bool>();
var picked = dialog.PushForResult<bool>(NavigationTarget.Create<ConfirmViewModel, string>("Discard?")); // input and a result

• Constructors are kept for trimming and NativeAOT ([DynamicallyAccessedMembers(PublicConstructors)] on T). [ActivatorUtilitiesConstructor] picks one of several constructors.
• A navigator without Services uses an empty provider. A constructor that needs services then fails as Failed(Preparing) (event 1061), like a throwing factory.
• A constructor parameter of type NavigationEntryContext receives the new entry's context; see The entry's context. It is never resolved from the container, and a service that depends on it fails to resolve.
• INavigationEntry.Services and NavigationEntryContext.Services return the provider that built the entry. After an entry with its own scope retires, resolving from it throws ObjectDisposedException.

Entry scopes. With RunicNavigatorOptions.CreateEntryScopes, the navigator creates one IServiceScope for each entry that a Create target builds, initial targets included. The factory and ActivatorUtilities resolve from it, and the entry's Services returns it. Retirement disposes the content, then the scope, so scoped and disposable transient dependencies live exactly as long as the entry. A construction that fails disposes the scope at once. Borrow and Own entries never get a scope.

Turn entry scopes on only for a navigator whose Services is the root provider, for example a singleton navigator. A navigator resolved from a scope, such as the one AddRunicNavigation() registers, must leave it off: CreateScope on a scoped provider makes a sibling scope of the root, so the entry's scoped dependencies would silently differ from the navigator's. The navigator can't tell the two providers apart. AddRunicNavigation() leaves it off; a window scope already bounds those dependencies.

Presenting regions

A host presents a region's Current. In Runic.Application.Views, a get-only NavigationRegion<TContent> property of a ViewModel is a content slot that the generated Bridge presents in the window's web View; see Content slots. Other hosts bind INavigationRegion, the non-generic view of a region, and INavigationEntry. Runic.Navigation.Wpf provides WPF hosts and a dispatcher model context. RunicNavigator.AttachPresentation is for presentation integrations only: retirement calls INavigationPresentation.Forget once for each attached presentation and owned entry.

Results from an entry

PushForResult<TResult>(target) pushes an entry and returns a NavigationResultRequest:

• Transition is the push's result.
• Completion ends Completed(value) only when the entry calls NavigationEntryContext.CompleteAsync(value) and the Back that it issues commits.
• Completion ends Dismissed on every other path: the push does not commit, the entry retires another way, the caller's token is cancelled after the commit (which also goes back from the entry), or the navigator closes.

A Back from CompleteAsync or from caller cancellation may leave the region empty, so a dialog region can start empty. A push onto an entry whose caller cancelled retires that entry instead of retaining it. That Back can still be rejected, for example when a push over the entry is already committing or the entry is no longer on top. Then the dismissed entry stays in the history: a later Back resumes it, and its CompleteAsync goes back, possibly emptying the region, and drops the value (event 1071).

A rejected CompleteAsync, for example one vetoed by a guard, leaves the request open, and the entry can try again. CompleteAsync accepts any value of the request's result type at run time, so true completes a bool? request and 5 an object request. Completions run their continuations asynchronously, after the commit turn. A completion is set when the entry starts retiring, so it can be observed while the entry's owned content is still being disposed.

NavigationEntryContext.DismissAsync() leaves an entry without a result. For the current entry it ends an open request Dismissed at once and goes back, which may leave the region empty, like a cancelled caller token. Called from the entry's own InitializeAsync, it cancels the push that creates the entry (Rejected(Cancelled)) and returns at once; don't await that push from the hook. For a retained entry it ends the request and leaves the entry in place; for a retired one it does nothing. Both return Rejected(NotCurrent). A dialog's Cancel uses it, so Cancel always ends the question, even when its Back is rejected because something was pushed over the dialog.

