Flutter Dart Code Review

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

• Implicit dynamic: Missing type annotations leading to dynamic — enable strict-casts, strict-inference, strict-raw-types
• Null safety misuse: Excessive ! (bang operator) instead of proper null checks or Dart 3 pattern matching (if (value case var v?))
• Type promotion failures: Using this.field where local variable promotion would work
• Catching too broadly: catch (e) without on clause; always specify exception types
• Catching Error: Error subtypes indicate bugs and should not be caught
• Unused async: Functions marked async that never await — unnecessary overhead
• late overuse: late used where nullable or constructor initialization would be safer; defers errors to runtime
• String concatenation in loops: Use StringBuffer instead of + for iterative string building
• Mutable state in const contexts: Fields in const constructor classes should not be mutable
• Ignoring Future return values: Use await or explicitly call unawaited() to signal intent
• var where final works: Prefer final for locals and const for compile-time constants
• Relative imports: Use package: imports for consistency
• Mutable collections exposed: Public APIs should return unmodifiable views, not raw List/Map
• Missing Dart 3 pattern matching: Prefer switch expressions and if-case over verbose is checks and manual casting
• Throwaway classes for multiple returns: Use Dart 3 records (String, int) instead of single-use DTOs
• print() in production code: Use dart:developer log() or the project's logging package; print() has no log levels and cannot be filtered

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

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

// Prefer (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);
}

// Prefer (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 link URLs validated and sanitized before navigation

Network security:

• HTTPS enforced for all API calls
• Certificate pinning considered for high-security apps
• Authentication tokens refreshed and expired properly
• No sensitive data logged or printed

10. Package/Dependency Review

Evaluating pub.dev packages:

• Check pub points score (aim for 130+/160)
• Check likes and popularity as community signals
• Verify the publisher is verified on pub.dev
• Check last publish date — stale packages (>1 year) are a risk
• Review open issues and response time from maintainers
• Check license compatibility with your project
• Verify platform support covers your targets

Version constraints:

• Use caret syntax (^1.2.3) for dependencies — allows compatible updates
• Pin exact versions only when absolutely necessary
• Run flutter pub outdated regularly to track stale dependencies
• No dependency overrides in production pubspec.yaml — only for temporary fixes with a comment/issue link
• Minimize transitive dependency count — each dependency is an attack surface

Monorepo-specific (melos/workspace):

• Internal packages import only from public API — no package:other/src/internal.dart (breaks Dart package encapsulation)
• Internal package dependencies use workspace resolution, not hardcoded path: ../../ relative strings
• All sub-packages share or inherit root analysis_options.yaml

11. Navigation and Routing

General principles (apply to any routing solution):

• One routing approach used consistently — no mixing imperative Navigator.push with a declarative router
• Route arguments are typed — no Map<String, dynamic> or Object? casting
• Route paths defined as constants, enums, or generated — no magic strings scattered in code
• Auth guards/redirects centralized — not duplicated across individual screens
• Deep links configured for both Android and iOS
• Deep link URLs validated and sanitized before navigation
• Navigation state is testable — route changes can be verified in tests
• Back behavior is correct on all platforms

12. Error Handling

Framework error handling:

• FlutterError.onError overridden to capture framework errors (build, layout, paint)
• PlatformDispatcher.instance.onError set for async errors not caught by Flutter
• ErrorWidget.builder customized for release mode (user-friendly instead of red screen)
• Global error capture wrapper around runApp (e.g., runZonedGuarded, Sentry/Crashlytics wrapper)

Error reporting:

• Error reporting service integrated (Firebase Crashlytics, Sentry, or equivalent)
• Non-fatal errors reported with stack traces
• State management error observer wired to error reporting (e.g., BlocObserver, ProviderObserver, or equivalent for your solution)
• User-identifiable info (user ID) attached to error reports for debugging

Graceful degradation:

• API errors result in user-friendly error UI, not crashes
• Retry mechanisms for transient network failures
• Offline state handled gracefully
• Error states in state management carry error info for display
• Raw exceptions (network, parsing) are mapped to user-friendly, localized messages before reaching the UI — never show raw exception strings to users

13. Internationalization (l10n)

Setup:

• Localization solution configured (Flutter's built-in ARB/l10n, easy_localization, or equivalent)
• Supported locales declared in app configuration

Content:

• All user-visible strings use the localization system — no hardcoded strings in widgets
• Template file includes descriptions/context for translators
• ICU message syntax used for plurals, genders, selects
• Placeholders defined with types
• No missing keys across locales

Code review:

• Localization accessor used consistently throughout the project
• Date, time, number, and currency formatting is locale-aware
• Text directionality (RTL) supported if targeting Arabic, Hebrew, etc.
• No string concatenation for localized text — use parameterized messages

14. Dependency Injection

Principles (apply to any DI approach):

• Classes depend on abstractions (interfaces), not concrete implementations at layer boundaries
• Dependencies provided externally via constructor, DI framework, or provider graph — not created internally
• Registration distinguishes lifetime: singleton vs factory vs lazy singleton
• Environment-specific bindings (dev/staging/prod) use configuration, not runtime if checks
• No circular dependencies in the DI graph
• Service locator calls (if used) are not scattered throughout business logic

15. Static Analysis

Configuration:

• analysis_options.yaml present with strict settings enabled
• Strict analyzer settings: strict-casts: true, strict-inference: true, strict-raw-types: true
• A comprehensive lint rule set is included (very_good_analysis, flutter_lints, or custom strict rules)
• All sub-packages in monorepos inherit or share the root analysis options

Enforcement:

• No unresolved analyzer warnings in committed code
• Lint suppressions (// ignore:) are justified with comments explaining why
• flutter analyze runs in CI and failures block merges

Key rules to verify regardless of lint package:

• prefer_const_constructors — performance in widget trees
• avoid_print — use proper logging
• unawaited_futures — prevent fire-and-forget async bugs
• prefer_final_locals — immutability at variable level
• always_declare_return_types — explicit contracts
• avoid_catches_without_on_clauses — specific error handling
• always_use_package_imports — consistent import style

State Management Quick Reference

The table below maps universal principles to their implementation in popular solutions. Use this to adapt review rules to whichever solution the project uses.

Sources

• Effective Dart: Style
• Effective Dart: Usage
• Effective Dart: Design
• Flutter Performance Best Practices
• Flutter Testing Overview
• Flutter Accessibility
• Flutter Internationalization
• Flutter Navigation and Routing
• Flutter Error Handling
• Flutter State Management Options