Dart language guardrails, patterns, and best practices for AI-assisted development.
Use when working with Dart files (.dart), pubspec.yaml, or when the user mentions Dart/Flutter.
Provides null safety patterns, async/await guidelines, Flutter integration conventions,
and testing standards specific to this project's coding standards.
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Dart language guardrails, patterns, and best practices for AI-assisted development.
Use when working with Dart files (.dart), pubspec.yaml, or when the user mentions Dart/Flutter.
Provides null safety patterns, async/await guidelines, Flutter integration conventions,
and testing standards specific to this project's coding standards.
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
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