Leaving with unsaved changes

A guard's true is not a commit: a later request can still supersede the departure, another guard can veto it, or it can fail. Work that must happen only when the departure happens goes to NavigationDeparture.OnCommitted:

public async ValueTask<bool> CanDepartAsync(NavigationDeparture departure, CancellationToken token)
{
    if (!await departure.ModelContext.InvokeAsync(() => Editor.IsDirty, token)) return true; // guards run outside turns
    if (!await AskAsync(token)) return false;
    departure.OnCommitted(Editor.DiscardChanges);
    return true;
}

• Actions run in the commit turn of the departure, in registration order, after the new navigation state is applied and before the regions raise PropertyChanged. They don't run when the transition is superseded, rejected, cancelled or fails.
• Register while the guard runs. OnCommitted throws InvalidOperationException after the guard returned.
• An action that throws is logged (event 1072); later actions still run, and the commit stands. Actions must not pump or await navigation.

LeaveConfirmation packages the whole pattern. It allows a departure that only retains the entry (unless askOnRetain), allows when there are no unsaved changes, asks otherwise, and discards with OnCommitted. With askOnRetain, a confirmed push over the entry discards too, although the entry stays in the history. Its yes stands while the departure it allowed is unsettled: when the guard reruns for the same entry, for a parent transition or a request that superseded the one it allowed, it answers without asking again, and discard still runs at most once. LeaveConfirmation.InDialog asks with a dialog pushed for a bool result into another region; only Completed(true) confirms, and the guard's token dismisses the dialog when the departure is superseded or the navigator closes. The dialog region must not be the guarded region, one of its ancestors or one of its descendants: that push would be Rejected(Reentrant).

LeaveConfirmation keeps the caller's context, so its confirm delegate stays on the UI thread. Without a hook scheduler the guard can run in the frame of the call that started the navigation. If that call blocks a UI thread on the navigation's result (for example with GetAwaiter().GetResult()), the guard's continuation can never run there, and the two deadlock. Await navigation results; don't block on them, as with any asynchronous code.

The Notes example asks this way. Here the navigator constructs the confirm dialog and passes it its entry:

// The document: the guard forwards to a LeaveConfirmation.
_leave = LeaveConfirmation.InDialog(navigation.Dialog,
    () => NavigationTarget.Create<IDialogViewModel>((_, entry) =>
        new ConfirmNavigationViewModel(entry, "Discard the unsaved edits?")),
    () => Editor.IsDirty, Editor.DiscardChanges);

public ValueTask<bool> CanDepartAsync(NavigationDeparture departure, CancellationToken token) =>
    _leave.CanDepartAsync(departure, token);

// The confirm answers with the entry it was constructed for.
public sealed partial class ConfirmNavigationViewModel(NavigationEntryContext entry, string message)
    : ObservableObject, IDialogViewModel
{
    public string Message => message;
    [RelayCommand] private Task Confirm() => entry.CompleteAsync(true).AsTask();
    [RelayCommand] private Task Cancel() => entry.DismissAsync().AsTask();
}

ReactiveUI apps can ask through an Interaction instead of a dialog region, with ReactiveLeaveConfirmation.WithInteraction from Runic.Navigation.ReactiveUI.

Work that a hook starts on a presentation's side, such as a question shown in a browser, should end with the hook. RunicNavigator.HookCancellationToken returns the token of the hook whose code runs on the calling asynchronous flow (the guard's token), or CancellationToken.None outside hooks. Runic.Application.Views ends a browser interaction request with it, so a superseded guard aborts the browser handler's signal.

Window close and disposal

DI disposes the navigator before the model context it depends on. Its DisposeAsync:

1. Refuses new requests and dismisses every open PushForResult request, so guards that await a result unblock.
2. Cancels in-flight transitions and waits for them, and for running cleanup, up to CloseTimeout (10 seconds on its TimeProvider). A late transition never commits.
3. Retires every remaining entry, region by region, pending entries included. Clearing a closing region normally runs in a model turn. These turns share one CloseTimeout, so disposal takes about twice CloseTimeout at most, plus owned content disposal. If a turn does not run in time, the stack is cleared outside a turn (event 1068).

Disposal does not run guards. Disposing a navigator cancels window-owned operations that its owned content started, and then disposes that content. To ask the guards before a user closes the window, opt in on the window: GuardClose on a RunicWindow<T> (Runic.Application.Views, Runic Desktop's embedded window) or GuardWindowClose on a WPF NavigationHost (Runic.Navigation.Wpf). Either clears the window's regions before it closes, and a guard that rejects or fails keeps the window open.

