Runic.Application.Views 0.7.0-preview.8

Runic.Application.Views

Typed .NET Windows and Views with generated TypeScript clients. C# ViewModels own application state, commands, and operation lifetimes; a React, Vue, Svelte, Angular, or plain TypeScript frontend renders them through a generated client. Every package's ID is its namespace: this package's types are in Runic.Application.Views, and the host adapters' in Runic.Application.Views.Desktop, Runic.Application.Views.CsWebUi and Runic.Application.Views.Wpf.

Install

Reference a host adapter; it brings this package and its build targets:

dotnet add package Runic.Application.Views.CsWebUi --prerelease  # CS-WebUI browser or WebView window
dotnet add package Runic.Application.Views.Desktop --prerelease  # or: Runic Desktop native host
dotnet add package CommunityToolkit.Mvvm                         # or ReactiveUI with Runic.Application.Views.ReactiveUI
dotnet add package Microsoft.Extensions.DependencyInjection      # ServiceCollection; the adapters need only the abstractions

To start a new application, run the guided creator (dnx Runic.Create@<VERSION>) or the dotnet new runic-app template, then dotnet tool restore and dotnet runic dev. The template defaults to a native Runic Desktop window and ReactiveUI ViewModels; the minimal Window below keeps to the shortest program, CS-WebUI (which opens a browser window) with CommunityToolkit.Mvvm, as in the First Window example.

Glossary

These words name one concept each across the Runic packages and frontends.

• Window: a top-level application window and its root ViewModel, declared as a partial RunicWindow<TViewModel>. The same declaration opens on every host with host.OpenWindowAsync<TWindow>(). A Window is also a View. Runic Desktop's DesktopWindow is the native window underneath one.
• View: a logical .NET presentation object for one ViewModel, a partial RunicView<TViewModel> with a typed DataContext. It owns no DOM: the frontend renders a component for it, which ViewOutlet selects in React, Vue, Svelte and Angular.
• Bridge: the code generated per ViewModel, C# and a typed TypeScript client, that carries snapshots, property writes, commands and operations between the ViewModel and its View over the wire protocol. Bridge* types and the RUNICBRIDGE diagnostics belong to this generated layer.
• Host: the adapter that opens Windows on a platform: RunicDesktopHost (Runic.Application.Views.Desktop) and RunicCsWebUiHost (Runic.Application.Views.CsWebUi), both an IRunicWindowHost. In WPF, RunicViewHost hosts one View inside the visual tree. An open Window's Host property returns its per-Window adapter, an IBridgeWindow.
• Presentation: what shows a Window's or View's web content on screen: an embedded WebView or an installed browser window, such as DesktopBridgeWindow.Presentation (a DesktopWindow) or the WebView of a RunicViewHost. A presentation closes and reopens without changing the ViewModel.
• Surface: Runic Desktop's DesktopSurface, which serves a Window's web content (DesktopSurfaceOptions.Content), capabilities and browser sessions, and opens presentations of it. DesktopBridgeWindow.Surface is the Window's surface.
• Owner: the native window that platform services, such as file dialogs, the clipboard and file launchers, attach to: DesktopNativeOwner for a Runic Desktop window, or an INativePickerOwner for another host. Native dialogs require one (OwnerPolicy.RequireOwner).

A minimal Window

In a project named MyApp:

// Counter.cs
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using Runic.Application.Views;

namespace MyApp;

public sealed partial class CounterViewModel : ObservableObject
{
    [ObservableProperty] private int count;

    [RelayCommand]
    private void Increment() => Count++;
}

// Selecting the Window makes the build generate Frontend/src/generated/counter.ts.
public sealed partial class CounterWindow : RunicWindow<CounterViewModel>;

// Program.cs
using Microsoft.Extensions.DependencyInjection;
using MyApp;
using Runic.Application.Views;
using Runic.Application.Views.CsWebUi;

var services = new ServiceCollection();
services.AddScoped<CounterViewModel>();
services.AddRunicViews();
using var provider = services.BuildServiceProvider();

// RunicDesktopHost.Run(provider, ...) from Runic.Application.Views.Desktop runs the same application natively.
return RunicCsWebUiHost.Run(provider, async host =>
{
    await using var window = await host.OpenWindowAsync<CounterWindow>();
    await window.WaitForCloseAsync();
    return 0;
});

import { connectCounter } from "./generated/counter.js";

const counter = await connectCounter();
counter.subscribe(state => { document.querySelector("#count")!.textContent = String(state.count); });
document.querySelector("#increment")!.addEventListener("click", () => void counter.increment());

Generated modules import the shared browser runtime, so the frontend installs @runic-artifex/views (npm install @runic-artifex/views@preview) and bundles its entry. connectCounter() returns a CounterClient; subscribe delivers the current state first, calls reject with the runtime's BridgeError, and dispose() releases the connection. @runic-artifex/react, @runic-artifex/vue, @runic-artifex/svelte (useView) and @runic-artifex/angular (injectView()) bind clients to component lifetimes.

Frontend/index.html loads webui.js and the host's client script (runic-cswebui.js or runic-desktop-views.js) before the module. The First Window example is this application with a complete frontend.

Windows, Views, and composition

A partial RunicWindow<TViewModel> declares a window and its root ViewModel; a partial RunicView<TViewModel> selects the content presented for a ViewModel property. [RunicViewContract("name")] adds a second View for the same ViewModel. The build compiles the application, inspects these classes after the MVVM source generators run, and generates C# attachments and one ordinary TypeScript module per ViewModel.

A View is a logical .NET presentation object with typed DataContext. Its browser counterpart can be a plain TypeScript function or a framework component. The browser acknowledges mount and unmount, so a content session creates a fresh View for each presentation and releases it when the outlet changes. The ViewModel belongs to its application or window DI scope and may survive View changes. Window-local routes and operation admission are host-neutral; native window creation belongs to the host adapter.

The generated composition class (<ProjectName>.RunicBridgeComposition, or RunicApplicationFrontendCompositionType) provides two registrations:

• AddRunicViews() registers the generated Bridges, every non-Window View as transient, and ServiceProviderViewLocator as the default IRunicViewLocator. Register an earlier IRunicViewLocator or View to replace a default.
• AddRunicBridges() registers only the Bridges, for applications that create Views themselves or use another locator, such as ReactiveRunicViewLocator.

ViewModels stay explicit registrations because their lifetime is an application decision; a Window's root ViewModel must be scoped. The registrations are generated code, so they need no reflection and are NativeAOT-safe.

The window binds its root ViewModel to the window's model context, and a Bridge attached directly to a WindowContentSession binds its ViewModel for the Bridge's lifetime, so no ViewModel needs a manual RunicModelContextRegistry.Bind. A root, content or Bridge ViewModel already bound to a different context is rejected with an InvalidOperationException that names its type.

Presented content is bound to the window's model context, and keeps its checked-field registries, only while it is attached. Replacing, clearing, or pruning it releases both; the window keeps a weak identity so presenting the same object again returns the same reference. When the browser still shows a suspended reference, as after Main = b; Main = a; in one command, its mount resumes on the re-attached bridge with a fresh View. Snapshot revisions increase across the whole window, so a re-attached route never publishes an older revision. The wire protocol describes these rules.

Generated clients expose a snapshot, subscriptions, typed property setters, commands, and disposal. Calls reject with typed errors. Accepted operations remain owned by .NET across a browser reload; a reconnect gets a new snapshot. A transport failure after acceptance can leave completion unknown, so callers should inspect state before retrying a non-idempotent command. The generated field-write API returns receipts and version conflicts; frontend form helpers must await pending edits before issuing Save.

The runtime emits state and replies with generated and explicit Utf8JsonWriter code. The build tool inspects the compiled application; release runtime serialization does not reflect over ViewModel members. The first-window example is exercised through Native AOT and Chromium in CI. The generator's errors are listed under Generator diagnostics.

Opening a Window

Every host opens Windows the same way. The application's entry point calls the host's Run method, RunicDesktopHost.Run (Runic.Application.Views.Desktop) or RunicCsWebUiHost.Run (Runic.Application.Views.CsWebUi), which runs the asynchronous application on the event loop the host needs and passes it an IRunicWindowHost:

static async Task<int> RunAsync(IRunicWindowHost host)
{
    await using var window = await host.OpenWindowAsync<MainWindow>(new RunicWindowOptions { Width = 1000, Height = 700 });
    await window.WaitForCloseAsync();
    return 0;
}

Switching host changes only the Run call, its package and its host options; the Window declaration and this body stay the same, with ReactiveUI or CommunityToolkit.Mvvm ViewModels.

