Skillintermediate

Flutter/Dart Code Review Best Practices

Comprehensive, library-agnostic checklist for reviewing Flutter/Dart applications. These principles apply regardless of which state management solution, routing library, or DI framework is used.

Claude Code Knowledge Pack7/10/2026

Overview

Flutter/Dart Code Review Best Practices

Comprehensive, library-agnostic checklist for reviewing Flutter/Dart applications. These principles apply regardless of which state management solution, routing library, or DI framework is used.

1. General Project Health

Project follows consistent folder structure (feature-first or layer-first)
Proper separation of concerns: UI, business logic, data layers
No business logic in widgets; widgets are purely presentational
pubspec.yaml is clean — no unused dependencies, versions pinned appropriately
analysis_options.yaml includes a strict lint set with strict analyzer settings enabled
No print() statements in production code — use dart:developer log() or a logging package
Generated files (.g.dart, .freezed.dart, .gr.dart) are up-to-date or in .gitignore
Platform-specific code isolated behind abstractions

2. Dart Language Pitfalls

3. Widget Best Practices

Widget decomposition:

No single widget with a build() method exceeding ~80-100 lines
Widgets split by encapsulation AND by how they change (rebuild boundaries)
Private _build*() helper methods that return widgets are extracted to separate widget classes (enables element reuse, const propagation, and framework optimizations)
Stateless widgets preferred over Stateful where no mutable local state is needed
Extracted widgets are in separate files when reusable

Const usage:

const constructors used wherever possible — prevents unnecessary rebuilds
const literals for collections that don't change (const [], const {})
Constructor is declared const when all fields are final

Key usage:

ValueKey used in lists/grids to preserve state across reorders
GlobalKey used sparingly — only when accessing state across the tree is truly needed
UniqueKey avoided in build() — it forces rebuild every frame
ObjectKey used when identity is based on a data object rather than a single value

Theming & design system:

Colors come from Theme.of(context).colorScheme — no hardcoded Colors.red or hex values
Text styles come from Theme.of(context).textTheme — no inline TextStyle with raw font sizes
Dark mode compatibility verified — no assumptions about light background
Spacing and sizing use consistent design tokens or constants, not magic numbers

Build method complexity:

No network calls, file I/O, or heavy computation in build()
No Future.then() or async work in build()
No subscription creation (.listen()) in build()
setState() localized to smallest possible subtree

4. State Management (Library-Agnostic)

These principles apply to all Flutter state management solutions (BLoC, Riverpod, Provider, GetX, MobX, Signals, ValueNotifier, etc.).

Architecture:

Business logic lives outside the widget layer — in a state management component (BLoC, Notifier, Controller, Store, ViewModel, etc.)
State managers receive dependencies via injection, not by constructing them internally
A service or repository layer abstracts data sources — widgets and state managers should not call APIs or databases directly
State managers have a single responsibility — no "god" managers handling unrelated concerns
Cross-component dependencies follow the solution's conventions:
- In Riverpod: providers depending on providers via ref.watch is expected — flag only circular or overly tangled chains
- In BLoC: blocs should not directly depend on other blocs — prefer shared repositories or presentation-layer coordination
- In other solutions: follow the documented conventions for inter-component communication

Immutability & value equality (for immutable-state solutions: BLoC, Riverpod, Redux):

State objects are immutable — new instances created via copyWith() or constructors, never mutated in-place
State classes implement == and hashCode properly (all fields included in comparison)
Mechanism is consistent across the project — manual override, Equatable, freezed, Dart records, or other
Collections inside state objects are not exposed as raw mutable List/Map

Reactivity discipline (for reactive-mutation solutions: MobX, GetX, Signals):

State is only mutated through the solution's reactive API (@action in MobX, .value on signals, .obs in GetX) — direct field mutation bypasses change tracking
Derived values use the solution's computed mechanism rather than being stored redundantly
Reactions and disposers are properly cleaned up (ReactionDisposer in MobX, effect cleanup in Signals)

State shape design:

Mutually exclusive states use sealed types, union variants, or the solution's built-in async state type (e.g. Riverpod's AsyncValue) — not boolean flags (isLoading, isError, hasData)
Every async operation models loading, success, and error as distinct states
All state variants are handled exhaustively in UI — no silently ignored cases
Error states carry error information for display; loading states don't carry stale data
Nullable data is not used as a loading indicator — states are explicit

// BAD — boolean flag soup allows impossible states
class UserState {
  bool isLoading = false;
  bool hasError = false; // isLoading && hasError is representable!
  User? user;
}

// GOOD (immutable approach) — sealed types make impossible states unrepresentable
sealed class UserState {}
class UserInitial extends UserState {}
class UserLoading extends UserState {}
class UserLoaded extends UserState {
  final User user;
  const UserLoaded(this.user);
}
class UserError extends UserState {
  final String message;
  const UserError(this.message);
}

// GOOD (reactive approach) — observable enum + data, mutations via reactivity API
// enum UserStatus { initial, loading, loaded, error }
// Use your solution's observable/signal to wrap status and data separately

Rebuild optimization:

State consumer widgets (Builder, Consumer, Observer, Obx, Watch, etc.) scoped as narrow as possible
Selectors used to rebuild only when specific fields change — not on every state emission
const widgets used to stop rebuild propagation through the tree
Computed/derived state is calculated reactively, not stored redundantly

Subscriptions & disposal:

All manual subscriptions (.listen()) are cancelled in dispose() / close()
Stream controllers are closed when no longer needed
Timers are cancelled in disposal lifecycle
Framework-managed lifecycle is preferred over manual subscription (declarative builders over .listen())
mounted check before setState in async callbacks
BuildContext not used after await without checking context.mounted (Flutter 3.7+) — stale context causes crashes
No navigation, dialogs, or scaffold messages after async gaps without verifying the widget is still mounted
BuildContext never stored in singletons, state managers, or static fields

Local vs global state:

Ephemeral UI state (checkbox, slider, animation) uses local state (setState, ValueNotifier)
Shared state is lifted only as high as needed — not over-globalized
Feature-scoped state is properly disposed when the feature is no longer active

5. Performance

Unnecessary rebuilds:

setState() not called at root widget level — localize state changes
const widgets used to stop rebuild propagation
RepaintBoundary used around complex subtrees that repaint independently
AnimatedBuilder child parameter used for subtrees independent of animation

Expensive operations in build():

No sorting, filtering, or mapping large collections in build() — compute in state management layer
No regex compilation in build()
MediaQuery.of(context) usage is specific (e.g., MediaQuery.sizeOf(context))

Image optimization:

Network images use caching (any caching solution appropriate for the project)
Appropriate image resolution for target device (no loading 4K images for thumbnails)
Image.asset with cacheWidth/cacheHeight to decode at display size
Placeholder and error widgets provided for network images

Lazy loading:

ListView.builder / GridView.builder used instead of ListView(children: [...]) for large or dynamic lists (concrete constructors are fine for small, static lists)
Pagination implemented for large data sets
Deferred loading (deferred as) used for heavy libraries in web builds

Other:

Opacity widget avoided in animations — use AnimatedOpacity or FadeTransition
Clipping avoided in animations — pre-clip images
operator == not overridden on widgets — use const constructors instead
Intrinsic dimension widgets (IntrinsicHeight, IntrinsicWidth) used sparingly (extra layout pass)

6. Testing

Test types and expectations:

Unit tests: Cover all business logic (state managers, repositories, utility functions)
Widget tests: Cover individual widget behavior, interactions, and visual output
Integration tests: Cover critical user flows end-to-end
Golden tests: Pixel-perfect comparisons for design-critical UI components

Coverage targets:

Aim for 80%+ line coverage on business logic
All state transitions have corresponding tests (loading → success, loading → error, retry, etc.)
Edge cases tested: empty states, error states, loading states, boundary values

Test isolation:

External dependencies (API clients, databases, services) are mocked or faked
Each test file tests exactly one class/unit
Tests verify behavior, not implementation details
Stubs define only the behavior needed for each test (minimal stubbing)
No shared mutable state between test cases

Widget test quality:

pumpWidget and pump used correctly for async operations
find.byType, find.text, find.byKey used appropriately
No flaky tests depending on timing — use pumpAndSettle or explicit pump(Duration)
Tests run in CI and failures block merges

7. Accessibility

Semantic widgets:

Semantics widget used to provide screen reader labels where automatic labels are insufficient
ExcludeSemantics used for purely decorative elements
MergeSemantics used to combine related widgets into a single accessible element
Images have semanticLabel property set

Screen reader support:

All interactive elements are focusable and have meaningful descriptions
Focus order is logical (follows visual reading order)

Visual accessibility:

Contrast ratio >= 4.5:1 for text against background
Tappable targets are at least 48x48 pixels
Color is not the sole indicator of state (use icons/text alongside)
Text scales with system font size settings

Interaction accessibility:

No no-op onPressed callbacks — every button does something or is disabled
Error fields suggest corrections
Context does not change unexpectedly while user is inputting data

8. Platform-Specific Concerns

iOS/Android differences:

Platform-adaptive widgets used where appropriate
Back navigation handled correctly (Android back button, iOS swipe-to-go-back)
Status bar and safe area handled via SafeArea widget
Platform-specific permissions declared in AndroidManifest.xml and Info.plist

Responsive design:

LayoutBuilder or MediaQuery used for responsive layouts
Breakpoints defined consistently (phone, tablet, desktop)
Text doesn't overflow on small screens — use Flexible, Expanded, FittedBox
Landscape orientation tested or explicitly locked
Web-specific: mouse/keyboard interactions supported, hover states present

9. Security

Secure storage:

Sensitive data (tokens, credentials) stored using platform-secure storage (Keychain on iOS, EncryptedSharedPreferences on Android)
Never store secrets in plaintext storage
Biometric authentication gating considered for sensitive operations

API key handling:

API keys NOT hardcoded in Dart source — use --dart-define, .env files excluded from VCS, or compile-time configuration
Secrets not committed to git — check .gitignore
Backend proxy used for truly secret keys (client should never hold server secrets)

Input validation:

All user input validated before sending to API
Form validation uses proper validation patterns
No raw SQL or string interpolation of user input
Deep