class Contact {
  final String name;
  final String? phone; // nullable — might not have phone
  final String? email;

  const Contact({required this.name, this.phone, this.email});

  @override
  String toString() => 'Contact(name: $name, phone: ${phone ?? "N/A"}, email: ${email ?? "N/A"})';
}

class ContactBook {
  final List<Contact> _contacts = [];

  void add(Contact contact) => _contacts.add(contact);

  Contact? findByName(String name) {
    try {
      return _contacts.firstWhere(
        (c) => c.name.toLowerCase() == name.toLowerCase(),
      );
    } on StateError {
      return null; // Not found — return null safely
    }
  }

  bool deleteByName(String name) {
    final contact = findByName(name);
    if (contact == null) return false;
    _contacts.remove(contact);
    return true;
  }

  void listAll() {
    if (_contacts.isEmpty) {
      print('Contact book is empty.');
      return;
    }
    for (final c in _contacts) print('  • $c');
  }
}

void main() {
  final book = ContactBook();
  book.add(Contact(name: 'Tarun', phone: '+91 98765 43210'));
  book.add(Contact(name: 'Alice', email: 'alice@example.com'));
  book.add(Contact(name: 'Bob', phone: '+1 555 0100', email: 'bob@example.com'));

  print('=== All Contacts ===');
  book.listAll();

  print('\n=== Search ===');
  final found = book.findByName('alice');
  print(found != null ? 'Found: $found' : 'Not found');

  print('\n=== Delete ===');
  print('Deleted Bob: ${book.deleteByName("Bob")}');
  print('Deleted Eve: ${book.deleteByName("Eve")}'); // false
  book.listAll();
}

import 'dart:async';

class NewsArticle {
  final String title;
  final String body;
  final DateTime publishedAt;
  final String? author; // nullable

  NewsArticle({
    required this.title,
    required this.body,
    required this.publishedAt,
    this.author,
  });

  @override
  String toString() =>
      '[${publishedAt.hour}:${publishedAt.minute.toString().padLeft(2,'0')}] '
      '${title} — ${author ?? "Anonymous"}';
}

class NewsFeedService {
  final List<NewsArticle> _fakeData = [
    NewsArticle(title: 'Flutter 4.0 Released!', body: 'Major rendering overhaul...', publishedAt: DateTime.now(), author: 'Tech Editor'),
    NewsArticle(title: 'Dart Null Safety Saves Lives', body: 'Billions of NPEs prevented...', publishedAt: DateTime.now()),
    NewsArticle(title: 'Riverpod 3.0 Ships', body: 'Breaking changes incoming...', publishedAt: DateTime.now(), author: 'Remi Rousselet'),
    NewsArticle(title: 'iOS 20 Announced', body: 'New APIs for Flutter devs...', publishedAt: DateTime.now()),
    NewsArticle(title: 'Android 16 Preview', body: 'Predictive back gesture...', publishedAt: DateTime.now(), author: 'Android Team'),
    NewsArticle(title: 'App Store Rejects 1M Apps', body: 'Quality crackdown...', publishedAt: DateTime.now()),
  ];

  // Future: fetch all at once (simulates REST API)
  Future<List<NewsArticle>> fetchLatest() async {
    await Future.delayed(const Duration(milliseconds: 300)); // network latency
    return _fakeData;
  }

  // Stream: emit one article at a time (simulates WebSocket/SSE)
  Stream<NewsArticle> fetchStream() async* {
    for (final article in _fakeData) {
      await Future.delayed(const Duration(milliseconds: 500));
      yield article; // yield sends the value to the stream listener
    }
  }
}

void main() async {
  final service = NewsFeedService();

  // 1. Batch fetch with Future
  print('=== Fetching latest (Future) ===');
  try {
    final articles = await service.fetchLatest();
    print('Fetched ${articles.length} articles.');
  } catch (e) {
    print('Error: $e');
  }

  // 2. Live stream — stop after 3 articles
  print('\n=== Live Stream (first 3 articles) ===');
  int count = 0;
  await for (final article in service.fetchStream()) {
    print('  $article');
    count++;
    if (count >= 3) break; // Stop listening after 3
  }
  print('Stream stopped after $count articles.');
}