• OpenWindowAsync<TWindow>(options, cancellationToken) creates the Window, checks that the generated Bridge is registered, resolves the root ViewModel in a new DI scope, assigns it to DataContext, and opens the native window. RunicWindowOptions sets the content root (by default www next to the application), the start page (index.html) and the initial size on every host. The Window owns the scope until it is disposed. A Window instance opens once.
• ValidateWindow<TWindow>(options) runs the same checks at startup without opening anything and returns a RunicWindowValidationResult: an error bridge-not-registered when AddRunicViews() is missing, a warning viewmodel-not-registered when the container does not report the ViewModel, and the host's own checks, such as Runic Desktop's window options. ThrowIfInvalid() throws a RunicWindowConfigurationException, the same exception OpenWindowAsync throws on every host.
• WaitForCloseAsync(cancellationToken) completes when the window closes or starts closing; the token stops only the wait. CloseAsync(timeout, cancellationToken) stops new operations, waits up to timeout for accepted ones, and returns a BridgeWindowCloseResult; its token also stops only the caller's wait. Disposal is asynchronous only.
• RequestCloseAsync(cancellationToken) closes the window as its close button does and returns whether it closed; with GuardClose it asks CanCloseAsync first (see Guarding a user close).
• Host returns the host adapter, an IBridgeWindow: DesktopBridgeWindow<TViewModel> (with Surface, Presentation and NativeOwner) or CsWebUiBridgeWindow<TViewModel> (with NativeWindow).

Opening a Window from a ViewModel

The host is passed only to the Run callback. To open Windows from a command or a service, register IRunicWindowService and take it in a constructor:

services.AddRunicWindowService();

With ReactiveUI, as in the multi-window example:

public sealed class ProjectViewModel : ReactiveObject
{
    public ProjectViewModel(IRunicWindowService windows)
    {
        // Opens another Window on the same project, in its own scope. The service releases it when it closes.
        NewWindowCommand = ReactiveCommand.CreateFromTask(async token =>
        {
            await windows.OpenWindowAsync<ProjectWindow>(cancellationToken: token);
        });
    }

    public ReactiveCommand<RxVoid, RxVoid> NewWindowCommand { get; }
}

With CommunityToolkit.Mvvm:

public sealed partial class ProjectViewModel(IRunicWindowService windows) : ObservableObject
{
    [RelayCommand]
    private async Task NewWindowAsync() => await windows.OpenWindowAsync<ProjectWindow>();
}

• OpenWindowAsync<TWindow>(options, cancellationToken) has the same parameters and checks as the host's, and opens the Window in its own DI scope on the host that runs the application, on Runic Desktop and CS-WebUI alike. On CS-WebUI, WaitForCloseAsync completes only when every browser window has closed.
• The service is a singleton, so ViewModels in every Window and singleton services can take it. A host binds it before it resolves the ViewModel of the first Window it opens, so it already works in that ViewModel's constructor. Before any Window has opened it throws InvalidOperationException.
• The service releases each Window it opened, with its scope and ViewModels, once the Window closes, so a command doesn't need to keep it. Keep the returned Window to wait for it or to close it; disposing it earlier is safe. A release failure is logged as event 1051.
• Runic has no owned or modal Windows, so there is no owner parameter. The new Window's ViewModel is resolved from DI like the first one's; to give it input, let it read a singleton service, as the multi-window example does with its shared project.
• The interface has one method, so a test registers a fake that records the requested Window types after the application's services.

A Window with its own page

Every Window opens index.html unless RunicWindowOptions.StartPage names another page. With Vite, each page is a build input, and listing inputs replaces the default index.html, so keep it in the list. The runic-app template's vite.config.ts already lists it; add the second page next to it:

// Frontend/vite.config.ts (Vite 8)
export default defineConfig({
  plugins: [svelte(), runic({ desktop: true })],
  build: {
    rolldownOptions: { input: ["index.html", "settings.html"] },
    outDir: "dist",
    emptyOutDir: true,
    target: "es2022"
  }
});

Create Frontend/settings.html beside index.html, for example as a copy that loads its own entry script, and open it with:

await windows.OpenWindowAsync<SettingsWindow>(new RunicWindowOptions { StartPage = "settings.html" });

The build writes www/settings.html beside www/index.html, and dotnet runic dev serves both. Vite resolves inputs against its root, the frontend directory. The object form, input: { main: "index.html", settings: "settings.html" }, names the output chunks; rollupOptions still works in Vite 8 but is deprecated. A page in a folder, such as pages/settings.html, works the same way with StartPage = "pages/settings.html": @runic-artifex/vite-plugin-runic points its host scripts (webui.js and the Runic client script) at the web root.

A page that the build does not emit fails before a window waits for it: on Runic Desktop, opening the Window throws RunicWindowConfigurationException with the code content-entry-not-found, which names the page and the pages the content root has; dotnet runic dev stops with RAPPDEV1009 when a build input's file is missing. CS-WebUI does not check the page before it opens the window.

Guarding a user close

A Window that sets GuardClose asks CanCloseAsync before a close the user starts (the title-bar close button, Alt+F4 or the window manager) and stays open when it returns false, like Window.Closing with e.Cancel = true in WPF and Avalonia, but asynchronous:

public sealed partial class EditorWindow : RunicWindow<EditorViewModel>
{
    public EditorWindow() => GuardClose = true;
}

The declaration is the same for ReactiveUI and CommunityToolkit.Mvvm ViewModels; only the guards and the commands below differ. The default CanCloseAsync clears each NavigationRegion<T> the Window's ViewModel exposes, in declaration order, so the departure guards of their entries decide, as WPF's NavigationHost.GuardWindowClose does (see Navigation). Override it to add a check of your own, and call the base method to keep the navigation guards. It starts in a turn of the window's model context, one decision runs at a time, and it may wait as long as a prompt needs; the event loop is never blocked. An exception keeps the window open and is logged as event 1052.

A guard can ask the user in the page: a ReactiveUI interaction it handles, such as ReactiveLeaveConfirmation.WithInteraction, reaches the web View's mounted handler when the Window has exactly one presentation with that handler mounted. It never reaches another Window. If the View unmounts or the Window closes during the question, the guard's answer is "keep", so the Window stays open (see Leave confirmation in a web View).

To close a Window from code the way the close button does, call RequestCloseAsync(cancellationToken). It returns true when the window closed and false when CanCloseAsync kept it open, like Window.Close() in WPF and Avalonia, which raises the cancellable Closing, but awaitable. Without GuardClose it closes. Keep the Window that OpenWindowAsync returned, including from IRunicWindowService, to call it from a command; a ViewModel closes its own Window through IRunicWindowContext (see Closing a ViewModel's own Window). With ReactiveUI, the command passes its token to the request. The Bridge starts the command in a turn of its own Window's model context, so the body sets state directly after await:

CloseEditorCommand = ReactiveCommand.CreateFromTask(async token =>
{
    if (!await _editor.RequestCloseAsync(token)) StatusText = "The editor has unsaved changes.";
});

With CommunityToolkit.Mvvm:

[RelayCommand]
private async Task CloseEditorAsync()
{
    if (!await _editor.RequestCloseAsync()) StatusText = "The editor has unsaved changes.";
}

A request while another request or a user close is being decided waits for the same decision; its token stops only the caller's wait. When the window may close, the visible window closes at once and disposal releases its scope, as after a user close.

• CloseAsync, disposal and a host shutdown (Ctrl+C, SIGTERM) are the application's own close: they do not ask, and they cancel a pending decision's token. A pending RequestCloseAsync then returns true.
• On Runic Desktop the embedded window asks, and RequestCloseAsync is its native close request. A Window that also has WindowOptions.ConfirmCloseAsync asks its own guard first, then that confirmation, for both.
• A presentation that cannot keep its window open opens the Window unguarded and logs event 1053 WindowCloseGuardUnavailable instead of failing: CS-WebUI, a browser window (an explicitly selected browser or the EmbeddedThenBrowser fallback), or a custom embedded-window host without the CloseConfirmation capability. A browser tab or window closes without asking the application. Save drafts as the user edits, or handle the close afterwards, where that matters. RequestCloseAsync still asks there, because the application is the one closing.

