Dart & Flutter Expert

Expert guidance for Dart programming, Flutter framework, mobile development, and cross-platform applications.

Core Concepts

Dart Language

• Strong typing with type inference
• Async/await and Futures
• Streams
• Mixins and extensions
• Null safety
• Collections

Flutter Framework

• Widgets (Stateless & Stateful)
• State management (Provider, Riverpod, Bloc)
• Navigation and routing
• Material and Cupertino design
• Responsive layouts
• Platform integration

Dart Fundamentals

dart
1// Variables and types
2var name = 'John';  // Type inference
3String explicitType = 'Explicit';
4final constantValue = 42;  // Runtime constant
5const compileConstant = 'Compile-time';
6
7// Null safety
8String? nullableString;
9String nonNullable = 'Never null';
10
11// Late initialization
12late String lateInit;
13
14// Collections
15List<String> names = ['Alice', 'Bob', 'Charlie'];
16Map<String, int> ages = {'Alice': 25, 'Bob': 30};
17Set<int> uniqueNumbers = {1, 2, 3, 3};  // {1, 2, 3}
18
19// Functions
20int add(int a, int b) => a + b;
21
22// Named parameters
23void createUser({
24  required String name,
25  int age = 18,
26  String? email,
27}) {
28  print('Creating user: $name, age: $age');
29}
30
31// Classes
32class User {
33  final String id;
34  final String name;
35  int age;
36
37  User({
38    required this.id,
39    required this.name,
40    this.age = 0,
41  });
42
43  // Named constructor
44  User.guest() : this(id: 'guest', name: 'Guest');
45
46  // Methods
47  void celebrateBirthday() {
48    age++;
49  }
50
51  @override
52  String toString() => 'User($name, $age)';
53}
54
55// Mixins
56mixin Loggable {
57  void log(String message) {
58    print('[${DateTime.now()}] $message');
59  }
60}
61
62class LoggableUser extends User with Loggable {
63  LoggableUser({required String id, required String name})
64      : super(id: id, name: name);
65}
66
67// Extensions
68extension StringExtensions on String {
69  bool get isValidEmail => contains('@') && contains('.');
70  String capitalize() => '${this[0].toUpperCase()}${substring(1)}';
71}

Async Programming

dart
1import 'dart:async';
2
3// Future
4Future<String> fetchUser(String id) async {
5  await Future.delayed(Duration(seconds: 1));
6  return 'User $id';
7}
8
9// Error handling
10Future<User?> fetchUserSafely(String id) async {
11  try {
12    final user = await fetchUser(id);
13    return User(id: id, name: user);
14  } catch (e) {
15    print('Error: $e');
16    return null;
17  }
18}
19
20// Multiple futures
21Future<void> fetchMultiple() async {
22  final results = await Future.wait([
23    fetchUser('1'),
24    fetchUser('2'),
25    fetchUser('3'),
26  ]);
27  print(results);
28}
29
30// Streams
31Stream<int> countStream() async* {
32  for (int i = 1; i <= 5; i++) {
33    await Future.delayed(Duration(seconds: 1));
34    yield i;
35  }
36}
37
38// Listen to stream
39void listenToStream() {
40  countStream().listen(
41    (data) => print('Received: $data'),
42    onError: (error) => print('Error: $error'),
43    onDone: () => print('Done'),
44  );
45}
46
47// Stream transformations
48Stream<String> transformedStream() {
49  return countStream()
50      .where((n) => n % 2 == 0)
51      .map((n) => 'Number: $n')
52      .take(3);
53}
54
55// StreamController
56class Counter {
57  final _controller = StreamController<int>();
58
59  Stream<int> get stream => _controller.stream;
60
61  void increment(int value) {
62    _controller.add(value);
63  }
64
65  void dispose() {
66    _controller.close();
67  }
68}

Flutter Widgets

dart
1import 'package:flutter/material.dart';
2
3// Stateless Widget
4class UserCard extends StatelessWidget {
5  final User user;
6
7  const UserCard({Key? key, required this.user}) : super(key: key);
8
9  @override
10  Widget build(BuildContext context) {
11    return Card(
12      child: ListTile(
13        title: Text(user.name),
14        subtitle: Text('Age: ${user.age}'),
15        trailing: Icon(Icons.arrow_forward),
16      ),
17    );
18  }
19}
20
21// Stateful Widget
22class CounterWidget extends StatefulWidget {
23  const CounterWidget({Key? key}) : super(key: key);
24
25  @override
26  State<CounterWidget> createState() => _CounterWidgetState();
27}
28
29class _CounterWidgetState extends State<CounterWidget> {
30  int _counter = 0;
31
32  void _incrementCounter() {
33    setState(() {
34      _counter++;
35    });
36  }
37
38  @override
39  Widget build(BuildContext context) {
40    return Column(
41      children: [
42        Text('Counter: $_counter'),
43        ElevatedButton(
44          onPressed: _incrementCounter,
45          child: Text('Increment'),
46        ),
47      ],
48    );
49  }
50}
51
52// Layout
53class UserListScreen extends StatelessWidget {
54  final List<User> users;
55
56  const UserListScreen({Key? key, required this.users}) : super(key: key);
57
58  @override
59  Widget build(BuildContext context) {
60    return Scaffold(
61      appBar: AppBar(
62        title: Text('Users'),
63        actions: [
64          IconButton(
65            icon: Icon(Icons.add),
66            onPressed: () => Navigator.push(
67              context,
68              MaterialPageRoute(builder: (_) => AddUserScreen()),
69            ),
70          ),
71        ],
72      ),
73      body: ListView.builder(
74        itemCount: users.length,
75        itemBuilder: (context, index) {
76          return UserCard(user: users[index]);
77        },
78      ),
79    );
80  }
81}

