Async Resources in Reactor

Reactor replaces the UseState + UseEffect + manual-cancellation dance with three hooks. Reads return AsyncValue<T>, writes return a Mutation handle. The hook owns the cancellation token, dedups across siblings, caches results in a shared QueryCache, and drops late results on unmount.

Available on any Component subclass directly; on RenderContext as extension methods for function components (Memo(ctx => ...) or RenderEachTime(ctx => ...)).

1. UseResource — single fetch

AsyncValue<T> UseResource<T>(
    Func<CancellationToken, Task<T>> fetcher,
    object[] deps,
    ResourceOptions? options = null);

AsyncValue<T> is a sealed 4-state record: Loading, Data(T Value), Error(Exception), Reloading(T Previous) (stale-while-revalidate). Pattern-match with a switch:

class UserCard : Component
{
    public override Element Render()
    {
        var user = UseResource(
            ct => Api.GetUserAsync(userId, ct),
            deps: new object[] { userId });

        return user switch
        {
            AsyncValue<User>.Loading   => TextBlock("Loading…"),
            AsyncValue<User>.Data d    => TextBlock(d.Value.Name).Bold(),
            AsyncValue<User>.Reloading r => TextBlock(r.Previous.Name).Opacity(0.5),
            AsyncValue<User>.Error e   => TextBlock($"Error: {e.Exception.Message}"),
            _ => Empty()
        };
    }
}

Or use user.Match<Element>(loading, data, error, reloading?) — reloading defaults to data so stale values stay visible.

Rules:

• Fetcher must be idempotent (reads only — the analyzer REACTOR_HOOKS_006 warns on names like Post*, Create*, Delete*, Update*, Save*, Send*). For writes, use UseMutation.
• Fetcher must observe ct — return a cancelled task or check ct.IsCancellationRequested. Cancellation fires on deps change, unmount, and cache invalidation.
• deps must be value-comparable. Don't pass new lambdas or freshly constructed collections each render — memoize with UseMemo or use scalar values.

ResourceOptions (all optional):

2. UseMutation — writes with optimistic updates

Mutation<TInput, TResult> UseMutation<TInput, TResult>(
    Func<TInput, CancellationToken, Task<TResult>> mutator,
    MutationOptions<TInput, TResult>? options = null);

The returned Mutation is stable across renders. Properties: IsPending, Error?, LastResult?. Call mutation.RunAsync(input) from click handlers; call mutation.Reset() to clear error/result.

var (todos, setTodos) = UseState<IReadOnlyList<Todo>>([]);

var addTodo = UseMutation<TodoInput, Todo>(
    mutator: (input, ct) => Api.AddTodoAsync(input, ct),
    options: new MutationOptions<TodoInput, Todo>(
        // Synchronous — lands in the next render before the network call.
        OnOptimistic: input =>
            setTodos([.. todos, new Todo(input.Title, IsTemporary: true)]),
        OnSuccess: (todo, _) =>
            setTodos([.. todos.Select(t => t.IsTemporary ? todo : t)]),
        OnError: (_, input) =>
            setTodos([.. todos.Where(t => t.Title != input.Title)]),
        // Any sibling UseResource subscribed to these keys refetches.
        InvalidateKeys: ["todos/list"]));

return VStack(
    Button("Add", () => _ = addTodo.RunAsync(new TodoInput("New")))
        .IsEnabled(!addTodo.IsPending));

• OnOptimistic runs synchronously on the caller. If it throws, the mutator never fires — prevents half-applied state.
• InvalidateKeys fires on success only, not on error.
• Overlapping RunAsync calls are allowed; each has its own token. Gate with IsPending if you need serial semantics.
• Unmount cancels the mutator's token. OnError is silent on cancellation.

3. UseInfiniteResource — cursor pagination

InfiniteResource<TItem> UseInfiniteResource<TItem, TCursor>(
    Func<TCursor?, CancellationToken, Task<Page<TItem, TCursor>>> fetchPage,
    object[] deps,
    InfiniteResourceOptions? options = null);  // PageSize default 50

Pull model. The virtualizer calls resource.ItemAt(i) per visible row; the hook fetches whichever page covers i, dedups, and caches. Unfetched or in-flight slots return null (render a shimmer).

var commits = UseInfiniteResource<Commit, string>(
    fetchPage: async (cursor, ct) =>
    {
        var (items, next, total) = await Api.GetCommitsPageAsync(cursor, ct);
        return new Page<Commit, string>(items, next, total);
    },
    deps: new object[] { repoId });

// In your virtualizer's renderItem:
var commit = commits.ItemAt(rowIndex);
return commit is null
    ? TextBlock("…").Opacity(0.4)
    : TextBlock($"{commit.Sha} — {commit.Message}");

Resource API:

Footer UI off LoadState:

commits.LoadState switch
{
    LoadState.Loading       => ProgressRing(),
    LoadState.Idle when !commits.HasMore => TextBlock("End of list"),
    LoadState.Error e       => Button("Retry", () => commits.Retry()),
    _ => Empty()
}

Notes:

• Cursor paging is serial — page N-1 must land before page N (the cursor lives in N-1's payload). For parallel offset paging, use an offset as the cursor type.
• Deps change cancels all in-flight pages, unsubscribes cache keys, and restarts from page 0.

4. Pending — bubble-up fallback

Render a single fallback for a subtree whose components each load independent resources:

PendingFactory.Pending(
    fallback: TextBlock("Loading dashboard…").Opacity(0.5),
    child: VStack(
        Component<UserHeader>(),    // has UseResource
        Component<RecentActivity>(),// has UseInfiniteResource
        Component<Stats>()          // has UseResource
    ))

• Both trees mount — the child's hooks run in the background.
• Fallback shows while any nested resource is in Loading.
• Reloading (refetch with data present) does not re-trigger the fallback. No flash on revalidation.

5. Gotchas

Non-idempotent fetcher (REACTOR_HOOKS_006). The analyzer flags UseResource(ct => api.PostComment(...)). Reads retry and refetch; use UseMutation for writes.

Unstable deps. deps: new object[] { userId } allocates a fresh array each render — the deps-change detector compares element-wise, so scalar userId is fine, but a fresh new Filter { ... } object is not. Memoize complex deps:

var deps = UseMemo(() => new object[] { filter, sort }, filter, sort);
var items = UseResource(ct => Api.SearchAsync(filter, sort, ct), deps);

Don't share CacheKey across unrelated hooks. Auto-key keeps siblings independent. Explicit CacheKey is for two distant subtrees that should observe the same entry.

Live-mutation overlay. UseInfiniteResource.Items is server-sourced and immutable — the hook re-renders when pages land, not when you edit a row. Keep optimistic edits in a separate Dictionary<int, T> overlay that takes precedence over Items[i] at read time.

AsyncValue.Match allocates one delegate per arm. In per-row render paths (inside a virtualized list), prefer a switch expression for exhaustiveness checking and zero per-call allocation.

6. See also

• docs/guide/async-resources-cookbook.md — task-oriented recipes.
• docs/reference/async-system.md — full state-machine reference, race analysis, known bugs/gaps.
• docs/specs/020-async-resources-design.md — the design spec.