Closing a ViewModel's own Window

A Quit or Done command closes the Window its ViewModel belongs to through IRunicWindowContext, which AddRunicWindowService() registers per scope. OpenWindowAsync binds it to the Window it opens before it resolves the Window's ViewModel, so the ViewModel takes it in its constructor and needs no holder that the code calling OpenWindowAsync fills in:

services.AddRunicWindowService();

With ReactiveUI:

public sealed partial class EditorViewModel : ReactiveObject
{
    public EditorViewModel(IRunicWindowContext window)
    {
        // Closes this Window as its close button does: GuardClose and the navigation guards decide.
        QuitCommand = ReactiveCommand.CreateFromTask(async token =>
        {
            if (!await window.RequestCloseAsync(token)) StatusText = "Save or discard the draft first.";
        });
    }

    [Reactive] public partial string StatusText { get; set; } = "";

    public ReactiveCommand<RxVoid, RxVoid> QuitCommand { get; }
}

With CommunityToolkit.Mvvm:

public sealed partial class EditorViewModel(IRunicWindowContext window) : ObservableObject
{
    [ObservableProperty] private string _statusText = "";

    [RelayCommand]
    private async Task QuitAsync()
    {
        if (!await window.RequestCloseAsync()) StatusText = "Save or discard the draft first.";
    }
}

• RequestCloseAsync(cancellationToken) is the Window's own RequestCloseAsync: it asks CanCloseAsync when the Window sets GuardClose, returns false with the Window open when that keeps it, and true once it closed. It works on Runic Desktop and on CS-WebUI, where the request is guarded although the browser's own close is not.
• The context belongs to the Window's DI scope. A scoped service that the ViewModel takes shares it; a navigation entry created with RunicNavigatorOptions.CreateEntryScopes has a scope of its own, so pass the context to it from the Window's ViewModel. Resolved outside a Window's scope, or called before the Window has opened, for example from the ViewModel's constructor, it throws InvalidOperationException.
• The interface has one method, so a test registers a fake that answers true or false after the application's services, or opens the Window and lets its guards decide.

Moved and renamed since 0.7.0-preview.6

The Window types and entry points of the two hosts were merged into this API, and package, build property and outlet names now match what they are. Earlier API shapes were removed rather than deprecated. Old build property names still work in 0.7.0-preview.7 with warning RUNICBRIDGE017, and Angular's RunicViewOutlet remains a deprecated alias; both are removed in the next preview. The old package IDs get no further releases, so replace them in PackageReference and Directory.Packages.props; namespaces are unchanged.

Before	Now
RunicWindow<T>(T dataContext) constructor	RunicWindow<T>(); OpenWindowAsync assigns DataContext
ReactiveRunicWindow<T> (Runic.Application.ReactiveUI, .Reactive)	RunicWindow<T>; Views keep ReactiveRunicView<T>
CsWebUiWindow<T>	RunicWindow<T>
provider.OpenWindow<TWindow, TViewModel>(factory) (CS-WebUI)	await host.OpenWindowAsync<TWindow>(options) on RunicCsWebUiHost
provider.OpenDesktopWindowAsync<TWindow, TViewModel>(desktop, surfaceOptions, factory, windowOptions)	await host.OpenWindowAsync<TWindow>(options) on RunicDesktopHost (SurfaceOptions, WindowOptions)
provider.ValidateWindow<TViewModel>(), which threw	host.ValidateWindow<TWindow>(options), which returns RunicWindowValidationResult
provider.ValidateDesktopWindow<TViewModel>(desktop, windowOptions)	host.ValidateWindow<TWindow>(options)
CsWebUiConfigurationException (Code, DiagnosticMessage, Remediation)	RunicWindowConfigurationException (Diagnostics)
DesktopConfigurationException from OpenDesktopWindowAsync	RunicWindowConfigurationException, with the Desktop exception as InnerException
DesktopEventLoop.Run(options, async desktop => ...) around OpenDesktopWindowAsync	RunicDesktopHost.Run(provider, options, async host => ...)
WebUiApplication.Wait() and WebUiApplication.Clean() in Program.cs	RunicCsWebUiHost.Run(provider, async host => ...)
window.Presentation.WaitForClose(), WebUiApplication.Wait()	await window.WaitForCloseAsync(cancellationToken)
IBridgeWindow.CloseAsync(TimeSpan)	IBridgeWindow.CloseAsync(TimeSpan, CancellationToken) and WaitForCloseAsync(CancellationToken)
SetRootFolder, SetSize, SetPort, Show, ShowWebView, StartServer on CsWebUiBridgeWindow<T> and CsWebUiWindow<T>	RunicWindowOptions, and UseWebView, ConfigureWindow and ShowWindow on RunicCsWebUiHost; NativeWindow remains
Log events 1050 CsWebUiWindowRegistrationMissing and 2001 DesktopWindowRegistrationMissing	Event 1050 WindowRegistrationMissing on every host
Package Runic.Application	Runic.Application.Views, its namespace
Package Runic.Application.CsWebUi	Runic.Application.Views.CsWebUi
Package Runic.Application.Desktop (assembly Runic.Application.Desktop.dll)	Runic.Application.Views.Desktop (Runic.Application.Views.Desktop.dll)
Package Runic.Application.Wpf (assembly Runic.Application.Wpf.dll)	Runic.Application.Views.Wpf (Runic.Application.Views.Wpf.dll)
Package Runic.Application.ReactiveUI	Runic.Application.Views.ReactiveUI
Package Runic.Application.ReactiveUI.Reactive	Runic.Application.Views.ReactiveUI.Reactive
Log category Runic.Application.Desktop (Desktop transport)	Runic.Application.Views.Desktop
RunicBridgeFrontendDir	RunicApplicationFrontendDirectory
RunicBridgeTypescriptDir	RunicApplicationFrontendGeneratedDirectory
RunicBridgeFrontendPackageManager	RunicApplicationFrontendPackageManager
RunicBridgeFrontendBuildCommand	RunicApplicationFrontendBuildCommand
RunicBridgeFrontendInstallCommand	RunicApplicationFrontendInstallCommand
RunicBridgeInstallFrontend	RunicApplicationFrontendInstallEnabled
RunicBridgeBuildFrontend	RunicApplicationFrontendBuildEnabled
RunicBridgeCopyFrontend	RunicApplicationFrontendCopyEnabled
RunicBridgeBuildEnabled	RunicApplicationFrontendGenerationEnabled
RunicBridgeCompositionType	RunicApplicationFrontendCompositionType
RunicBridgeModelAssembly	RunicApplicationFrontendModelAssembly
RunicBridgeReactiveUiFlavor	RunicApplicationFrontendReactiveUiFlavor
RunicBridgeDocumentation	RunicApplicationFrontendDocumentationEnabled
RunicBridgeBootstrap (set in the bootstrap pass)	RunicApplicationFrontendBootstrap; the old name is still set in this preview
RunicViewsWindowProject	RunicApplicationFrontendWindowProject
RunicBridgeFrontendInput items	RunicApplicationFrontendInput items
RunicViewOutlet (@runic-artifex/angular)	ViewOutlet, as in React, Vue and Svelte; the element stays <runic-view-outlet>

Runic Desktop's window lifecycle is asynchronous too: see the Runic.Desktop guide for DesktopWindow.WaitForCloseAsync and signed window positions.

The First Window, CommunityToolkit Notes, and Reactive Notes examples exercise the packaged graph and generated client.

Threading: state after await

A Window's ViewModels belong to its model context. The bridge reads state, applies property writes and starts commands in that context's turns, one at a time. The rule for both MVVM flavours is:

Set ViewModel state directly in a command, also after await. Leave the model context only for work that doesn't touch ViewModel state, and commit its result with IRunicModelContext.InvokeAsync.

[RelayCommand] // or ReactiveCommand.CreateFromTask(SaveAsync)
private async Task SaveAsync(CancellationToken token)
{
    Status = "Saving";
    await _store.SaveAsync(Title, token);
    Status = "Saved";               // runs in a model-context turn
}

• A command body that the bridge starts, such as a CommunityToolkit [RelayCommand] or AsyncRelayCommand, or a ReactiveUI ReactiveCommand.CreateFromTask, runs in a turn. Every await in it, nested ones included, resumes in a later turn of the same context, as code on a UI thread resumes on that thread. Writes after await are applied in the order in which the continuations become ready, and no other turn runs between two awaits of one body.
• Code after ConfigureAwait(false), in Task.Run, in a timer or in another background callback runs outside the context. Do CPU-bound or blocking work there, then commit: await modelContext.InvokeAsync(() => Items = loaded). Don't use ConfigureAwait(false) in ViewModel code that writes state after it. Code you start outside a turn, such as a test that calls ExecuteAsync directly, also commits with InvokeAsync.
• Don't block a turn on asynchronous work (.Result, .Wait(), GetAwaiter().GetResult()): its continuation needs the same context, so it deadlocks, as it would on a UI thread.
• When the context closes, for example because the window closed, pending continuations run on the thread pool outside any turn, so the command still finishes; nothing presents its later writes.
• With WPF, DispatcherModelContext is the UI dispatcher, so the same rule holds there. Navigation hooks are not commands; see Runic.Navigation threading.

