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()

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