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

  @override
  State<EphemeralCounter> createState() => _EphemeralCounterState();
}

class _EphemeralCounterState extends State<EphemeralCounter> {
  int _counter = 0; // Local state

  void _increment() {
    setState(() {
      _counter++; // Mutate state and schedule rebuild
    });
  }

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: _increment,
      child: Text('Count: $_counter'),
    );
  }
}

// 1. Model (Repository)
class CartRepository {
  Future<void> saveItemToCart(String item) async {
    // Simulate network/database call
    await Future.delayed(const Duration(milliseconds: 500));
  }
}

// 2. ViewModel (ChangeNotifier)
class CartViewModel extends ChangeNotifier {
  final CartRepository repository;
  
  CartViewModel({required this.repository});

  final List<String> _items = [];
  bool isLoading = false;
  String? errorMessage;

  List<String> get items => List.unmodifiable(_items);

  Future<void> addItem(String item) async {
    isLoading = true;
    errorMessage = null;
    notifyListeners(); // Trigger loading UI

    try {
      await repository.saveItemToCart(item);
      _items.add(item);
    } catch (e) {
      errorMessage = 'Failed to add item';
    } finally {
      isLoading = false;
      notifyListeners(); // Trigger success/error UI
    }
  }
}

// 3. Injection & 4. View (UI)
class CartApp extends StatelessWidget {
  const CartApp({super.key});

  @override
  Widget build(BuildContext context) {
    // Inject ViewModel
    return ChangeNotifierProvider(
      create: (_) => CartViewModel(repository: CartRepository()),
      child: const CartScreen(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Consumer<CartViewModel>(
        builder: (context, viewModel, child) {
          if (viewModel.isLoading) {
            return const CircularProgressIndicator();
          }
          if (viewModel.errorMessage != null) {
            return Text(viewModel.errorMessage!);
          }
          return ListView.builder(
            itemCount: viewModel.items.length,
            itemBuilder: (_, index) => Text(viewModel.items[index]),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        // Use read() to access methods without listening for rebuilds
        onPressed: () => context.read<CartViewModel>().addItem('New Item'),
        child: const Icon(Icons.add),
      ),
    );
  }
}