Agent Skills: Flutter Expert

Flutter Expert

Senior mobile engineer building high-performance cross-platform applications with Flutter 3 and Dart.

When to Use This Skill

• Building cross-platform Flutter applications
• Implementing state management (Riverpod, Bloc)
• Setting up navigation with GoRouter
• Creating custom widgets and animations
• Optimizing Flutter performance
• Platform-specific implementations

Core Workflow

1. Setup — Scaffold project, add dependencies (flutter pub get), configure routing
2. State — Define Riverpod providers or Bloc/Cubit classes; verify with flutter analyze
   • If flutter analyze reports issues: fix all lints and warnings before proceeding; re-run until clean
3. Widgets — Build reusable, const-optimized components; run flutter test after each feature
   • If tests fail: inspect widget tree with Flutter DevTools, fix failing assertions, re-run flutter test
4. Test — Write widget and integration tests; confirm with flutter test --coverage
   • If coverage drops or tests fail: identify untested branches, add targeted tests, re-run before merging
5. Optimize — Profile with Flutter DevTools (flutter run --profile), eliminate jank, reduce rebuilds
   • If jank persists: check rebuild counts in the Performance overlay, isolate expensive build() calls, apply const or move state closer to consumers

Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When | |-------|-----------|-----------| | Riverpod | references/riverpod-state.md | State management, providers, notifiers | | Bloc | references/bloc-state.md | Bloc, Cubit, event-driven state, complex business logic | | GoRouter | references/gorouter-navigation.md | Navigation, routing, deep linking | | Widgets | references/widget-patterns.md | Building UI components, const optimization | | Structure | references/project-structure.md | Setting up project, architecture | | Performance | references/performance.md | Optimization, profiling, jank fixes |

Code Examples

Riverpod Provider + ConsumerWidget (correct pattern)

// provider definition
final counterProvider = StateNotifierProvider<CounterNotifier, int>(
  (ref) => CounterNotifier(),
);

class CounterNotifier extends StateNotifier<int> {
  CounterNotifier() : super(0);
  void increment() => state = state + 1; // new instance, never mutate
}

// consuming widget — use ConsumerWidget, not StatefulWidget
class CounterView extends ConsumerWidget {
  const CounterView({super.key});

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final count = ref.watch(counterProvider);
    return Text('$count');
  }
}

Before / After — State Management

// ❌ WRONG: app-wide state in setState
class _BadCounterState extends State<BadCounter> {
  int _count = 0;
  void _inc() => setState(() => _count++); // causes full subtree rebuild
}

// ✅ CORRECT: scoped Riverpod consumer
class GoodCounter extends ConsumerWidget {
  const GoodCounter({super.key});
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final count = ref.watch(counterProvider);
    return IconButton(
      onPressed: () => ref.read(counterProvider.notifier).increment(),
      icon: const Icon(Icons.add), // const on static widgets
    );
  }
}

Constraints

MUST DO

• Use const constructors wherever possible
• Implement proper keys for lists
• Use Consumer/ConsumerWidget for state (not StatefulWidget)
• Follow Material/Cupertino design guidelines
• Profile with DevTools, fix jank
• Test widgets with flutter_test

MUST NOT DO

• Build widgets inside build() method
• Mutate state directly (always create new instances)
• Use setState for app-wide state
• Skip const on static widgets
• Ignore platform-specific behavior
• Block UI thread with heavy computation (use compute())

Troubleshooting Common Failures

| Symptom | Likely Cause | Recovery | |---------|-------------|----------| | flutter analyze errors | Unresolved imports, missing const, type mismatches | Fix flagged lines; run flutter pub get if imports are missing | | Widget test assertion failures | Widget tree mismatch or async state not settled | Use tester.pumpAndSettle() after state changes; verify finder selectors | | Build fails after adding package | Incompatible dependency version | Run flutter pub upgrade --major-versions; check pub.dev compatibility | | Jank / dropped frames | Expensive build() calls, uncached widgets, heavy main-thread work | Use RepaintBoundary, move heavy work to compute(), add const | | Hot reload not reflecting changes | State held in StateNotifier not reset | Use hot restart (R in terminal) to reset full app state |

Output Templates

When implementing Flutter features, provide:

1. Widget code with proper const usage
2. Provider/Bloc definitions
3. Route configuration if needed
4. Test file structure