Dart

void main() {
  String name = 'Sufi';
  int age = 30;
  double price = 9.99;
  bool active = true;
  final immutable = DateTime.now();
  const PI = 3.14159;

// Nullable types String? nullable = null; String safe = nullable ?? 'default'; print(nullable?.length);

// String interpolation print('Hello, $name! Age: $age'); print('Upper: ${name.toUpperCase()}');

// Collections var list = [1, 2, 3, 4, 5]; var map = {'key': 'value', 'name': 'Sufi'}; var extended = [...list, 6, 7]; // spread } ```

class User {
  final String id;
  String name;
  String? bio;

User({required this.id, required this.name, this.bio});

User.anonymous() : id = 'anon', name = 'Anonymous';

factory User.fromJson(Map json) { return User(id: json['id'] as String, name: json['name'] as String, bio: json['bio'] as String?); }

bool get hasProfile => bio != null && bio!.isNotEmpty; Map toJson() => {'id': id, 'name': name, 'bio': bio};

@override String toString() => 'User($name)'; }

mixin Loggable { void log(String message) => print('[LOG] $message'); }

class Service with Loggable { void doWork() { log('Working...'); } } ```

Future<String> fetchUser(int id) async {
  await Future.delayed(Duration(milliseconds: 300));
  return 'User $id';
}

Future loadParallel() async { final results = await Future.wait([fetchUser(1), fetchUser(2), fetchUser(3)]); print(results); }

// Stream multiple async values Stream countDown(int from) async* { for (int i = from; i >= 0; i--) { await Future.delayed(Duration(seconds: 1)); yield i; } }

void listenToStream() { countDown(5).listen( (value) => print(value), onDone: () => print('Done!'), ); } ```

import 'package:flutter/material.dart';

class UserCard extends StatelessWidget { final String name; final String email; final VoidCallback onTap;

const UserCard({super.key, required this.name, required this.email, required this.onTap});

@override Widget build(BuildContext context) { return Card( elevation: 4, shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)), child: InkWell( onTap: onTap, borderRadius: BorderRadius.circular(12), child: Padding( padding: const EdgeInsets.all(16), child: Row(children: [ CircleAvatar(child: Text(name[0].toUpperCase())), const SizedBox(width: 12), Expanded(child: Column( crossAxisAlignment: CrossAxisAlignment.start, children: [ Text(name, style: const TextStyle(fontWeight: FontWeight.bold)), Text(email, style: const TextStyle(color: Colors.grey)), ], )), const Icon(Icons.chevron_right), ]), ), ), ); } } ```

// ChangeNotifier
class CartNotifier extends ChangeNotifier {
  final List<CartItem> _items = [];
  List<CartItem> get items => List.unmodifiable(_items);
  double get total => _items.fold(0, (sum, item) => sum + item.price);

void addItem(CartItem item) { _items.add(item); notifyListeners(); } void removeItem(String id) { _items.removeWhere((i) => i.id == id); notifyListeners(); } }

// Riverpod final cartProvider = StateNotifierProvider((ref) => CartNotifier());

final userProvider = FutureProvider.family((ref, id) async { return ref.read(userRepositoryProvider).getUser(id); });

class CartScreen extends ConsumerWidget { @override Widget build(BuildContext context, WidgetRef ref) { final cart = ref.watch(cartProvider); return ListView.builder( itemCount: cart.items.length, itemBuilder: (_, i) => ListTile(title: Text(cart.items[i].name)), ); } } ```