Dart 3 Records and Patterns in Flutter: solved exercise

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dependencies:
  flutter:
    sdk: flutter

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import 'package:flutter/material.dart';

// ══════════════════════════════════════════════════════════════════════════════
// 1. RECORDS — anonymous struct with static typing
// ══════════════════════════════════════════════════════════════════════════════

// Record with positional fields
typedef Point = (double x, double y);

// Record with named fields
typedef Product = ({String name, double price, int stock});

Product fetchProduct() =>
    (name: 'Flutter Course', price: 29.99, stock: 100);

// Function returning multiple values without creating a class
(String city, int population) getCapital(String country) {
  return switch (country) {
    'Spain'   => ('Madrid', 3_300_000),
    'France'  => ('Paris', 2_100_000),
    'Germany' => ('Berlin', 3_700_000),
    _         => ('Unknown', 0),
  };
}

// ══════════════════════════════════════════════════════════════════════════════
// 2. SEALED CLASSES — closed hierarchy with guaranteed exhaustiveness
// ══════════════════════════════════════════════════════════════════════════════

sealed class Shape {}

class Circle    extends Shape { final double radius;  Circle(this.radius); }
class Rectangle extends Shape { final double width, height; Rectangle(this.width, this.height); }
class Triangle  extends Shape { final double base, height;  Triangle(this.base, this.height); }

// switch expression is exhaustive with sealed — the compiler verifies it
double area(Shape shape) => switch (shape) {
  Circle(:var radius)                    => 3.14159 * radius * radius,
  Rectangle(:var width, :var height)     => width * height,
  Triangle(:var base, :var height)       => 0.5 * base * height,
};

// Generic async state with sealed class
sealed class LoadState<T> {}
class Loading<T> extends LoadState<T> {}
class Success<T> extends LoadState<T> { final T data; Success(this.data); }
class Failure<T> extends LoadState<T> { final String message; Failure(this.message); }

// ══════════════════════════════════════════════════════════════════════════════
// 3. PATTERNS — list and map destructuring
// ══════════════════════════════════════════════════════════════════════════════

String describeList(List<int> numbers) => switch (numbers) {
  []                              => 'Empty list',
  [var single]                    => 'One element: $single',
  [var first, var second]         => 'Two elements: $first and $second',
  [var first, ..., var last]      => 'Several: first $first, last $last',
};

String readConfig(Map<String, dynamic> config) {
  return switch (config) {
    {'mode': 'debug',   'version': var v} => 'DEBUG v$v',
    {'mode': 'release', 'version': var v} => 'RELEASE v$v',
    _ => 'Unknown configuration',
  };
}

// ══════════════════════════════════════════════════════════════════════════════
// Flutter app
// ══════════════════════════════════════════════════════════════════════════════

Future<void> main() => runApp(const MyApp());

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Dart 3 Features',
      theme: ThemeData(useMaterial3: true, colorSchemeSeed: Colors.deepPurple),
      home: const Dart3Page(),
    );
  }
}

class Dart3Page extends StatelessWidget {
  const Dart3Page({super.key});

  @override
  Widget build(BuildContext context) {
    // ── 1. Records ────────────────────────────────────────────────────────────
    final product = fetchProduct();
    final (city, population) = getCapital('Spain'); // positional destructuring

    // ── 2. Shapes ─────────────────────────────────────────────────────────────
    final shapes = <Shape>[
      Circle(5),
      Rectangle(4, 6),
      Triangle(3, 8),
    ];

    // ── 3. LoadState ──────────────────────────────────────────────────────────
    final LoadState<String> state = Success('Data loaded successfully');

    // ── 4. Lists and maps ─────────────────────────────────────────────────────
    final listExamples = [
      describeList([]),
      describeList([42]),
      describeList([1, 2]),
      describeList([10, 20, 30, 40, 50]),
    ];

    final mapExample = readConfig({'mode': 'release', 'version': '2.0.0'});

    return Scaffold(
      appBar: AppBar(title: const Text('Dart 3: Records & Patterns')),
      body: ListView(
        padding: const EdgeInsets.all(16),
        children: [
          // Named record
          _DemoCard(
            title: '📦 Named record fields',
            content: 'Product: ${product.name}\n'
                'Price: \$${product.price}\n'
                'Stock: ${product.stock}',
          ),
          // Positional record + destructuring
          _DemoCard(
            title: '🗺 Positional record + destructuring',
            content: 'Capital: $city\nPopulation: $population',
          ),
          // Sealed + switch expression
          _DemoCard(
            title: '🔷 Sealed class: shapes and areas',
            content: shapes.map((s) {
              final name = switch (s) {
                Circle()    => 'Circle',
                Rectangle() => 'Rectangle',
                Triangle()  => 'Triangle',
              };
              return '$name → area ${area(s).toStringAsFixed(2)}';
            }).join('\n'),
          ),
          // LoadState
          _DemoCard(
            title: '🔄 Sealed LoadState<T>',
            content: switch (state) {
              Loading()              => 'Loading...',
              Success(:var data)     => '✅ $data',
              Failure(:var message)  => '❌ $message',
            },
          ),
          // List patterns
          _DemoCard(
            title: '📋 List patterns',
            content: listExamples.join('\n'),
          ),
          // Map patterns
          _DemoCard(
            title: '🗂 Map patterns',
            content: mapExample,
          ),
        ],
      ),
    );
  }
}

class _DemoCard extends StatelessWidget {
  final String title;
  final String content;
  const _DemoCard({required this.title, required this.content});

  @override
  Widget build(BuildContext context) {
    return Card(
      margin: const EdgeInsets.only(bottom: 12),
      child: Padding(
        padding: const EdgeInsets.all(16),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Text(title,
                style: const TextStyle(
                    fontWeight: FontWeight.bold, fontSize: 15)),
            const SizedBox(height: 8),
            Text(content, style: const TextStyle(fontFamily: 'monospace')),
          ],
        ),
      ),
    );
  }
}