Read-only and settable state

The web frontend is the View, so the usual MVVM rule applies: the ViewModel's C# accessibility is the contract, and the View decides what it binds two-way. A property with a public getter is state every View can read. A property with a public setter is also what any View, whether WPF, Avalonia or the web frontend, may set, so its generated client gets a set<Property> member. A private set keeps the property read-only to every View. Runic adds no attribute of its own.

Give status that the ViewModel computes, such as IsDirty, Error, a status message or a collection, a private setter, and keep form fields publicly settable. With CommunityToolkit.Mvvm 8.4, declare partial properties:

public sealed partial class EditorViewModel : ObservableObject
{
    // Form field: the client has setTitle.
    [ObservableProperty]
    public partial string Title { get; set; }

    // Status: read-only to every View, so there is no setIsDirty or setError.
    [ObservableProperty]
    public partial bool IsDirty { get; private set; }

    [ObservableProperty]
    public partial string? Error { get; private set; }
}

A field such as [ObservableProperty] private bool isDirty; generates a property with a public setter, so the web frontend could set it too. With ReactiveUI, give the property a private setter, either written out with RaiseAndSetIfChanged or generated with [Reactive]:

public sealed partial class EditorViewModel : ReactiveObject
{
    private bool _isDirty;

    // Form field: the client has setTitle.
    [Reactive]
    public partial string Title { get; set; } = "";

    public bool IsDirty
    {
        get => _isDirty;
        private set => this.RaiseAndSetIfChanged(ref _isDirty, value);
    }

    // The same with the ReactiveUI source generator.
    [Reactive]
    public partial string? Error { get; private set; }
}

The comment on each generated set<Property> member names the C# setter it calls. To ask the ViewModel to change its status, call a command. The React, Vue, Svelte and Angular guides show reading state one way and binding a settable property two way. The dotnet new runic-app template's Counter page has one of each.

Generated TypeScript

Each ViewModel gets a module named after it (EditorViewModel becomes editor.ts) with its state interface, client interface and connect function. Every enum, DTO and union in the contract becomes one exported type named after its C# type, declared once in types.ts beside the modules. Next to each client, editor.mock.ts exports a typed test double, mockEditor; see Testing. Applications import it only from tests, so it is not bundled. Each module imports the named types it uses and re-exports them, so import type { DocumentPane } from "./generated/document.js" works too:

/// <summary>A pane of the document page.</summary>
public enum DocumentPane { Editor, Preview }

/// <summary>The pane the document currently shows.</summary>
public DocumentPane ActivePane => ...;

/** A pane of the document page. */
export type DocumentPane = "Editor" | "Preview";

• Numbers follow JavaScript's precision: int, double and smaller numeric types are number, but long, ulong and BigInteger are bigint, because a number is exact only up to 2^53, and decimal is a string. The wire carries 64-bit and big integers as strings; the generated client converts them, and the member's TSDoc says so. Use BigInt(value) and Number(value) to convert at the edges, or model a small count as int.
• An enum is a union of its wire names (the C# names, or [RunicAlias]), so a switch over it can be exhaustive.
• A DTO is an interface with its wire member names. A [RunicUnion] is a union of its cases, discriminated by $case.
• Two C# types with the same name, or a name that collides with a generated declaration such as EditorState, are qualified with their namespace segments, for example NotesItem and TasksItem. Names are stable while no new collision appears: adding a same-named type, even in another ViewModel, qualifies the existing one too. Import the name your frontend uses and expect a compile error, not a silent change, when that happens. Generic DTOs append their arguments, for example PageOfNoteRow or BoxOfArrayOfInt32, and an enum without cases is never. A C# type named like a TypeScript reserved word is qualified the same way.
• A generic DTO member declared T is nullable where the type argument is: value: string in Box<string> and value: string | null in Box<string?>, and the generated .NET reader rejects null for the former. A member declared T? is always nullable. Members inherited from a generic base class follow the derived type's arguments, and a type without annotations, such as [RunicCommandInput(typeof(Box<string>))] or a union case, is non-nullable throughout. Uses whose members differ get separate declarations, for example BoxOfString and BoxOfNullableOfString; a type used only as Box<string?> keeps the plain name BoxOfString.
• A content property is the union of the PageReference types of the ViewModels it can present, such as DocumentPageReference | HomePageReference, with | null when it is nullable. A NavigationRegion<TContent> slot always includes | null, because a region can be empty; see Navigation. The frameworks' ViewRegistry<State["main"]> accepts such a nullable type.
• <summary> comments on ViewModels, state properties, commands, interactions, DTO types and members, and enum cases become TSDoc. A summary on a partial property such as [ObservableProperty] public partial bool IsDirty { get; private set; } or [Reactive] public partial string Name { get; set; } is read from its declaring part in source, found through the assembly's PDB (source paths that a CI or deterministic build maps, such as /_/, resolve through the project's source roots), because the XML documentation file keeps the generated part's <inheritdoc/>. The bootstrap pass writes the XML documentation file for this; a project that already writes one keeps its own. Set RunicApplicationFrontendDocumentationEnabled=false to skip it. A separate model assembly (RunicApplicationFrontendModelAssembly) needs its XML documentation file beside it. A command generated from a method, such as a CommunityToolkit [RelayCommand], uses the method's comment. The C#-only comments an MVVM library's source generator writes are not copied; a generated cancel command is documented as cancelling its command.
• A command argument keeps the parameter name of its CommunityToolkit [RelayCommand] method. Other commands name it after the argument's C# type, or input for scalars.

The generated C# bridge calls public runtime types such as ViewModelBridge<T>, PropertyDescriptor<T>, CommandDescriptor<T>, BridgeWire, BridgeJson and ReactiveCommandExecution. They are marked [EditorBrowsable(Never)], so they stay out of completion lists; they are not application API.

Generator diagnostics

The generator reports every ViewModel's first problem in one build. An error names the type and member, and points at its source line when the inspected assembly has a portable PDB (the default). A member generated by a source generator, such as a CommunityToolkit [ObservableProperty], points at its [RelayCommand] method or its declaring class. Each error and warning ends with a link to its entry in the diagnostics catalog for the release you use.

RUNICBRIDGE013 is a warning and repeats on builds that reuse the cached generation; every other code fails the build.

The build targets run the generator and the frontend install and build commands without wrapping their failures in MSB3073. Errors and warnings that a command prints in the canonical file(line,column): error CODE: text form, such as the generator's, tsc's and vue-tsc's, are logged once with their code and location. When svelte-check fails, its errors and warnings are logged at their file, line and column too, without a code, from its default output or --output machine; warnings of a check that passes stay in the command output. A failed command that reports no such error gets one summary error instead: RUNICBRIDGE001 for the generator, RUNICBRIDGE016 for the frontend commands.

