Hari 22: State Management — Provider

Mengapa State Management?

setState() bekerja baik untuk state lokal, tapi untuk state yang perlu dibagi antar banyak widget, kita perlu solusi seperti Provider.

// pubspec.yaml: provider: ^6.1.1
import "package:flutter/material.dart";
import "package:provider/provider.dart";

class CartProvider extends ChangeNotifier {
  List> _items = [];
  int get itemCount => _items.length;
  double get total => _items.fold(0, (sum, item) => sum + item["harga"]);

  void tambah(String nama, double harga) {
    _items.add({"nama": nama, "harga": harga});
    notifyListeners(); // beritahu widget untuk rebuild
  }

  void hapus(int index) {
    _items.removeAt(index);
    notifyListeners();
  }

  List> get items => List.unmodifiable(_items);
}

void main() => runApp(
  ChangeNotifierProvider(
    create: (_) => CartProvider(),
    child: MaterialApp(home: TokoPage()),
  ),
);

class TombolBeli extends StatelessWidget {
  final String nama;
  final double harga;
  const TombolBeli(this.nama, this.harga, {super.key});

  @override Widget build(BuildContext context) {
    final cart = context.read();
    return ElevatedButton(
      onPressed: () => cart.tambah(nama, harga),
      child: Text("Tambah ke Keranjang"),
    );
  }
}

class CartBadge extends StatelessWidget {
  @override Widget build(BuildContext context) {
    final count = context.watch().itemCount;
    return Badge(label: Text("$count"), child: Icon(Icons.shopping_cart));
  }
}