Dart：什么是它，Flutter语言的特点和编程

// Dart中的基本类型和声明
void main() {
  // 显式类型化
  int age = 25;
  double price = 19.99;
  String name = 'Dart';
  bool isValid = true;

  // Null safety —— 变量可以为null
  int? nullableAge = null;
  int result = nullableAge ?? 0; // Null-aware运算符

  // 带泛型的集合
  List<String> items = ['Dart', 'Flutter', 'Mobile'];
  Map<String, dynamic> config = {
    'version': '3.0',
    'mode': 'debug',
  };

  // 级联表示法
  final list = <String>[]
    ..add('one')
    ..add('two')
    ..sort();
}

// Record —— 匿名不可变类型
(String, int) user = ('Alice', 30);
print(user.$1); // 'Alice'

// Dart中的类、继承和mixin
abstract class Animal {
  void sound(); // 抽象方法
}

mixin Swimmer {
  void swim() => print('Swimming...');
}

mixin Flyer {
  void fly() => print('Flying...');
}

class Duck extends Animal with Swimmer, Flyer {
  @override
  void sound() => print('Quack!');
}

void main() {
  final duck = Duck();
  duck.sound(); // Quack!
  duck.swim();  // Swimming...
  duck.fly();   // Flying...
}

// 命名构造函数
class Config {
  final String env;

  Config.development() : env = 'dev';
  Config.production()  : env = 'prod';
}

// 使用Future和Stream异步加载数据
import 'dart:convert';
import 'package:http/http.dart' as http;

class ApiService {
  final String baseUrl;

  ApiService(this.baseUrl);

  // 用于单个请求的Future
  Future<Map<String, dynamic>> fetchUser(int id) async {
    final response = await http.get(
      Uri.parse('$baseUrl/users/$id'),
    );

    if (response.statusCode == 200) {
      return jsonDecode(response.body);
    } else {
      throw Exception('Failed to load user: ${response.statusCode}');
    }
  }

  // 用于数据序列的Stream
  Stream<int> countStream(int max) async* {
    for (var i = 1; i <= max; i++) {
      await Future.delayed(Duration(seconds: 1));
      yield i;
    }
  }
}

void main() async {
  final api = ApiService('https://api.example.com');

  // 使用await
  final user = await api.fetchUser(1);
  print('User: ${user['name']}');

  // 使用await for处理Stream
  await for (final count in api.countStream(5)) {
    print('Count: $count');
  }
}

// Dart中的简单Flutter应用程序
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Dart Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  int _counter = 0;

  void _increment() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Dart + Flutter')),
      body: Center(
        child: Text('Counter: $_counter'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _increment,
        child: Icon(Icons.add),
      ),
    );
  }
}

# pubspec.yaml —— Dart项目的配置
name: my_flutter_app
description: A new Flutter project.
version: 1.0.0

environment:
  sdk: '>=3.0.0 <4.0.0'

dependencies:
  flutter:
    sdk: flutter
  http: ^1.2.0
  provider: ^6.1.0
  shared_preferences: ^2.2.0
  json_annotation: ^4.8.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  build_runner: ^2.4.0
  json_serializable: ^6.7.0
  flutter_lints: ^3.0.0