Flutter: Dart Language Crash Course

import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;

// Simplified class definition (for demonstration)
class Product {
  final int id;
  final String name;
  final double price;
  final String? imageUrl;
  const Product({required this.id, required this.name, required this.price, this.imageUrl});
  factory Product.fromJson(Map<String, dynamic> json) => Product(
    id: json['id'] as int,
    name: json['name'] as String,
    price: (json['price'] as num).toDouble(),
    imageUrl: json['image_url'] as String?,
  );
  Product copyWith({String? name, double? price}) => Product(
    id: id, name: name ?? this.name, price: price ?? this.price, imageUrl: imageUrl,
  );
  static List<Product> fromJsonList(List data) => data.map((j) => Product.fromJson(j as Map<String, dynamic>)).toList();
}

class CartItem {
  final Product product;
  final double price;
  final int quantity;
  CartItem({required this.product, required this.price, this.quantity = 1});
}

// Strong typing prevents price bugs
double calculateTotal(List<CartItem> items) {
  return items.fold(0.0, (sum, item) => sum + item.price * item.quantity);
}

// async/await eliminates callback hell
Future<List<Product>> fetchProducts() async {
  final response = await http.get(Uri.parse('/api/v1/products'));
  return Product.fromJsonList(jsonDecode(response.body));
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

// var: type inferred, mutable
var productName = 'Flutter T-Shirt';
productName = 'Dart Hoodie'; // OK

// final: runtime constant, immutable
final double totalPrice = 29.99 * 3;
// totalPrice = 100.0; // Error: final variable

// const: compile-time constant
const double discountRate = 0.15;
const String storeName = 'ShopApp';

// late: deferred initialization
late String userToken;
void login() {
  userToken = 'eyJhbGciOiJIUzI1NiIs...';
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

import 'package:flutter/material.dart';

// Product class definition shown above

// List: ordered collection
List<Product> products = [
  Product(id: 1, name: 'Laptop', price: 1299.99),
  Product(id: 2, name: 'Phone', price: 899.99),
];

// Map: key-value pairs
Map<String, double> prices = {
  'Laptop': 1299.99,
  'Phone': 899.99,
  'Tablet': 599.99,
};

// Set: unique items
Set<String> categories = {'Electronics', 'Clothing', 'Books'};

// Access and modify
products.add(Product(id: 3, name: 'Tablet', price: 599.99));
prices['Headphones'] = 199.99;
categories.add('Electronics'); // Ignored, already exists

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

import 'package:flutter/material.dart';

// Product class definition shown above

// collection-if: conditional items
bool isAdmin = true;
var menuItems = [
  'Home',
  'Products',
  'Cart',
  if (isAdmin) 'Admin Panel',
];

// collection-for: expand items
var productNames = ['Laptop', 'Phone', 'Tablet'];
var descriptions = [
  for (var name in productNames) '$name - Best Price'
];
// ['Laptop - Best Price', 'Phone - Best Price', 'Tablet - Best Price']

// Combined in Widget list
Widget buildProductList(List<Product> products) {
  return Column(
    children: [
      for (var p in products)
        ListTile(title: Text(p.name), trailing: Text('\$${p.price}')),
    ],
  );
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

import 'package:flutter/material.dart';

// Named parameters (recommended for Flutter)
Widget buildPriceTag({
  required double price,
  String currency = 'USD',
  double fontSize = 16.0,
}) {
  return Text(
    '$currency \$${price.toStringAsFixed(2)}',
    style: TextStyle(fontSize: fontSize),
  );
}

// Call with named params
buildPriceTag(price: 99.99);
buildPriceTag(price: 49.99, currency: 'EUR', fontSize: 20.0);

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

import 'package:flutter/material.dart';

// Product class definition shown above

// Anonymous function (lambda)
var expensiveItems = products.where((p) => p.price > 500.0).toList();

// Closure: captures surrounding variables
double discountThreshold = 100.0;
var discountedItems = products.map((p) {
  // Closure captures discountThreshold
  if (p.price > discountThreshold) {
    return p.copyWith(price: p.price * 0.9);
  }
  return p;
}).toList();

// Callback in Flutter widget
ElevatedButton(
  onPressed: () {
    // Closure captures context variables
    addToCart(product);
  },
  child: const Text('Add to Cart'),
)