ID	Problem	Fix
RUNICBRIDGE001	Invalid generator invocation or build configuration, such as a malformed RunicApplicationFrontendCompositionType; also an internal generator error.	Correct the build property. Report an internal error with the ViewModel that triggers it.
RUNICBRIDGE002	A CommunityToolkit ObservableValidator ViewModel in a Native AOT publish.	Publish framework-dependent until its validation is verified under AOT.
RUNICBRIDGE003	A state, command, or interaction value type is not a supported bridge value. The message names the member path, for example EditorViewModel.Current.value.	Use a supported scalar, collection, public DTO, [RunicUnion] or [RunicBridgeCodec] type. Exclude computed DTO properties with [RunicIgnore], or opt into unconditional [JsonIgnore] as described under Shared DTO exclusions. A NavigationResult<T> holds .NET content: return its Outcome, a NavigationOutcome, as CreateBackCommand() does.
RUNICBRIDGE004	Two generated names collide: ViewModel names, presentation kinds, state wire names, the reserved revision and validation fields, interactions, routes, client members, or generated files (including a hand-written types.ts). Also a command whose client member is reserved in a TypeScript object: new (a construct signature), then (a thenable client), constructor, __proto__, toJSON, toString or valueOf.	Rename one member, or set a state wire name with [RunicAlias]. Rename a reserved command; the message suggests a name, such as CreateCommand for NewCommand.
RUNICBRIDGE005	The model assembly or one of its dependencies could not be loaded.	Check the bootstrap output and package versions.
RUNICBRIDGE006	The assembly has no Window or View class (an error only when generation is required, see Build properties), one is not public, top-level, concrete and closed, or a View contract is invalid, duplicated or missing.	Make the class public and top-level; give each [RunicViewContract] a unique letters-and-digits name.
RUNICBRIDGE007	A ViewModel does not implement INotifyPropertyChanged, is not a public top-level class, or has no state, command or interaction.	Change the ViewModel declaration.
RUNICBRIDGE008	A state property has no public getter or an empty wire name, ViewModel content or a NavigationRegion<TContent> slot has a public setter, a ViewModel collection's item type has no registered View, or a region slot is a NavigationRegion<object> or has no ViewModel with a registered View that is a TContent.	Add a getter, make content read-only to the web view, register a View for the item or content type, or give the region a specific content interface or base class.
RUNICBRIDGE009	A command's name does not end with Command, its shape is unsupported, or a non-ReactiveUI command has [RunicCommandResult].	Rename the command or use a supported CommunityToolkit, ReactiveUI or [RunicCommandInput] command.
RUNICBRIDGE010	A [RunicCollection] member is not a read-only, non-nullable collection of DTO rows, or its key is not a non-nullable string, Guid or Int32 row property.	Change the collection or its key.
RUNICBRIDGE011	A ReactiveUI interaction has no public getter or has a public setter.	Expose the interaction as a get-only property.
RUNICBRIDGE012	A [RunicFailure] is on a member that is not a Bridge command or its [RelayCommand] method, is on both the property and the method, or names object, an exception or Nullable<T>.	Declare one failure type per command: any Bridge value type, typically a DTO, an enum or a [RunicUnion]. A failure type the Bridge cannot encode is RUNICBRIDGE003 at {Model}.{Command}.failure.
RUNICBRIDGE013 (warning)	A public, concrete ViewModel in the application assembly can be put in a ViewModel content property, ViewModel collection or NavigationRegion<TContent> slot, but has no View, so pushing or presenting it throws NotSupportedException naming the slot and the type. Broad slot types are not checked: framework types (System, ReactiveUI, CommunityToolkit), a type the slot's owner or a Window's ViewModel also derives from or implements, and a type that more than eight ViewModels without a View could fill.	Add a View, such as public sealed partial class SettingsView : RunicView<SettingsViewModel>; (with the slot's [RunicViewContract], if any). If the type is never presented there, suppress the warning with <NoWarn>$(NoWarn);RUNICBRIDGE013</NoWarn>.
RUNICBRIDGE014	A project declares Runic Views, but RunicApplicationFrontendDirectory does not exist or, with the default build command, has no package.json. Reported by the build targets.	Create the frontend there, point RunicApplicationFrontendDirectory at your frontend package, or set RunicApplicationFrontendBuildEnabled=false when another tool builds it.
RUNICBRIDGE015 (warning)	A project declares a Runic Window or View, but generation is off because it references no host adapter and not Runic.Application.Views directly, so no bridge, AddRunicViews() or client is generated. Reported by the build targets.	Set RunicApplicationFrontendGenerationEnabled=true to generate in that project, or false when another project generates its Views.
RUNICBRIDGE016	The frontend install or build command failed without reporting an error in the canonical file(line,column): error CODE: text form or as a svelte-check error (a Vite or package-manager error, for example), or the install created no node_modules for declared dependencies. The command's own output is printed above the error. Reported by the build targets.	Fix the problem the command reports, or run dotnet runic doctor to check the package manager and lock file.
RUNICBRIDGE017 (warning)	The project, an import or the command line sets a build property or item by its 0.7.0-preview.6 name (RunicBridge*, RunicViewsWindowProject). The old name still applies in this preview. Reported by the build targets.	Rename it to the RunicApplicationFrontend* name in the message; see the rename table.

Build properties

The build targets ship in this package and apply to every project that references a host adapter. A project needs none of these properties unless it departs from the conventional Frontend folder. Every property starts with RunicApplicationFrontend, as do the development-server properties of dotnet runic dev. The RunicBridge* and RunicViewsWindowProject names of 0.7.0-preview.6 and earlier still apply in this preview with warning RUNICBRIDGE017; the rename table maps them. Underscore-prefixed properties, RunicApplicationHostAdapter (which the host adapters set) and the RunicViews* target names are not settings.

Property	Default	Purpose
RunicApplicationFrontendDirectory	$(MSBuildProjectDirectory)/Frontend	Frontend package directory.
RunicApplicationFrontendGeneratedDirectory	<frontend>/src/generated	Generated TypeScript clients.
RunicApplicationFrontendPackageManager	packageManager in package.json, then pnpm-lock.yaml, bun.lock, else npm	Selects the default build and install commands.
RunicApplicationFrontendBuildCommand	npm run build, pnpm run build, or bun run --bun build	Production frontend build.
RunicApplicationFrontendInstallCommand	npm ci, pnpm install --frozen-lockfile, or bun install --frozen-lockfile, each with --ignore-scripts	Runs before the frontend build when package.json or a lock file changed since the last install (stamp: obj/runic-bridge-frontend-install.stamp), or when declared dependencies have no node_modules. A package.json without dependencies needs no node_modules.
RunicApplicationFrontendInstallEnabled	true	Set false when a workspace install owns the frontend packages.
RunicApplicationFrontendBuildEnabled	true	Set false when another tool owns the frontend build. TypeScript is still generated.
RunicApplicationFrontendCopyEnabled	true	Copies <frontend>/dist to www/ in the build output. Publish always copies it.
RunicApplicationFrontendCompositionType	<project name>.RunicBridgeComposition (host adapters)	Generated composition class.
RunicApplicationFrontendModelAssembly	the project itself	Inspect a separately built ViewModel assembly instead of a bootstrap build.
RunicApplicationFrontendReactiveUiFlavor	none	primitives or reactive for ReactiveUI projects.
RunicApplicationFrontendGenerationEnabled	see below	true requires Bridge generation, false turns it off.
RunicApplicationFrontendDocumentationEnabled	true	false skips the XML documentation file that the bootstrap pass writes for comments in the generated TypeScript.
RunicApplicationFrontendWindowProject	false	true marks a Window project that dotnet runic dev and doctor run.

dotnet build and dotnet publish copy the built frontend as loose files to www/ next to the executable; they do not embed it. dotnet runic dev sets RunicApplicationFrontendBuildEnabled=false and RunicApplicationFrontendCopyEnabled=false while its development server serves the frontend. The dotnet runic README lists the development-server properties it reads.

Bridge generation turns on by default for a project that references Runic.Application.Views directly, references a host adapter (Runic.Application.Views.Desktop, Runic.Application.Views.CsWebUi or Runic.Application.Views.Wpf) directly or transitively, or builds in this repository. Each adapter's build props set RunicApplicationHostAdapter to true, which is what the targets check. A project that reaches Runic.Application.Views only through another project, such as a library that references a model project, does not generate; if it declares a Window or View anyway, the build warns with RUNICBRIDGE015. A test project (IsTestProject is true, which Microsoft.NET.Test.Sdk and TUnit set) that references a Runic application uses the application's generated Bridges: when it declares no Window or View of its own and has no frontend package.json, RunicApplicationFrontendInput items or RunicApplicationFrontendModelAssembly, generation stays off there without a diagnostic or a bootstrap build. Any other project keeps the diagnostics below. That default is optional: when the assembly declares no Runic Window or View, for example an application that only uses the navigator, the build generates nothing (and removes stale generated C# and TypeScript), skips the frontend install, build and copy steps, and succeeds with a normal-importance message. A publish that reuses an earlier build (--no-build) still copies an existing <frontend>/dist to www/. Generation is required, and RUNICBRIDGE006 stays an error for an assembly without a Window, when the project sets RunicApplicationFrontendGenerationEnabled to true, or when it shows that it expects output: a package.json in RunicApplicationFrontendDirectory, RunicApplicationFrontendInput items, RunicApplicationFrontendCompositionType or RunicApplicationFrontendModelAssembly. Projects that reference Runic.Application.Views.CsWebUi or Runic.Application.Views.Desktop, other than those test projects, always require generation, because the adapter sets RunicApplicationFrontendCompositionType. WPF projects (UseWPF) stay optional, because a WPF application often keeps its Views in a WPF-free model project. A navigator-only application should set RunicApplicationFrontendGenerationEnabled to false: that also skips the nested bootstrap build, which optional mode still pays for.

