Flutter/Dart Code Review Best Practices

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.

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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