Flutter Dev

Flutter Development Expert

Expert guidance for Flutter and Dart development following official Flutter team best practices.

Core Development Principles

Code Philosophy

• Apply SOLID principles throughout the codebase
• Write concise, modern, technical Dart code with functional and declarative patterns
• Favor composition over inheritance for building complex widgets and logic
• Prefer immutable data structures, especially for widgets (use StatelessWidget when possible)
• Separate ephemeral state from app state using appropriate state management
• Keep functions short with single purpose (strive for less than 20 lines)
• Use meaningful, descriptive names - avoid abbreviations

Project Structure Assumptions

• Standard Flutter project structure with lib/main.dart as entry point
• Organize by logical layers: Presentation (widgets/screens), Domain (business logic), Data (models/API clients), Core (utilities/extensions)
• For larger projects: organize by feature with presentation/domain/data subfolders per feature

Interaction Guidelines

When generating code:

• Provide explanations for Dart-specific features (null safety, futures, streams)
• If request is ambiguous, ask for clarification on functionality and target platform
• When suggesting new dependencies from pub.dev, explain their benefits
• Use dart format tool for consistent formatting
• Use dart fix tool to automatically fix common errors
• Use the Dart linter with recommended rules to catch issues

Code Quality Standards

Styling Rules

• Line length: 80 characters or fewer
• Naming conventions:
  • PascalCase for classes
  • camelCase for members/variables/functions/enums
  • snake_case for files
• No trailing comments
• Use arrow syntax for simple one-line functions

Error Handling

• Anticipate and handle potential errors - never fail silently
• Use try-catch blocks with appropriate exception types
• Use custom exceptions for code-specific situations
• Proper async/await usage with robust error handling

Documentation

• Add dartdoc comments to all public APIs (classes, constructors, methods, top-level functions)
• Write clear comments for complex/non-obvious code
• Use /// for doc comments
• Start with single-sentence summary ending with period
• Comment why code is written a certain way, not what it does

Dart Best Practices

Type System & Null Safety

• Write soundly null-safe code
• Leverage Dart's null safety features
• Avoid ! operator unless value is guaranteed non-null
• Use pattern matching features where they simplify code
• Use records to return multiple types when defining a class is cumbersome
• Prefer exhaustive switch statements/expressions (no break needed)

Async Programming

• Use Futures, async, await for single asynchronous operations
• Use Streams for sequences of asynchronous events
• Ensure proper error handling in async operations

Class & Library Organization

• Define related classes within the same library file
• For large libraries: export smaller private libraries from single top-level library
• Group related libraries in same folder

Flutter Best Practices

Widget Design

• Widgets (especially StatelessWidget) are immutable
• When UI needs to change, Flutter rebuilds widget tree
• Prefer composing smaller widgets over extending existing ones
• Use small, private Widget classes instead of private helper methods returning widgets
• Break down large build() methods into smaller, reusable private Widget classes
• Use const constructors whenever possible to reduce rebuilds

Performance Optimization

• Use ListView.builder or SliverList for long lists (lazy-loaded)
• Use compute() to run expensive calculations in separate isolate (e.g., JSON parsing)
• Avoid expensive operations (network calls, complex computations) in build() methods
• Use const constructors in build() methods to minimize rebuilds

Responsive Design

• Use LayoutBuilder or MediaQuery for responsive UIs
• Ensure mobile responsive design across different screen sizes
• Test on mobile and web platforms

State Management

Prefer Flutter's built-in solutions unless third-party explicitly requested:

Built-in Solutions (in order of simplicity)

1. ValueNotifier + ValueListenableBuilder - Simple, local state with single value

final ValueNotifier<int> _counter = ValueNotifier<int>(0);

ValueListenableBuilder<int>(
  valueListenable: _counter,
  builder: (context, value, child) => Text('Count: $value'),
);

2. ChangeNotifier + ListenableBuilder - Complex/shared state across widgets

class CounterModel extends ChangeNotifier {
  int _count = 0;
  int get count => _count;
  void increment() {
    _count++;
    notifyListeners();
  }
}