When the Views live in a referenced project, the application copies that project's built <frontend>/dist to its own www/ on build and publish, so the application itself can set RunicApplicationFrontendGenerationEnabled to false. The application's own frontend wins over a referenced one with the same file. WPF's temporary markup-compile project (*_wpftmp) never runs the generator; it compiles the code that the real project generated.

A single-project application is compiled twice: a bootstrap pass with an empty generated composition, which the generator inspects, then the real build with the generated code. RunicApplicationFrontendBootstrap is true only in the bootstrap pass.

Typed commands and checked writes

CommunityToolkit IRelayCommand<T> and IAsyncRelayCommand<T> use the same supported input shapes and generated codecs as ReactiveUI commands: scalars, nullable values, DTOs, collections, unions, and explicit custom codecs. Generated canX(input) queries use the actual parameter. Async Toolkit commands expose startX, completion, recovery, and cancellation, with no invented result value. Toolkit cancellation calls the command's Cancel() and therefore targets its current execution; it does not isolate concurrent invocations of the same command instance.

[RelayCommand(IncludeCancelCommand = true)] is supported too. Toolkit's generated SaveCancelCommand exposes parameterless saveCancel() and canSaveCancel state in the web client. It executes the actual generated cancel command, so availability follows Toolkit's CanBeCanceled and cancellation targets the same native SaveCommand. The existing startSave operation still owns admission, completion, recovery, and View lifetime cancellation. Recognition requires Toolkit's generator metadata, the paired async command, and its cancellation-token method with IncludeCancelCommand = true; other plain ICommand properties still require [RunicCommandInput].

Checked-write receipts decode their values just like state: for example, an Int64 receipt's snapshot.value or conflict's incoming.value is a bigint. An identical request ID and payload replays its receipt. Reusing an ID with a different baseline version, baseline value, or new value produces a conflict. Each field retains at most 64 receipts within a default 256 KiB encoded-value and receipt-metadata budget. Evicted or oversized receipts leave bounded request-ID tombstones; reconcile authoritative state before issuing a new ID. The write may have succeeded even when its receipt could not be retained.

Batch synchronous model updates

using (BridgeSnapshotBatch.Begin(document))
{
    document.Title = imported.Title;
    document.Content = imported.Content;
    document.Language = imported.Language;
}

Nested scopes on the same model capture and publish one final snapshot per attached bridge when the outer scope ends. Notifications still advance the revision, and direct command/checked-write replies capture current state immediately. A batch controls snapshot work; it does not make mutations atomic or own the model scheduler. Enter the model's execution context as usual and keep the batch around synchronous changes, after awaiting I/O. The translations editor uses this during bulk document reconciliation and command result updates.

Structured validation

Models and nested DTOs may implement INotifyDataErrorInfo. Generated snapshots then expose validation.hasErrors, validation.truncated, and validation.errors, alongside existing property error string arrays. Each error contains a path of wire property names, list indexes, or dictionary keys and a message. Root entity errors use an empty path. Aliases are preserved, so a postal-code error might have the path ["profile", "postal-code"].

Return a BridgeValidationMessage from GetErrors to supply a stable Code, Severity, or relative CLR MemberPaths. Ordinary strings and ValidationResult remain supported. Generated metadata resolves paths without runtime reflection. Traversal and messages are bounded; truncated reports when validation details are incomplete. Nested ErrorsChanged notifications publish new validation state, and removed objects lose their subscriptions. This projection works across model frameworks; it does not run validation rules itself.

Use [RunicIgnore] on validation-only DTO properties such as a computed HasErrors; their values are represented by the validation projection.

Shared DTO exclusions

Public readable DTO properties are part of the Bridge contract, including computed properties. A DTO reader needs a public constructor matching its included properties, or public setters with a public parameterless constructor. Use [RunicIgnore] to leave a property on .NET without exposing it to the Bridge. By default, [JsonIgnore] affects System.Text.Json serialization only; it does not exclude a Bridge property.

For shared DTOs that already use System.Text.Json attributes, opt in from the assembly declaring the ViewModel:

using Runic.Application.Views;

[assembly: RunicBridgeJsonIgnore]

For example, a separate Core assembly can keep its DTO free of Runic dependencies:

using System.Text.Json.Serialization;

public sealed record HistoryQuery(string Message, string Author)
{
    [JsonIgnore]
    public bool IsFiltered => Message.Length != 0 || Author.Length != 0;
}

The opt-in excludes [JsonIgnore] and [JsonIgnore(Condition = JsonIgnoreCondition.Always)] throughout that ViewModel's DTO graphs, including nested DTOs, collections, command values, interactions and declared failures. Generated codecs, TypeScript, validation traversal and the Hot Reload fingerprint use the same exclusions. The Core assembly and the assembly declaring only a Window need no marker. Each ViewModel uses its own declaring assembly's policy.

Never, WhenWritingNull and WhenWritingDefault remain included; Bridge fields have a fixed contract and are emitted even when null or default. [RunicIgnore] always excludes a property, including one also marked [JsonIgnore(Condition = JsonIgnoreCondition.Never)]. Root ViewModel properties still use [RunicIgnore]; this opt-in applies to DTO properties only. Adding the marker can change an existing DTO wire contract and requires rebuilding the .NET application and generated frontend together.

Failure detail in development

A command, setter, checked write or operation that throws replies with a bounded message such as "Save failed." and is logged (see Logging and telemetry). In development the error also carries detail: {type, message, stack}, which the TypeScript runtime exposes as BridgeError.detail and the Vite plugin shows in DevTools. Development means DOTNET_ENVIRONMENT (or else ASPNETCORE_ENVIRONMENT) is Development, which dotnet runic dev sets. Set BridgeDiagnostics.IncludeFailureDetail to true or false to choose explicitly. Detail can contain file paths and application data, so do not enable it for a distributed build.

Declared failures

A command declares its expected failure type with [RunicFailure(typeof(SaveFailure))] on the command property or on its CommunityToolkit [RelayCommand] method, and signals it by throwing RunicFailureException(new TitleRequired()). The Bridge replies domain-failed with the encoded failure, in every environment and never with detail; an operation ends with the domain-failed status. Use [RunicUnion] for several cases. A value that is not the declared type, one that cannot be encoded, or one over 4 KiB is reported as an ordinary failure (event 1008). A synchronous command reports only failures thrown synchronously from Execute, so an async void plain ICommand cannot declare a failure that it throws after its first await. Operations exist only for asynchronous CommunityToolkit and ReactiveUI commands.

[RunicUnion(typeof(TitleRequired), typeof(TitleTaken))]
public abstract record SaveFailure;
[RunicUnionCase("titleRequired")] public sealed record TitleRequired : SaveFailure;
[RunicUnionCase("titleTaken")] public sealed record TitleTaken(string ExistingTitle) : SaveFailure;

[RelayCommand, RunicFailure(typeof(SaveFailure))]
private async Task SaveAsync(CancellationToken token)
{
    if (string.IsNullOrWhiteSpace(Title)) throw new RunicFailureException(new TitleRequired());
    // ...
}

The generated client's save() then returns Promise<BridgeOutcome<EditorState, SaveFailure>> instead of Promise<EditorState>, its operation is BridgeOperation<void, SaveFailure>, and the generated mock encodes the failure. A ReactiveUI command also reports the failure on ThrownExceptions; the generated Bridge observes it there and does not log it, so the ViewModel needs no subscription.

Navigation

Navigation comes from the Runic.Navigation package, which this package depends on. Its types, and the model context types, are in the Runic.Navigation namespace. Its README covers regions, transitions, ownership, results and disposal. Register it per window scope with services.AddRunicNavigation(), and give the window's session the navigator's model context; AddRunicNavigation() does that for windows opened from the scope.

Content slots

Expose a region as a get-only NavigationRegion<TContent> property, and the generator treats it as a content slot. The snapshot presents Current, or clears the slot when the region is empty. The wire value and TypeScript type are those of a nullable content property (for example DocumentPageReference | HomePageReference | null), so moving a non-null content property to a region adds | null, and the frontend must render the empty state. A ViewRegistry<State["main"]> accepts the nullable slot type and checks its non-null kinds. TContent must be an interface or base class of ViewModels with registered Views. A NavigationRegion<object> slot, or one with a public setter, is RUNICBRIDGE008.