WhenIdleAsync() waits until no transition or retirement is running.

Threading

Without a scheduler, as with RunicModelContext, hooks (factories, guards, initialize and resume) run outside turns on the thread that runs the transition, and owned content is disposed there too. A model context whose model has thread affinity, such as a UI dispatcher context, can also implement IRunicModelHookScheduler. The navigator then runs each hook and each owned content disposal as its own operation on the model's thread: never inline in the caller, never inside a turn. Hook operations don't count as turns. Once the context is closed, disposal runs on the thread pool. A decorator that wraps a scheduling context must implement and forward the interface too.

An initialize hook may set the state of content created for its entry (NavigationTarget.Create, or a new instance handed over with Own) directly, also after await: nothing presents that content before the entry commits, and the navigator commits in a turn after the awaited hook. Commit with ModelContext.InvokeAsync state that is already presented or that other work also writes, such as a Borrow target, a service or another page, and read or write state that way in guards and resume hooks, which run while the content may be presented.

A context that can close on its own, for example when its UI thread shuts down, can also implement IRunicModelContextLifetime. The navigator subscribes to its Closed token and starts closing when it fires: later requests are Rejected(Closed) at once, and transitions in flight are cancelled as closed. Without it, the navigator learns about the close only when a turn or hook fails with ObjectDisposedException, so a request can wait behind a transition whose hook ignores its cancellation. Decorators forward this interface too.

During DisposeAsync, a scheduled disposal that hasn't started by the clearing turns' deadline runs on the thread pool instead, exactly once, so disposal completes even when the model's thread is blocked. Don't block the model's thread on DisposeAsync (for example GetAwaiter().GetResult() in a WPF OnExit): call Dispose() there, which starts disposal without waiting. A cleanup that starts after disposal finished (for example content that a factory returns late) waits on the model's thread without a bound. If that thread never runs it again and the context never closes, that content and its model-context lease are never released. Close the context when the application shuts down, for example on Dispatcher.ShutdownStarted in WPF.

The navigator itself never continues on the model's thread: after awaiting a hook, a turn or other user code, it moves to the thread pool when the completion ran in the model context. A caller that awaits a navigation result resumes on its own context as usual.

Pitfalls

• A hook that disposes its own navigator stalls disposal for CloseTimeout, because disposal waits for the transition that runs the hook.
• A hook that awaits WhenIdleAsync() deadlocks: the navigator is not idle while that hook runs.
• A child guard that commits a change in its own region supersedes the parent's transition, because the parent's commit sees the child's version change.
• A request from a hook into the same region, one of its descendants or one of its ancestors is Rejected(Reentrant). For example, a parent's InitializeAsync cannot push into its own child region, and a CurrentPane guard cannot call Main.BackAsync() on the region that holds its document. Issue the request after the transition commits. A hook's code is the hook's own frame and its continuations, including await Task.Yield() and awaits that resume through the SynchronizationContext the hook started with: requests from there stay Rejected(Reentrant). Input handlers that a hook's nested message loop dispatches, for example while a guard shows a modal window, are not the hook. One gap fails safe: a handler that the nested loop dispatched and that awaits past the loop's end, while the transition is still in flight, may continue as the hook and get Rejected(Reentrant).
• A service that creates a region with a container-built initial target whose constructor needs that service is a dependency cycle. For example, MainNavigation creates its region with Create<HomeViewModel>(), and HomeViewModel(MainNavigation navigation) resolves MainNavigation again, which creates another region. CreateRegion detects the re-entry for the same owner type and initial target type on the same thread, on any navigator, and throws InvalidOperationException instead of overflowing the stack. One holder type reused at two levels with different targets is fine. Create the region empty and start it once the service exists, for example with navigation.Region.Start(NavigationTarget.Create<HomeViewModel>()) in the window ViewModel's constructor (see The root page).