ListenableBuilder(
  listenable: counterModel,
  builder: (context, child) => Text('${counterModel.count}'),
);

3. Streams + StreamBuilder - Sequences of asynchronous events
4. Futures + FutureBuilder - Single async operations

Advanced Patterns

• MVVM: Model-View-ViewModel pattern for robust applications
• Dependency Injection: Use manual constructor injection for explicit dependencies
• Provider: Only if explicitly requested for DI beyond manual injection

Navigation

GoRouter (Recommended)

Use go_router for declarative navigation, deep linking, and web support:

// Add dependency
flutter pub add go_router

// Configure router
final GoRouter _router = GoRouter(
  routes: [
    GoRoute(
      path: '/',
      builder: (context, state) => const HomeScreen(),
      routes: [
        GoRoute(
          path: 'details/:id',
          builder: (context, state) {
            final String id = state.pathParameters['id']!;
            return DetailScreen(id: id);
          },
        ),
      ],
    ),
  ],
);

// Use in MaterialApp
MaterialApp.router(routerConfig: _router);

• Configure redirect property for authentication flows
• Use for deep-linkable routes

Navigator (Built-in)

Use for short-lived screens not needing deep links (dialogs, temporary views):

Navigator.push(context, MaterialPageRoute(builder: (context) => DetailsScreen()));
Navigator.pop(context);

Package Management

Using pub Tool

• Add dependency: flutter pub add <package_name>
• Add dev dependency: flutter pub add dev:<package_name>
• Add override: flutter pub add override:<package_name>:1.0.0
• Remove dependency: dart pub remove <package_name>

External Packages

• Search pub.dev for suitable, stable packages
• Explain benefits when suggesting new dependencies

Data Handling

JSON Serialization

Use json_serializable and json_annotation:

import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart';

@JsonSerializable(fieldRename: FieldRename.snake)
class User {
  final String firstName;
  final String lastName;
  
  User({required this.firstName, required this.lastName});
  
  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
}

Code Generation

• Ensure build_runner is dev dependency
• Run after modifications: dart run build_runner build --delete-conflicting-outputs

Logging

Use dart:developer for structured logging:

import 'dart:developer' as developer;

// Simple messages
developer.log('User logged in successfully.');

// Structured error logging
try {
  // code
} catch (e, s) {
  developer.log(
    'Failed to fetch data',
    name: 'myapp.network',
    level: 1000, // SEVERE
    error: e,
    stackTrace: s,
  );
}

UI & Theming

Material Design 3 & ThemeData

Use ColorScheme.fromSeed() for harmonious palettes:

MaterialApp(
  theme: ThemeData(
    colorScheme: ColorScheme.fromSeed(
      seedColor: Colors.deepPurple,
      brightness: Brightness.light,
    ),
    textTheme: TextTheme(
      displayLarge: TextStyle(fontSize: 57, fontWeight: FontWeight.bold),
      bodyMedium: TextStyle(fontSize: 14, height: 1.4),
    ),
  ),
  darkTheme: ThemeData(
    colorScheme: ColorScheme.fromSeed(
      seedColor: Colors.deepPurple,
      brightness: Brightness.dark,
    ),
  ),
  themeMode: ThemeMode.system,
);

Custom Theme Extensions

For custom design tokens beyond standard ThemeData:

@immutable
class MyColors extends ThemeExtension<MyColors> {
  const MyColors({required this.success, required this.danger});
  final Color? success;
  final Color? danger;
  
  @override
  ThemeExtension<MyColors> copyWith({Color? success, Color? danger}) {
    return MyColors(success: success ?? this.success, danger: danger ?? this.danger);
  }
  
  @override
  ThemeExtension<MyColors> lerp(ThemeExtension<MyColors>? other, double t) {
    if (other is! MyColors) return this;
    return MyColors(
      success: Color.lerp(success, other.success, t),
      danger: Color.lerp(danger, other.danger, t),
    );
  }
}

// Register in ThemeData
theme: ThemeData(
  extensions: [MyColors(success: Colors.green, danger: Colors.red)],
),

// Use in widgets
Container(color: Theme.of(context).extension<MyColors>()!.success)

Fonts

Use google_fonts package for custom fonts:

flutter pub add google_fonts

final TextTheme appTextTheme = TextTheme(
  displayLarge: GoogleFonts.oswald(fontSize: 57, fontWeight: FontWeight.bold),
  titleLarge: GoogleFonts.roboto(fontSize: 22, fontWeight: FontWeight.w500),
  bodyMedium: GoogleFonts.openSans(fontSize: 14),
);

Images & Assets

Declare in pubspec.yaml:

flutter:
  uses-material-design: true
  assets:
    - assets/images/

// Local images
Image.asset('assets/images/placeholder.png')

// Network images (always include error handling)
Image.network(
  'https://example.com/image.png',
  loadingBuilder: (context, child, progress) {
    if (progress == null) return child;
    return Center(child: CircularProgressIndicator());
  },
  errorBuilder: (context, error, stackTrace) => Icon(Icons.error),
)

Layout Best Practices

Flexible Layouts

• Expanded: Fill remaining space in Row/Column
• Flexible: Shrink to fit (don't combine with Expanded)
• Wrap: Auto-wrap to next line on overflow
• SingleChildScrollView: For fixed-size content larger than viewport
• ListView.builder/GridView.builder: For long lists (lazy loading)
• FittedBox: Scale/fit single child within parent
• LayoutBuilder: Responsive layouts based on available space

Stack Layouts

• Positioned: Precisely place child by anchoring to edges
• Align: Position using alignments (e.g., Alignment.center)

Overlays

Use OverlayPortal for UI elements on top of everything:

final _controller = OverlayPortalController();

OverlayPortal(
  controller: _controller,
  overlayChildBuilder: (context) => Positioned(
    top: 50,
    left: 10,
    child: Card(child: Text('Overlay content')),
  ),
  child: ElevatedButton(
    onPressed: _controller.toggle,
    child: Text('Toggle'),
  ),
)

Visual Design Principles

Design Guidelines

• Build beautiful, intuitive UIs following modern design
• Ensure responsive across screen sizes (mobile & web)
• Provide intuitive navigation
• Use typography hierarchy (hero text, headlines, keywords)
• Apply subtle background textures for premium feel
• Use multi-layered shadows for depth
• Incorporate icons for enhanced understanding
• Interactive elements have shadows with color glow effects

Color Guidelines

• 60-30-10 Rule: 60% primary/neutral, 30% secondary, 10% accent
• Contrast Ratios (WCAG 2.1):
  • Normal text: 4.5:1 minimum
  • Large text (18pt or 14pt bold): 3:1 minimum
• Avoid complementary colors for text/background (causes eye strain)
• Use complementary colors sparingly for accents

Typography

• Limit to 1-2 font families
• Prioritize legibility (sans-serif for UI body text)
• Line height: 1.4x-1.6x font size
• Line length: 45-75 characters for body text
• Avoid all caps for long-form text

Testing

Test Types

• Unit Tests: package:test for domain logic, data layer, state management
• Widget Tests: package:flutter_test for UI components
• Integration Tests: package:integration_test for end-to-end flows

Testing Best Practices

• Follow Arrange-Act-Assert (Given-When-Then) pattern
• Prefer package:checks for more expressive assertions
• Prefer fakes/stubs over mocks
• If mocks necessary: use mockito or mocktail
• Aim for high test coverage
• Write testable code: use file, process, platform packages for dependency injection

Running Tests

flutter test

Accessibility (A11y)

• Color Contrast: Text minimum 4.5:1 ratio against background
• Dynamic Text Scaling: Test UI with increased system font size
• Semantic Labels: Use Semantics widget for clear, descriptive labels
• Screen Reader Testing: Test with TalkBack (Android) and VoiceOver (iOS)

Analysis & Linting

Include in analysis_options.yaml:

include: package:flutter_lints/flutter.yaml

linter:
  rules:
    # Add additional lint rules here

Additional Resources

For detailed patterns and examples, see:

• references/navigation_patterns.md - Advanced navigation patterns with GoRouter
• references/state_patterns.md - Comprehensive state management examples
• references/theme_patterns.md - Advanced theming and styling patterns