The generated Bridge observes the region's Current and binds the window session to the navigator, so retiring owned content forgets its routes. A navigator serves one window, and the session must use the navigator's model context. Nothing is forwarded by hand, and ViewOutlets and test drivers (View<T>(vm => vm.Main)) work unchanged. Going back presents the same PageReference id with the retained model, and the outlet mounts a fresh View. Browser reload, remount and disconnect never touch entries.

Stale-route window. A commit retires and forgets departing owned content right away, while the frontend sees the new Current only with the next state capture. Until then it may still hold the old reference. An invocation on that reference is rejected like any forgotten route, and the outlet may briefly show the old View or nothing. The next capture converges.

Logging and telemetry

OpenWindowAsync passes the scope's ILoggerFactory to the Window's WindowContentSession when one is registered, for example with services.AddLogging(...). A session constructed directly takes it as loggerFactory. Without a factory, each entry is written to System.Diagnostics.Trace (TraceError, or TraceWarning for warnings) as its formatted message followed by the exception; a failed View mount goes to standard error instead.

Every entry carries the exception, in every environment: logs belong to the operator, whereas BridgeDiagnostics governs only what a Bridge reply shows to web content. Exception messages can contain application data, so treat log output accordingly. The message properties are listed per event.

Event ID	Name	Level	Logged when	Properties
1000	BridgeCommandFailed	Error	A command throws.	Model, Member, Route, ErrorType
1001	BridgeSetterFailed	Error	A property setter throws.	Model, Member, Route, ErrorType
1002	BridgeFieldWriteFailed	Error	A checked field write throws.	Model, Member, Route, ErrorType
1003	BridgeOperationAdmissionFailed	Error	Admitting an operation throws.	Model, Member, Route, ErrorType
1004	BridgeOperationFailed	Error	An admitted operation throws.	Member, ErrorType
1005	BridgeOperationCancellationCallbackFailed	Warning	A cancellation callback throws while a Window closes.	ErrorType
1006	BridgeCommandDomainFailed	Debug	A command throws its declared failure.	Model, Member, Route, FailureType
1007	BridgeOperationDomainFailed	Debug	An admitted operation throws its declared failure.	Member, FailureType
1008	BridgeDomainFailureNotEncoded	Error	A RunicFailureException from a command or operation is undeclared, cannot be encoded or is over 4 KiB, so it is reported as failed. When the encoder threw, the entry carries both exceptions.	Model, Member, Route, FailureType, Reason
1010	BridgeSnapshotCaptureFailed	Error	A state snapshot writer throws.	Model, Route, ErrorType
1011	BridgeSnapshotDeliveryFailed	Error	A host rejects a state or delta frame.	Model, Route, ErrorType
1012	BridgeCollectionKeysRejected	Error	A [RunicCollection] has a null row or a null, empty or duplicate key, so the route withholds its state.	Model, Field, Key, Route
1013	AcceptedWorkFailed	Error	A task owned by an AcceptedWorkScope faults.	ErrorType
1014	AcceptedWorkDomainFailed	Debug	A task owned by an AcceptedWorkScope throws a declared failure.	FailureType
1020	ViewMountFailed	Error	A .NET View fails to mount.	Route, ErrorType
1021	ViewRemountFailed	Error	A View fails to mount again after a reconnect.	Route, ErrorType
1033	ModelContextReleaseFailed	Error	A window session's own model context fails to shut down in the background.	ErrorType
1040	RoutedRegionRouteIncompatible	Error	A ReactiveUI ReactiveRoutedRegion<T> receives a ViewModel that is not a T, so it presents no content.	Region, Model
1041	RoutedRegionRouterFailed	Error	The router observed by a ReactiveRoutedRegion<T> fails; the region keeps its last content.	Region, ErrorType
1042	ReactiveCommandFailed	Error	A bridged ReactiveUI command reports on ThrownExceptions an exception that no Bridge call received, while a generated Bridge presents it, or a command or object observed with ObserveBridgeExceptions(logger) reports one; declared RunicFailureExceptions and cancellations are not logged.	Source (Model.Command for the Bridge; the command expression or sourceName for ObserveBridgeExceptions), ErrorType
1043	ReactiveSchedulerOutsideModelTurn	Warning	Work is scheduled on the model-context main-thread scheduler that AddRunicReactiveModelContext() installs, outside a model turn, so it runs on the scheduler it replaced. Logged once per ViewModel type found on the call stack, and once for work with no ViewModel there, such as a timer. Runic Desktop, CS-WebUI and RunicWindowTestHost.Create pass the application's logger factory when a window opens; before that, or without one, it goes to Trace.	Source, Fallback
1050	WindowRegistrationMissing	Error	ValidateWindow or OpenWindowAsync finds a Window's generated Bridge unregistered, on any host.	Code, DiagnosticMessage, Remediation
1051	WindowReleaseFailed	Error	IRunicWindowService fails to release a Window it opened, after that Window closed.	Window, ErrorType
1052	WindowCloseGuardFailed	Error	A guarded Window's CanCloseAsync throws; the window stays open. A departure guard that throws is logged by the navigator as 1060 instead.	Window, ErrorType
1053	WindowCloseGuardUnavailable	Warning	A Window with GuardClose opens on a presentation that cannot keep its window open, such as CS-WebUI or a browser, so a user close does not ask CanCloseAsync.	Window, Presentation
1090	ReactiveSchedulerKept	Information	A Runic Desktop, CS-WebUI or test-host window opens while ReactiveUI's main-thread scheduler is one set before AddRunicReactiveModelContext(), so Runic's was not installed. Once per scheduler type; a UI dispatcher's scheduler, such as WPF's, is not logged.	Scheduler
1091	ReactiveSchedulerReplaced	Warning	Such a window opens after another scheduler replaced the installed one, for example a later RxAppBuilder BuildApp() with WithPlatformServices(). Call WithRunicModelContext() on that builder. Once per scheduler type; a UI dispatcher's scheduler is not logged.	Scheduler
2000	DesktopSnapshotDeliveryFailed	Error	Runic Desktop cannot run a state delivery script.	Route, ErrorType
2002	DesktopWindowCloseAfterOpen	Warning	RUNIC_APPLICATION_CLOSE_AFTER_OPEN=1 closed a Desktop window right after it opened.	Variable
2003	DesktopPortalDiagnostic	Warning or Debug	On Linux, xdg-desktop-portal reports a diagnostic for the notifications AddRunicPlatformServices() registers: Warning for a rejected identity, an unavailable portal or a denied request, Debug for an accepted request or registration.	Code, DiagnosticMessage, Remediation
3000	WindowCloseCancellationCallbackFailed	Error	A Runic Desktop close-cancellation callback throws.	ErrorType
3001	WindowCloseConfirmationFailed	Error	A Runic Desktop close confirmation throws; the Window stays open.	ErrorType
3002	DesktopConfigurationInvalid	Error	DesktopHost.Validate finds a check that fails the window request.	Code, Option, DiagnosticMessage, Remediation
3003	DesktopConfigurationLimited	Warning	DesktopHost.Validate finds an option the presentation ignores or narrows, or a browser fallback opens without a permission grant.	Code, Option, DiagnosticMessage, Remediation
3004	BrowserLaunchStalled	Warning	A browser was still running but had requested nothing after ConnectionTimeout, so Runic Desktop relaunched it (DesktopHostOptions.BrowserLaunchAttempts).	Attempt (the attempt that stalled), Attempts, TimeoutSeconds
3005	BridgeHandshakeExpired	Warning	A Bridge WebSocket sent no token check, so Runic Desktop closed it and the page reconnects: after 10 seconds, or after 2 seconds when a newer WebSocket needs the only connection.	ConnectionId, SilentSeconds, Reason

Model is the ViewModel type name, Member the command or property, and Route the Bridge route (a content presentation's route is per instance, such as content12). A declared failure is an expected outcome, so it is logged at Debug, still with its exception; FailureType is the failure value's type.

Events 1000-1012, 1020, 1021 and 1050-1052 use the category Runic.Application.Views (RunicViewsTelemetry.LogCategory). Event 1033 here is a window session's own model context failing to shut down, and uses the session's logger. Events 1013 and 1014 use the logger passed to new AcceptedWorkScope(logger), or Trace without one. Navigation (1060-1075) and the model context (1030-1033) log under Runic.Navigation and Runic.Navigation.RunicModelContext; see the Runic.Navigation logging section. Events 2000 and 2002 use Runic.Application.Views.Desktop. Events 3000-3005 use Runic.Desktop and need DesktopHostOptions.LoggerFactory.

