Dart async expert

Skill almasumdev/awesome-flutter-agent-skills/.github/skills/concurrency_and_networking/dart-async-expert

Curated agent skills, conventions, and workflows for building Flutter apps with AI coding agents.

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Review and fix Dart async code — Futures, async/await, error handling, cancellation, and isolates. Use this when auditing asynchronous code for correctness and performance.

SKILL.md

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Dart Async Expert

Instructions

Write async code that is predictable, testable, and does not block the UI thread.

1. Prefer `async`/`await` Over `.then`

// Good
final user = await repo.fetchUser(id);
final prefs = await repo.fetchPrefs(user.id);

// Avoid chained thens — harder to read, harder to handle errors.

2. Run Independent Work in Parallel

If two futures do not depend on each other, launch them concurrently:

final (user, prefs) = await (repo.fetchUser(id), repo.fetchPrefs(id)).wait; // Dart 3
// or
final results = await Future.wait([repo.fetchUser(id), repo.fetchPrefs(id)]);

3. Error Handling

Use try / on TypedException / catch. Do not swallow errors with bare catch (_).
Convert low-level exceptions to domain ones at the repository boundary.
Future.wait([...]) fails fast on the first error — use eagerError: false if you need all errors collected.

4. Cancellation

Dart Future is not cancellable by itself. Patterns:

Dio: pass a CancelToken and call cancelToken.cancel() in dispose.
Streams: keep the StreamSubscription and .cancel() it.
Custom: pass a boolean/Completer-based signal and check it at await points.

5. Don't Block the Event Loop

CPU-bound work (parsing large JSON, crypto, image decode) must run in an isolate. Use Isolate.run(() => expensive()) (Dart 3) or compute(fn, arg).
A Future does not mean "other thread" — it still runs on the UI isolate unless you explicitly spawn one.

6. Timeouts

final response = await api.fetch().timeout(
  const Duration(seconds: 10),
  onTimeout: () => throw TimeoutException('fetch'),
);

Prefer per-call timeouts over a single global one — failures are easier to attribute.

7. Microtasks vs Timers

scheduleMicrotask runs before any further I/O. Use sparingly.
Timer, Future.delayed(Duration.zero) yield to the event loop. Use to avoid "setState during build" when unavoidable.

8. Common Pitfalls

Forgetting await — the future runs, but errors become unhandled and the caller continues immediately. Enable unawaited_futures lint.
async inside initState — wrap the body or use an unawaited(_init()) call plus proper error handling.
Performing await across BuildContext without checking if (!mounted) return; afterwards.
Holding references to BuildContext or State in long-lived async closures → leaks.

9. Checklist

Every Future-returning call is awaited or explicitly unawaited(...).
Independent calls use .wait / Future.wait.
CPU-bound work runs in an isolate.
BuildContext is not used after await without a mounted check.
Cancellation paths are handled for in-flight work on dispose.