Model context

IRunicModelContext runs short synchronous turns one at a time, in posting order. Navigation commits run as turns, and region PropertyChanged handlers run inside them. InvokeAsync runs a turn and returns its result; called from a turn of the same context, it runs inline. TryPost queues a turn without waiting. RunicModelContext is the default implementation, and AddRunicNavigation() registers one per scope unless one is registered.

RunicModelContext runs each turn under a SynchronizationContext that queues posted callbacks as new turns, so an await in async code started in a turn, such as a command body, resumes in a later turn, as on a UI thread. Continuations are queued in the order they become ready. Code after ConfigureAwait(false) or in Task.Run leaves the context. RunicModelContext.Current is the context whose turn is running on the current thread, or null outside a turn; adapters use it to route work scheduled in a turn back to that turn's context. Once the context is closed, continuations run on the thread pool outside any turn, so the code still finishes. Navigation hooks don't run in turns (see Threading).

RunicModelContextRegistry associates each model object with the context that owns its changes. Bind registers objects with an application-owned context. Acquire creates a context for a model graph, reuses it for later leases of the same graph, and disposes it after the last lease is released. A second context for an object that is already bound is rejected.

Disposing a RunicModelContext rejects queued turns and waits for the running turn. It implements IRunicModelContextLifetime: its Closed token is cancelled after admission stops and before queued turns are rejected, so navigators close immediately, like those using the WPF dispatcher context. A posted turn that disposal drops is logged (event 1031). Exceptions from close callbacks are reported through UnhandledTurnException (event 1030) and do not interrupt shutdown.

Testing

Test a ViewModel through a real navigator rather than a test double. The navigator needs no UI host: construct it with a RunicModelContext, push the ViewModel, and drive it through its commands. The ViewModel then receives a real NavigationEntryContext, and its results, Backs, guards and INavigationAware notifications behave as in the application.

[Fact]
public async Task ConfirmCompletesTheRequest()
{
    await using var model = new RunicModelContext();
    await using var navigator = new RunicNavigator(new RunicNavigatorOptions { ModelContext = model, Services = services });
    var dialog = navigator.CreateRegion<object>(this);

    var request = dialog.PushForResult<ConfirmViewModel, bool>();
    Assert.True((await request.Transition).IsCommitted);
    await ((ConfirmViewModel)dialog.Current!).Confirm();

    Assert.Equal(true, (await request.Completion as NavigationCompletion<bool>.Completed)?.Value);
    Assert.Null(dialog.Current); // the dialog went back and retired
}

• Services provides the ViewModel's constructor dependencies, fakes included. Without it, constructors that need services fail as Failed(Preparing).
• Every navigation result is returned, so assert on it: a guard's veto is Rejected(Guard), a throwing hook is Failed.
• Code that reads ViewModel state the navigation changed should run in a turn (model.InvokeAsync) when other threads write it.
• RunicNavigator.UnretiredEntryCount counts entries that have not finished retiring. After DisposeAsync, it is 0 when every entry retired.

Logging

Errors and warnings carry the exception. Rejections, supersessions, joined Backs and dismissed results log at Debug. The properties are Region (the TContent type name), RegionId (a per-navigator number that tells apart regions with the same TContent), Operation, EntryType, Step, Reason, Property (event 1063, the region property whose handler threw), Notification (event 1075) and ErrorType, never values.

• Events 1060-1075 use the category Runic.Navigation (RunicNavigator.LogCategory), from RunicNavigatorOptions.LoggerFactory or the scope's ILoggerFactory with AddRunicNavigation().
• Events 1030-1033 use the logger of the RunicModelContext: ILogger<RunicModelContext> (category Runic.Navigation.RunicModelContext) when DI or a Runic.Application.Views window session with a logger factory created it. Event 1033 is also logged by the registry when a context it created fails to shut down.
• Without a logger factory, each entry is written to System.Diagnostics.Trace (TraceError, or TraceWarning for warnings) as its formatted message followed by the exception.

Navigation reserves events 1060-1079: 1060-1069 for transitions and cleanup, and 1070-1079 for results and later navigation events, of which 1076-1079 are not used yet. Events 1044-1049 are reserved for the ReactiveUI navigation adapter.