// Khai báo biến
var name = 'Nguyễn Văn A';  // Kiểu được suy ra
String city = 'Hà Nội';     // Kiểu tường minh
final age = 25;             // Không thể thay đổi sau khi gán
const PI = 3.14;            // Hằng số biên dịch

// Hàm trong Dart
int add(int a, int b) {
  return a + b;
}

// Hàm mũi tên (Arrow function)
int multiply(int a, int b) => a * b;

// Lớp trong Dart
class Person {
  String name;
  int age;
  
  Person(this.name, this.age);  // Constructor ngắn gọn
  
  void introduce() {
    print('Xin chào, tôi là $name, $age tuổi.');
  }
}

// StatelessWidget - Widget không có trạng thái
import 'package:flutter/material.dart';

class MyStatelessWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text('Hello, Flutter!'),
    );
  }
}

// StatefulWidget - Widget có trạng thái
class MyStatefulWidget extends StatefulWidget {
  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  int _counter = 0;
  
  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text('Số lần nhấn: $_counter'),
        ElevatedButton(
          onPressed: _incrementCounter,
          child: Text('Tăng'),
        ),
      ],
    );
  }
}

// Ví dụ về Column và Row
import 'package:flutter/material.dart';

Widget build(BuildContext context) {
  return Column(
    mainAxisAlignment: MainAxisAlignment.center,
    crossAxisAlignment: CrossAxisAlignment.start,
    children: [
      Text('Dòng 1'),
      Text('Dòng 2'),
      Row(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
          Icon(Icons.star),
          Icon(Icons.star),
          Icon(Icons.star),
        ],
      ),
    ],
  );
}

// Định nghĩa model
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class Counter {
  int value = 0;
  
  void increment() {
    value++;
  }
}

// Sử dụng trong app
void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => Counter(),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Provider Example'),
        ),
        body: Center(
          child: CounterDisplay(),
        ),
      ),
    );
  }
}

// Tiêu thụ trong Widget
class CounterDisplay extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text(
      '${context.watch<Counter>().value}',
      style: Theme.of(context).textTheme.headline4,
    );
  }
}

// Điều hướng cơ bản
import 'package:flutter/material.dart';

Navigator.push(
  context,
  MaterialPageRoute(builder: (context) => SecondScreen()),
);

// Điều hướng có tên (Named Routes)
Navigator.pushNamed(context, '/second');

// Định nghĩa routes
MaterialApp(
  routes: {
    '/': (context) => HomeScreen(),
    '/second': (context) => SecondScreen(),
    '/detail': (context) => DetailScreen(),
  },
);

// Sử dụng package http
import 'package:http/http.dart' as http;
import 'dart:convert';

Future<List<Post>> fetchPosts() async {
  final response = await http.get(Uri.parse('https://jsonplaceholder.typicode.com/posts'));
  
  if (response.statusCode == 200) {
    List<dynamic> body = jsonDecode(response.body);
    return body.map((item) => Post.fromJson(item)).toList();
  } else {
    throw Exception('Failed to load posts');
  }
}

// Định nghĩa model
class Post {
  final int id;
  final String title;
  final String body;
  
  Post({required this.id, required this.title, required this.body});
  
  factory Post.fromJson(Map<String, dynamic> json) {
    return Post(
      id: json['id'],
      title: json['title'],
      body: json['body'],
    );
  }
}

// Sử dụng SharedPreferences
import 'package:shared_preferences/shared_preferences.dart';

// Lưu dữ liệu
Future<void> saveData() async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.setString('username', 'NguyenVanA');
  await prefs.setInt('age', 25);
  await prefs.setBool('isLoggedIn', true);
}

// Đọc dữ liệu
Future<void> readData() async {
  final prefs = await SharedPreferences.getInstance();
  final username = prefs.getString('username') ?? 'Guest';
  final age = prefs.getInt('age') ?? 0;
  final isLoggedIn = prefs.getBool('isLoggedIn') ?? false;
  
  print('Username: $username, Age: $age, Logged in: $isLoggedIn');
}

// Animation cơ bản với AnimatedContainer
import 'package:flutter/material.dart';

AnimatedContainer(
  duration: Duration(seconds: 1),
  curve: Curves.fastOutSlowIn,
  width: _expanded ? 200 : 100,
  height: _expanded ? 100 : 50,
  color: _expanded ? Colors.blue : Colors.red,
  child: Center(child: Text('Nhấn vào tôi')),
);

// Controller-based Animation
class _MyAnimationState extends State<MyAnimation> with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<double> _animation;
  
  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: const Duration(seconds: 2),
      vsync: this,
    );
    _animation = CurvedAnimation(
      parent: _controller,
      curve: Curves.easeInOut,
    );
    _controller.forward();
  }
  
  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
  
  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: _animation,
      child: const Text('Fade In Animation'),
    );
  }
}

// Unit test
import 'package:flutter_test/flutter_test.dart';

void main() {
  test('Counter value should be incremented', () {
    final counter = Counter();
    counter.increment();
    expect(counter.value, 1);
  });
}

// Widget test
void main() {
  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
    await tester.pumpWidget(MyApp());
    expect(find.text('0'), findsOneWidget);
    await tester.tap(find.byIcon(Icons.add));
    await tester.pump();
    expect(find.text('1'), findsOneWidget);
  });
}

// Tránh rebuild không cần thiết với const
import 'package:flutter/material.dart';

const MyWidget(
  key: Key('my_widget'),
  child: Text('Hello'),
);

// Tối ưu với ListView.builder cho danh sách dài
ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) {
    return ListTile(
      title: Text(items[index].title),
    );
  },
);

// lib/models/todo.dart
class Todo {
  final String id;
  final String title;
  bool isCompleted;
  
  Todo({
    required this.id,
    required this.title,
    this.isCompleted = false,
  });
}

// lib/screens/home_screen.dart
import 'package:flutter/material.dart';
import 'package:todo_app/models/todo.dart';

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

class _HomeScreenState extends State<HomeScreen> {
  final List<Todo> _todos = [];
  final _textController = TextEditingController();
  
  void _addTodo() {
    if (_textController.text.isEmpty) return;
    
    setState(() {
      _todos.add(Todo(
        id: DateTime.now().toString(),
        title: _textController.text,
      ));
      _textController.clear();
    });
  }
  
  void _toggleTodo(String id) {
    setState(() {
      final todo = _todos.firstWhere((todo) => todo.id == id);
      todo.isCompleted = !todo.isCompleted;
    });
  }
  
  void _removeTodo(String id) {
    setState(() {
      _todos.removeWhere((todo) => todo.id == id);
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Todo App'),
      ),
      body: Column(
        children: [
          Padding(
            padding: const EdgeInsets.all(16.0),
            child: Row(
              children: [
                Expanded(
                  child: TextField(
                    controller: _textController,
                    decoration: InputDecoration(
                      hintText: 'Thêm công việc mới...',
                    ),
                  ),
                ),
                IconButton(
                  icon: Icon(Icons.add),
                  onPressed: _addTodo,
                ),
              ],
            ),
          ),
          Expanded(
            child: _todos.isEmpty
                ? Center(child: Text('Chưa có công việc nào!'))
                : ListView.builder(
                    itemCount: _todos.length,
                    itemBuilder: (context, index) {
                      final todo = _todos[index];
                      return ListTile(
                        title: Text(
                          todo.title,
                          style: TextStyle(
                            decoration: todo.isCompleted
                                ? TextDecoration.lineThrough
                                : null,
                          ),
                        ),
                        leading: Checkbox(
                          value: todo.isCompleted,
                          onChanged: (_) => _toggleTodo(todo.id),
                        ),
                        trailing: IconButton(
                          icon: Icon(Icons.delete),
                          onPressed: () => _removeTodo(todo.id),
                        ),
                      );
                    },
                  ),
          ),
        ],
      ),
    );
  }
}

// lib/main.dart
import 'package:flutter/material.dart';
import 'package:todo_app/screens/home_screen.dart';

void main() {
  runApp(TodoApp());
}

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