// variables and types
String name = "Rahul";
int age = 25;
double price = 75000.50;
bool isActive = true;
List<String> tags = ["flutter", "dart", "mobile"];
Map<String, dynamic> user = {"name": "Rahul", "age": 25};

// Null safety — explicit nullable types
String? nullableName = null;    // ? means can be null
String definedName = "Rahul";  // Cannot be null

// Functions
String greet(String name) {
    return "Hello, $name!";
}

// Arrow function
String greetShort(String name) => "Hello, $name!";

// Named and optional parameters
void createUser({required String name, String role = "user", int? age}) {
    print("$name is a $role");
}

createUser(name: "Rahul", role: "admin", age: 25);
createUser(name: "Priya");  // role defaults to "user", age is null

// Async/await (similar to JavaScript)
Future<String> fetchUser(int id) async {
    final response = await http.get(Uri.parse("https://api.example.com/users/$id"));
    return jsonDecode(response.body)["name"];
}

// Classes
class User {
    final String name;
    final String email;
    int _loginCount = 0;  // Private field (underscore prefix)

    User({required this.name, required this.email});

    void login() => _loginCount++;
    int get loginCount => _loginCount;

    @override
    String toString() => "User($name, $email)";
}

// Dart: Static typing
String name = 42;  // ERROR at compile time
// JavaScript: name = 42; would be fine (runtime issue)

// Dart: Sound null safety
String name = null;  // ERROR — name is non-nullable
String? nullable = null;  // OK — explicitly nullable

// Dart: No implicit coercion
print(1 + "2");  // ERROR in Dart
// JavaScript: 1 + "2" = "12" (confusing)

import "package:flutter/material.dart";

// StatelessWidget: UI never changes after creation
class WelcomeCard extends StatelessWidget {
    final String username;
    final String role;

    const WelcomeCard({
        super.key,
        required this.username,
        required this.role
    });

    @override
    Widget build(BuildContext context) {
        return Card(
            elevation: 4,
            margin: const EdgeInsets.all(16),
            child: Padding(
                padding: const EdgeInsets.all(20),
                child: Column(
                    mainAxisSize: MainAxisSize.min,
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: [
                        Text(
                            "Welcome, $username!",
                            style: const TextStyle(
                                fontSize: 24,
                                fontWeight: FontWeight.bold,
                            ),
                        ),
                        const SizedBox(height: 8),
                        Chip(
                            label: Text(role),
                            backgroundColor: Colors.blue.shade100,
                        ),
                    ],
                ),
            ),
        );
    }
}

class CounterWidget extends StatefulWidget {
    const CounterWidget({super.key});

    @override
    State<CounterWidget> createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
    int _count = 0;

    void _increment() {
        setState(() {          // setState triggers UI rebuild
            _count++;
        });
    }

    @override
    Widget build(BuildContext context) {
        return Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
                Text(
                    "Count: $_count",
                    style: const TextStyle(fontSize: 48, fontWeight: FontWeight.bold),
                ),
                const SizedBox(height: 20),
                Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                        FloatingActionButton(
                            onPressed: () => setState(() => _count--),
                            child: const Icon(Icons.remove),
                        ),
                        const SizedBox(width: 20),
                        FloatingActionButton(
                            onPressed: _increment,
                            child: const Icon(Icons.add),
                        ),
                    ],
                ),
            ],
        );
    }
}

// Layout widgets
Column(children: [...])           // Vertical arrangement
Row(children: [...])              // Horizontal arrangement
Stack(children: [...])            // Overlapping widgets
Container(child: ...)             // Box with styling
Padding(padding: ..., child: ...) // Add spacing
Center(child: ...)                // Center a child
Expanded(child: ...)              // Fill available space
SizedBox(width: 100, height: 50)  // Fixed size box

// Text and images
Text("Hello Flutter!")
RichText(text: TextSpan(...))
Image.network("https://...")
Image.asset("assets/logo.png")
Icon(Icons.home)

// User input
TextField(controller: ..., onChanged: ...)
ElevatedButton(onPressed: ..., child: Text("Click"))
TextButton(onPressed: ..., child: Text("Cancel"))
Checkbox(value: ..., onChanged: ...)
Switch(value: ..., onChanged: ...)
Slider(value: ..., onChanged: ...)