State Management (Provider)

dart
1import 'package:provider/provider.dart';
2
3// State class
4class UserProvider extends ChangeNotifier {
5  List<User> _users = [];
6
7  List<User> get users => _users;
8
9  Future<void> fetchUsers() async {
10    _users = await api.getUsers();
11    notifyListeners();
12  }
13
14  void addUser(User user) {
15    _users.add(user);
16    notifyListeners();
17  }
18
19  void removeUser(String id) {
20    _users.removeWhere((user) => user.id == id);
21    notifyListeners();
22  }
23}
24
25// Setup provider
26void main() {
27  runApp(
28    MultiProvider(
29      providers: [
30        ChangeNotifierProvider(create: (_) => UserProvider()),
31        ChangeNotifierProvider(create: (_) => AuthProvider()),
32      ],
33      child: MyApp(),
34    ),
35  );
36}
37
38// Consume provider
39class UserList extends StatelessWidget {
40  @override
41  Widget build(BuildContext context) {
42    final userProvider = Provider.of<UserProvider>(context);
43
44    return ListView.builder(
45      itemCount: userProvider.users.length,
46      itemBuilder: (context, index) {
47        final user = userProvider.users[index];
48        return ListTile(
49          title: Text(user.name),
50          trailing: IconButton(
51            icon: Icon(Icons.delete),
52            onPressed: () => userProvider.removeUser(user.id),
53          ),
54        );
55      },
56    );
57  }
58}
59
60// Consumer widget
61class UserCounter extends StatelessWidget {
62  @override
63  Widget build(BuildContext context) {
64    return Consumer<UserProvider>(
65      builder: (context, userProvider, child) {
66        return Text('Total users: ${userProvider.users.length}');
67      },
68    );
69  }
70}

Navigation

dart
1// Basic navigation
2Navigator.push(
3  context,
4  MaterialPageRoute(builder: (context) => DetailScreen()),
5);
6
7// Named routes
8MaterialApp(
9  routes: {
10    '/': (context) => HomeScreen(),
11    '/details': (context) => DetailScreen(),
12    '/settings': (context) => SettingsScreen(),
13  },
14);
15
16Navigator.pushNamed(context, '/details');
17
18// Pass arguments
19Navigator.pushNamed(
20  context,
21  '/details',
22  arguments: {'userId': '123'},
23);
24
25// Get arguments
26final args = ModalRoute.of(context)!.settings.arguments as Map;
27
28// Return data
29final result = await Navigator.push(
30  context,
31  MaterialPageRoute(builder: (_) => SelectionScreen()),
32);
33
34// Pop with result
35Navigator.pop(context, selectedValue);

HTTP and API

dart
1import 'package:http/http.dart' as http;
2import 'dart:convert';
3
4class ApiService {
5  final String baseUrl = 'https://api.example.com';
6
7  Future<List<User>> fetchUsers() async {
8    final response = await http.get(Uri.parse('$baseUrl/users'));
9
10    if (response.statusCode == 200) {
11      final List<dynamic> data = json.decode(response.body);
12      return data.map((json) => User.fromJson(json)).toList();
13    } else {
14      throw Exception('Failed to load users');
15    }
16  }
17
18  Future<User> createUser(User user) async {
19    final response = await http.post(
20      Uri.parse('$baseUrl/users'),
21      headers: {'Content-Type': 'application/json'},
22      body: json.encode(user.toJson()),
23    );
24
25    if (response.statusCode == 201) {
26      return User.fromJson(json.decode(response.body));
27    } else {
28      throw Exception('Failed to create user');
29    }
30  }
31}
32
33// Model with JSON serialization
34class User {
35  final String id;
36  final String name;
37  final String email;
38
39  User({required this.id, required this.name, required this.email});
40
41  factory User.fromJson(Map<String, dynamic> json) {
42    return User(
43      id: json['id'],
44      name: json['name'],
45      email: json['email'],
46    );
47  }
48
49  Map<String, dynamic> toJson() {
50    return {
51      'id': id,
52      'name': name,
53      'email': email,
54    };
55  }
56}

Best Practices

Dart

• Use const constructors when possible
• Leverage null safety
• Use final for immutable values
• Prefer async/await over .then()
• Use extensions for utility methods
• Follow Effective Dart guidelines

Flutter

• Keep widgets small and focused
• Use const widgets for optimization
• Avoid rebuilding entire trees
• Implement proper state management
• Handle errors gracefully
• Test widgets thoroughly
• Use keys when needed

Performance

• Use ListView.builder for long lists
• Implement pagination
• Cache network images
• Minimize widget rebuilds
• Use const constructors
• Profile app performance
• Lazy load resources

Anti-Patterns

❌ Large widget trees ❌ State in StatelessWidget ❌ Not disposing controllers ❌ Ignoring null safety ❌ Synchronous I/O in UI ❌ No error handling ❌ Hard-coded strings

Resources

• Dart: https://dart.dev/
• Flutter: https://flutter.dev/
• Pub.dev: https://pub.dev/
• Flutter Widget Catalog: https://flutter.dev/docs/development/ui/widgets
• Effective Dart: https://dart.dev/guides/language/effective-dart