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 ```dart // 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 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: