Dart Guide

Applies to: Dart 3.0+, Sound Null Safety, Flutter, Server-side Dart

Core Principles

1. Sound Null Safety: The type system guarantees non-nullable by default; nullable types are explicit
2. Async by Design: Use Futures, Streams, and async/await for all I/O and event-driven logic
3. Immutability First: Prefer final locals, const constructors, and immutable data structures
4. Composition Over Inheritance: Use mixins, extension methods, and sealed class hierarchies
5. Effective Dart: Follow official Effective Dart style for naming, documentation, and API design

Guardrails

Version & Dependencies

• Use Dart 3.0+ with sound null safety enabled (no // @dart=2.x opt-outs)
• Manage dependencies with pubspec.yaml; run dart pub get after changes
• Pin dependency versions with caret syntax (^1.2.0) for libraries, exact for apps
• Run dart pub outdated periodically to check for updates
• Audit transitive dependencies with dart pub deps

Code Style

• Run dart format . before every commit (line length: 80 characters)
• Run dart analyze with strict mode before committing
• Libraries/packages: lowercase_with_underscores (my_package)
• Classes/enums/typedefs: UpperCamelCase (UserProfile)
• Functions/variables/parameters: lowerCamelCase (fetchUser)
• Constants: lowerCamelCase (defaultTimeout, not DEFAULT_TIMEOUT)
• Private members: prefix with _ (_internalState)
• Follow Effective Dart guidelines

Null Safety

• Never use ! (null assertion) without a preceding null check or guarantee
• Use ? for nullable types only at API boundaries (JSON, platform channels)
• Prefer ?? (if-null) and ?. (null-aware access) over explicit null checks
• Use required keyword for non-optional named parameters
• Use late only when initialization is guaranteed before access (avoid for laziness)

// BAD: crashes at runtime if null
final name = json['name'] as String;

// GOOD: explicit nullable handling
final name = json['name'] as String? ?? 'Unknown';

// GOOD: required parameter enforces non-null at call site
void createUser({required String name, required String email}) { ... }

// BAD: late used for laziness without guarantees
late final String config;

// GOOD: late with guaranteed initialization in constructor body
late final Database _db;
void init(Database db) => _db = db;

Async/Await

• Always await Futures or return them; never fire-and-forget without error handling
• Use async* and yield for Stream generators
• Set timeouts on all external calls with .timeout(Duration(...))
• Catch specific exceptions, not bare catch (e)
• Use Completer only when bridging callback-based APIs to Futures

// BAD: fire-and-forget loses errors
void save() {
  repository.save(data); // Future ignored
}

// GOOD: propagate or handle
Future<void> save() async {
  await repository.save(data);
}

// GOOD: timeout on external calls
final response = await http.get(uri).timeout(
  const Duration(seconds: 10),
  onTimeout: () => throw TimeoutException('Request timed out'),
);

Immutability

• Use final for all local variables that are not reassigned
• Use const constructors for compile-time constant objects (widgets, values)
• Prefer UnmodifiableListView or immutable collections from package:collection
• Use records for lightweight immutable data groupings
• Mark classes @immutable when all fields are final

Project Structure

myproject/
├── lib/
│   ├── src/                   # Private implementation
│   │   ├── models/            # Data classes, records, sealed types
│   │   ├── services/          # Business logic, use cases
│   │   ├── repositories/      # Data access layer
│   │   └── utils/             # Shared utilities, extensions
│   └── my_package.dart        # Public barrel export file
├── bin/                       # CLI entry points (for command-line apps)
│   └── main.dart
├── test/                      # Tests (mirrors lib/src/ structure)
│   ├── models/
│   ├── services/
│   └── repositories/
├── pubspec.yaml               # Dependencies and metadata
├── analysis_options.yaml      # Linter and analyzer configuration
└── README.md

• lib/src/ for private implementation; export public API from lib/my_package.dart
• test/ mirrors lib/src/ directory structure
• bin/ only for executable entry points
• No business logic in main.dart (delegate to services)

Key Patterns

Records and Pattern Matching (Dart 3)

// Records: lightweight immutable tuples with named fields
typedef UserSummary = ({String name, String email, int age});

UserSummary fetchSummary() => (name: 'Alice', email: 'a@b.com', age: 30);

// Destructuring with pattern matching
final (name: userName, email: _, age: userAge) = fetchSummary();

// Switch expressions with patterns
String classify(Object value) => switch (value) {
  int n when n < 0   => 'negative',
  int n when n == 0  => 'zero',
  int()              => 'positive',
  String s           => 'string: $s',
  (int x, int y)     => 'point ($x, $y)',
  _                  => 'unknown',
};

Sealed Classes and Exhaustive Matching

sealed class Result<T> {
  const Result();
}

final class Success<T> extends Result<T> {
  final T value;
  const Success(this.value);
}

final class Failure<T> extends Result<T> {
  final String message;
  final Object? error;
  const Failure(this.message, [this.error]);
}

// Exhaustive switch: compiler enforces all subtypes handled
String describe<T>(Result<T> result) => switch (result) {
  Success(:final value) => 'Success: $value',
  Failure(:final message) => 'Failed: $message',
};

Extension Methods and Types

// Extension methods: add functionality to existing types
extension StringValidation on String {
  bool get isValidEmail => RegExp(r'^[\w-.]+@[\w-]+\.\w+$').hasMatch(this);
  String truncate(int maxLength) =>
      length <= maxLength ? this : '${substring(0, maxLength)}...';
}

// Extension types (Dart 3.3): zero-cost type wrappers
extension type UserId(String value) {
  UserId.generate() : value = Uuid().v4();
  bool get isValid => value.isNotEmpty;
}

// Usage: compile-time type safety, zero runtime overhead
void fetchUser(UserId id) { ... }
fetchUser(UserId('abc-123'));
// fetchUser('raw-string'); // Compile error

Late Keyword (Correct Usage)

class ApiClient {
  // GOOD: late for dependency injection with guaranteed init
  late final HttpClient _client;

  void configure(HttpClient client) {
    _client = client;
  }

  // GOOD: late final for expensive one-time computation
  late final Map<String, dynamic> _config = _loadConfig();

  Map<String, dynamic> _loadConfig() {
    // expensive operation, runs once on first access
    return jsonDecode(File('config.json').readAsStringSync());
  }
}

Stream Patterns

// async* generator for producing streams
Stream<int> countDown(int from) async* {
  for (var i = from; i >= 0; i--) {
    await Future.delayed(const Duration(seconds: 1));
    yield i;
  }
}

// Stream transformations
final results = inputStream
    .where((event) => event.isValid)
    .map((event) => event.transform())
    .handleError((error) => logger.warning('Stream error: $error'))
    .distinct();

Isolates for Heavy Computation

import 'dart:isolate';

// Simple one-shot isolate computation
Future<List<int>> computePrimes(int limit) async {
  return await Isolate.run(() => _sieveOfEratosthenes(limit));
}

List<int> _sieveOfEratosthenes(int limit) {
  final sieve = List.filled(limit + 1, true);
  sieve[0] = sieve[1] = false;
  for (var i = 2; i * i <= limit; i++) {
    if (sieve[i]) {
      for (var j = i * i; j <= limit; j += i) {
        sieve[j] = false;
      }
    }
  }
  return [for (var i = 2; i <= limit; i++) if (sieve[i]) i];
}

Testing

Standards

• Test files: *_test.dart in test/ (mirror lib/src/ structure)
• Test functions: test('description of behavior', () { ... })
• Group related tests: group('ClassName', () { ... })
• Use package:test for pure Dart, package:flutter_test for widgets
• Use package:mockito with @GenerateMocks for type-safe mocking
• Coverage target: >80% for business logic, >60% overall
• Test names describe behavior: 'returns empty list when no users exist'

Unit Test Pattern

import 'package:test/test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';

@GenerateMocks([UserRepository])
import 'user_service_test.mocks.dart';

void main() {
  late MockUserRepository mockRepo;
  late UserService service;

  setUp(() {
    mockRepo = MockUserRepository();
    service = UserService(mockRepo);
  });

  group('UserService', () {
    group('getUser', () {
      test('returns user when found', () async {
        final expected = User(id: '1', name: 'Alice');
        when(mockRepo.findById('1')).thenAnswer((_) async => expected);

        final result = await service.getUser('1');

        expect(result, isA<Success<User>>());
        expect((result as Success).value, equals(expected));
        verify(mockRepo.findById('1')).called(1);
      });

      test('returns failure when user not found', () async {
        when(mockRepo.findById('99')).thenAnswer((_) async => null);

        final result = await service.getUser('99');

        expect(result, isA<Failure>());
        expect((result as Failure).message, contains('not found'));
      });
    });
  });
}

Async and Stream Testing

test('stream emits values in order', () {
  expect(
    countDown(3),
    emitsInOrder([3, 2, 1, 0, emitsDone]),
  );
});

test('completes within timeout', () async {
  final result = await fetchData().timeout(const Duration(seconds: 5));
  expect(result, isNotNull);
});

Tooling

Essential Commands

dart format .                        # Format all Dart files
dart analyze                         # Static analysis (lint + type checks)
dart test                            # Run all tests
dart test --coverage=coverage        # Run tests with coverage
dart run bin/main.dart               # Run CLI application
dart compile exe bin/main.dart       # AOT compile to native executable
dart pub get                         # Install dependencies
dart pub outdated                    # Check for dependency updates
dart pub deps                        # Show dependency tree
dart fix --apply                     # Auto-apply suggested fixes

Analysis Options Configuration

# analysis_options.yaml
include: package:lints/recommended.yaml

analyzer:
  language:
    strict-casts: true
    strict-inference: true
    strict-raw-types: true
  errors:
    missing_return: error
    dead_code: warning

linter:
  rules:
    - prefer_final_locals
    - prefer_const_constructors
    - prefer_const_declarations
    - avoid_dynamic_calls
    - unawaited_futures
    - always_declare_return_types
    - annotate_overrides
    - avoid_print
    - prefer_single_quotes
    - require_trailing_commas
    - unnecessary_lambdas
    - prefer_expression_function_bodies

Pub Configuration

# pubspec.yaml
name: my_package
description: A concise description of the package.
version: 1.0.0

environment:
  sdk: ^3.0.0

dependencies:
  http: ^1.2.0
  json_annotation: ^4.8.0

dev_dependencies:
  test: ^1.25.0
  mockito: ^5.4.0
  build_runner: ^2.4.0
  json_serializable: ^6.7.0
  lints: ^4.0.0
  coverage: ^1.7.0

References

For detailed patterns and examples, see:

• references/patterns.md -- Stream patterns, Isolate communication, sealed class hierarchies

External References

• Effective Dart
• Dart Language Tour
• Dart 3 Records and Patterns
• Dart Null Safety
• Dart Concurrency (Isolates)
• dart_test package
• mockito for Dart
• Dart Lints