Events 1040-1043, 1090 and 1091 come from Runic.Application.Views.ReactiveUI (and its .Reactive flavor) and also use Runic.Application.Views. Events 1040 and 1041 need the ReactiveRoutedRegion<T>(router, loggerFactory) constructor, which a ViewModel can call with an ILoggerFactory injected from DI, as examples/notes-reactive-views does. Without a factory, the region writes the same messages to Trace as described above. Region is the region's ViewModel type name T. The Bridge logs event 1042 through the Window's logger factory, with the ViewModel type and command property, such as EditorViewModel.SaveCommand, as Source; an exception that a Bridge call received is logged as 1000 or 1004 instead. ObserveBridgeExceptions(logger) replaces that logging for its command and uses the logger passed to it; Source is the observed command's expression, such as SaveCommand, or sourceName.

Bridge calls are traced by the ActivitySource and measured by the Meter named Runic.Application.Views (RunicViewsTelemetry.ActivitySourceName and MeterName). With OpenTelemetry, call AddSource(...) and AddMeter(...) with these names. Tag values are generated contract names, outcomes and exception types, never arguments or state.

Span	Covers
runic.bridge.command	A synchronous or awaited command.
runic.bridge.set	A property setter.
runic.bridge.write	A checked field write.
runic.bridge.operation.start	Admitting an operation.
runic.bridge.operation	Running an admitted operation.

Spans are named <kind> <Model>.<Member> and have the tags runic.bridge.kind, runic.bridge.model, runic.bridge.member, runic.bridge.route and runic.bridge.outcome. The outcome is ok, rejected, cancelled, failed, domain_failed or disconnected. An admission can also be unavailable, capacity, duplicate or expired, and an operation whose result could not be delivered is delivery_failed. A failed span has the status Error and error.type. A domain_failed call is an expected outcome: its span status stays Unset, it has no error.type, and runic.bridge.failures does not count it.

Instrument	Type	Unit	Tags
runic.bridge.calls	Counter	{call}	kind, model, member, outcome, error.type
runic.bridge.call.duration	Histogram	s	kind, model, member, outcome, error.type
runic.bridge.failures	Counter	{failure}	kind (a call kind, snapshot.capture, snapshot.delivery, collection.keys or mount), model, member, error.type
runic.bridge.snapshot.frames	Counter	{frame}	model, runic.bridge.frame (state, delta or failure)
runic.bridge.snapshot.size	Histogram	By	model, frame
runic.bridge.snapshot.delivery.duration	Histogram	s	model, frame
runic.bridge.snapshot.recoveries	Counter	{snapshot}	model
runic.bridge.snapshot.queue.depth	ObservableUpDownCounter	{frame}	none

Route names are span tags but not metric tags, because content routes identify one instance. The duration histograms advise buckets from 0.5 ms to 10 s, and the size histogram from 256 B to 4 MiB. The queue depth reports the frames waiting for slow hosts across all Bridges when it is observed.

Incremental generation

The generator caches successful multi-view generation in its C# obj output directory. It hashes the model/dependency assemblies, generator assemblies, output-affecting options, and generated output contents. Missing or edited outputs invalidate the cache; unchanged generated files keep their timestamps. Set RUNIC_BRIDGE_CODEGEN_FORCE=1 or RUNIC_BRIDGE_CODEGEN_CACHE=0 to bypass it.

The configured frontend build command (RunicApplicationFrontendBuildCommand) runs only when one of its inputs is newer than obj/<Configuration>/<TargetFramework>/runic-bridge-frontend.stamp: files under RunicApplicationFrontendDirectory (excluding node_modules, dist, build, bin, obj and dot-directories), the generated TypeScript, and the project file. Generated TypeScript is rewritten only when its content changes, so C#-only edits skip the frontend build. Add RunicApplicationFrontendInput items, from a target that runs before RunicViewsGenerateBridge, for inputs outside the frontend directory such as workspace packages or generated assets; the translations editor is an example. Delete the stamp or rebuild to force a frontend build. Set RunicApplicationFrontendBuildEnabled=false when another tool, such as a running dev server, owns the frontend build; dotnet runic dev does this while its development server runs.

Generated C# and TypeScript files start with // <auto-generated />. Only such files are removed from the output directories when a ViewModel disappears, so RunicApplicationFrontendGeneratedDirectory may point into a directory with hand-written modules.

Accepted application work

Cancellation requests that a command stop; its invocation can finish before the underlying task completes. In particular, ReactiveUI bridge execution can settle a cancelled invocation and release its subscription while an uncooperative task continues recovery. WindowContentSession.BeginCloseAsync and its result's Completion drain bridge invocations. Their completion does not establish that independently accepted application work, durable changes, or recovery has finished.

Own such work with an AcceptedWorkScope in the scoped model or service that owns its resources. Pass a factory so ownership is registered before the task starts, and include recovery, publication, and cleanup in that task:

private readonly AcceptedWorkScope _acceptedWork;

public NotesModel(ILogger<NotesModel> logger) => _acceptedWork = new(logger);

public Task SaveAsync(CancellationToken cancellationToken) =>
    _acceptedWork.RunAsync(() => SaveAndRecoverAsync(cancellationToken));

public Task SaveAndCloseAsync(CancellationToken cancellationToken) =>
    _acceptedWork.RunAsync(async () =>
    {
        await SaveAndRecoverAsync(cancellationToken);
        // Closing disposes this model; that drain does not wait for this task.
        await _window.CloseAsync();
    });

public async ValueTask DisposeAsync()
{
    await _acceptedWork.DisposeAsync();
    // Release the model's resources after its accepted tasks finish.
}

RunAsync invokes its factory synchronously on the calling thread and returns the original task, preserving its result, failure, or cancellation. Concurrent drain includes accepted factories that have not yet returned a task. A null task or synchronous factory exception releases that admission and throws to the caller. Calls after drain starts throw InvalidOperationException without invoking the factory.

DrainAsync(cancellationToken) closes admission and waits for actual task completion, including faulted and cancelled tasks. Its cancellation token only stops that observer's wait; it neither cancels nor abandons owned work. DisposeAsync is idempotent and waits without cancellation. Keep resources alive through this drain even if a bridge invocation or an earlier drain observer has already been cancelled.

Each owned task keeps its original outcome, and drain does not throw for it. Because an invocation may stop observing a task before it fails, the scope logs each fault once: event 1013 at Error, or 1014 at Debug for a declared RunicFailureException (see Logging and telemetry). Cancellations are not logged. Pass the model's logger to the constructor; without one, entries go to System.Diagnostics.Trace.

A drain or disposal started from accepted work of the same scope, such as the "Save and close" task above, waits for all other accepted work but not for the calling task or the accepted tasks awaiting it, which cannot finish until the drain does. This applies to code that inherits the task's execution context, including awaited continuations and Task.Run. Code that continues in that task after the drain must not use the resources the owner released. If the close is queued to another thread without the execution context, do not await its disposal from the accepted task; let the task return instead.

The scope does not serialize operations, dispatch model changes, request cancellation, or implement recovery. The application still owns those policies and must return a task that covers all work requiring its resources.

Troubleshooting

• The frontend cannot resolve ./generated/... imports. The build writes the generated modules to RunicApplicationFrontendGeneratedDirectory (Frontend/src/generated by default). Build the .NET project once, or start dotnet runic dev, before you type-check or run the frontend on its own.
• The build fails with RUNICBRIDGE.... The error names the type and member and ends with a link to the ID's entry in the diagnostics catalog, which says how to fix it. See Generator diagnostics.
• The frontend dependencies were not installed. The build installs them when package.json or a lock file changed since the last install, and when declared dependencies have no node_modules. If that install did not run or failed, install them with the frontend's package manager, or run dotnet runic doctor to check the package manager and lock file. Set RunicApplicationFrontendInstallEnabled=false when a workspace install owns them.
• The build says only one ReactiveUI flavor can be used. Runic.Application.Views.ReactiveUI and Runic.Application.Views.ReactiveUI.Reactive cannot both be referenced, and the .Reactive adapter needs Runic.Navigation.ReactiveUI.Reactive. Keep the pair for one flavor; see Runic.Application.Views.ReactiveUI.Reactive.