// Lists
ListView.builder(
    itemCount: items.length,
    itemBuilder: (context, index) => ListTile(title: Text(items[index])),
)

GridView.builder(
    gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
    itemBuilder: (context, index) => ProductCard(product: products[index]),
)

Traditional mobile development:
Change code → Stop app → Full recompile (30-60 seconds) → Restart → Navigate back to screen
Multiply by 100 changes per day → Hours wasted

Flutter Hot Reload:
Change code → Press r in terminal → < 1 second → UI updates (state preserved!)
You stay on the same screen, form data intact, scroll position kept

Hot Reload (r):
→ Injects updated code into the running Dart VM
→ Widget tree rebuilds
→ State is PRESERVED
→ < 1 second
→ Use for: UI changes, logic changes within current state

Hot Restart (R or Shift+R):
→ Restarts the Dart VM completely
→ State is LOST (starts fresh)
→ 2-5 seconds
→ Use for: main() changes, new global state, initializer changes

import "package:flutter/material.dart";

void main() {
    runApp(const MyApp());
}

class MyApp extends StatelessWidget {
    const MyApp({super.key});

    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            title: "FutureTechZone",
            theme: ThemeData(
                useMaterial3: true,
                colorScheme: ColorScheme.fromSeed(
                    seedColor: const Color(0xFF0066CC),
                    brightness: Brightness.light,
                ),
                fontFamily: "Inter",
                textTheme: const TextTheme(
                    headlineLarge: TextStyle(fontSize: 32, fontWeight: FontWeight.bold),
                    bodyLarge: TextStyle(fontSize: 16),
                ),
            ),
            darkTheme: ThemeData(
                useMaterial3: true,
                colorScheme: ColorScheme.fromSeed(
                    seedColor: const Color(0xFF0066CC),
                    brightness: Brightness.dark,
                ),
            ),
            themeMode: ThemeMode.system,   // Follows device setting
            home: const HomePage(),
        );
    }
}

class HomePage extends StatelessWidget {
    const HomePage({super.key});

    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
                title: const Text("FutureTechZone"),
                actions: [
                    IconButton(
                        icon: const Icon(Icons.search),
                        onPressed: () {},
                    ),
                    IconButton(
                        icon: const Icon(Icons.notifications),
                        onPressed: () {},
                    ),
                ],
            ),
            body: const ArticleList(),
            bottomNavigationBar: NavigationBar(
                destinations: const [
                    NavigationDestination(icon: Icon(Icons.home), label: "Home"),
                    NavigationDestination(icon: Icon(Icons.explore), label: "Explore"),
                    NavigationDestination(icon: Icon(Icons.bookmark), label: "Saved"),
                    NavigationDestination(icon: Icon(Icons.person), label: "Profile"),
                ],
            ),
            floatingActionButton: FloatingActionButton.extended(
                onPressed: () {},
                icon: const Icon(Icons.add),
                label: const Text("Write Article"),
            ),
        );
    }
}

import "package:flutter/cupertino.dart";

// Use Cupertino widgets for native iOS look
CupertinoApp(
    home: CupertinoPageScaffold(
        navigationBar: const CupertinoNavigationBar(
            middle: Text("FutureTechZone"),
        ),
        child: Column(
            children: [
                CupertinoTextField(placeholder: "Search..."),
                CupertinoButton(
                    onPressed: () {},
                    child: const Text("Subscribe"),
                ),
                CupertinoSwitch(
                    value: true,
                    onChanged: (val) {},
                ),
            ],
        ),
    ),
)