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

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

// Product class definition shown above

Future<List<Product>> fetchProducts({int page = 1}) async {
  try {
    final response = await http.get(
      Uri.parse('https://api.shopapp.com/v1/products?page=$page'),
    );
    if (response.statusCode == 200) {
      final List data = jsonDecode(response.body);
      return data.map((json) => Product.fromJson(json)).toList();
    } else {
      throw Exception('Failed to load products: ${response.statusCode}');
    }
  } catch (e) {
    // Handle network errors
    rethrow;
  }
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

import 'dart:async';

// Simplified class definition (for demonstration)
class Product {
  final String name;
  final double price;
  const Product({required this.name, required this.price});
}

class CartItem {
  final Product product;
  int quantity;
  CartItem({required this.product, this.quantity = 1});
}

// StreamController for cart updates
class CartBloc {
  final _cartStream = StreamController<List<CartItem>>.broadcast();

  Stream<List<CartItem>> get cartStream => _cartStream.stream;
  List<CartItem> _items = [];

  void addToCart(Product product) {
    _items.add(CartItem(product: product, quantity: 1));
    _cartStream.sink.add(_items);
  }

  void dispose() {
    _cartStream.close();
  }
}

// Listen to stream
cartBloc.cartStream.listen((items) {
  print('Cart updated: ${items.length} items, total: \$${calculateTotal(items)}');
});

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

class Product {
  final int id;
  final String name;
  final double price;
  final String? imageUrl;
  final double rating;

  // Main constructor with named params
  const Product({
    required this.id,
    required this.name,
    required this.price,
    this.imageUrl,
    this.rating = 0.0,
  });

  // Named constructor from JSON
  factory Product.fromJson(Map<String, dynamic> json) {
    return Product(
      id: json['id'] as int,
      name: json['name'] as String,
      price: (json['price'] as num).toDouble(),
      imageUrl: json['image_url'] as String?,
      rating: (json['rating'] as num?)?.toDouble() ?? 0.0,
    );
  }

  // Copy with pattern for immutable updates
  Product copyWith({String? name, double? price}) {
    return Product(
      id: id,
      name: name ?? this.name,
      price: price ?? this.price,
      imageUrl: imageUrl,
      rating: rating,
    );
  }
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

// Product class definition shown above

// Mixin: reusable behavior without inheritance
mixin PriceFormatter {
  String formatPrice(double price, {String currency = 'USD'}) {
    return '$currency \$${price.toStringAsFixed(2)}';
  }

  String formatDiscount(double original, double discounted) {
    double percent = ((original - discounted) / original * 100);
    return '${percent.toStringAsFixed(0)}% OFF';
  }
}

// Apply mixin to class
class ProductCard with PriceFormatter {
  final Product product;
  ProductCard(this.product);

  String get priceLabel => formatPrice(product.price);
  String get discountLabel => formatDiscount(100.0, product.price);
}

// Abstract class: cannot be instantiated
abstract class Repository<T> {
  Future<List<T>> getAll();
  Future<T> getById(int id);
  Future<T> create(T item);
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.

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

// Model
class Product {
  final int id;
  final String name;
  final double price;
  final String category;

  const Product({
    required this.id,
    required this.name,
    required this.price,
    required this.category,
  });

  factory Product.fromJson(Map<String, dynamic> json) => Product(
        id: json['id'] as int,
        name: json['name'] as String,
        price: (json['price'] as num).toDouble(),
        category: json['category'] as String,
      );
}

// Repository with async/await
class ProductRepository {
  final String baseUrl = 'https://api.shopapp.com/v1';

  Future<List<Product>> fetchProducts({int page = 1, int limit = 20}) async {
    final response = await http.get(
      Uri.parse('$baseUrl/products?page=$page&limit=$limit'),
    );
    if (response.statusCode != 200) {
      throw Exception('API error: ${response.statusCode}');
    }
    final List data = jsonDecode(response.body)['items'] as List;
    return data.map((j) => Product.fromJson(j as Map<String, dynamic>)).toList();
  }

  Future<Product> fetchById(int id) async {
    final response = await http.get(Uri.parse('$baseUrl/products/$id'));
    return Product.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  }
}

// Usage
void main() async {
  final repo = ProductRepository();
  final products = await repo.fetchProducts(page: 1);
  for (var p in products) {
    print('${p.name}: \$${p.price.toStringAsFixed(2)}');
  }
}

> Output: Run locally with Flutter SDK (Flutter 3.x / Dart 3.x). The Piston server does not have Flutter installed; use `flutter run` on your local machine for comparison. Actual UI/state may vary slightly across platforms.