Flutter Provider example

provider.dart

import 'dart:collection';

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => ShoppingCartProvider(),
      child: const MyApp(),
    ),
  );
}

const List<Item> inventory = [
  Item(name: "Shirt", cost: 12.99),
  Item(name: "Shoes", cost: 120.00),
  Item(name: "Pants", cost: 63.74),
  Item(name: "Hat", cost: 8.23),
];

class Inventory extends StatelessWidget {
  const Inventory({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Consumer<ShoppingCartProvider>(
      builder:
          (BuildContext context, ShoppingCartProvider cart, Widget? child) {
        return ListView(
          shrinkWrap: true,
          physics: const ClampingScrollPhysics(),
          children: [
            Text(
              'Inventory',
              style: Theme.of(context).textTheme.displayMedium,
            ),
            ListView.builder(
              shrinkWrap: true,
              physics: const ClampingScrollPhysics(),
              itemCount: inventory.length,
              itemBuilder: (BuildContext context, int i) => Card(
                child: ListTile(
                  title: Text(inventory[i].name),
                  subtitle: Text('\$${inventory[i].cost.toStringAsFixed(2)}'),
                  trailing: IconButton(
                    icon: const Icon(Icons.add_shopping_cart),
                    onPressed: () => cart.add(inventory[i]),
                  ),
                ),
              ),
            ),
          ],
        );
      },
    );
  }
}

class Item {
  final double cost;
  final String name;

  const Item({
    required this.cost,
    required this.name,
  });
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Padding(
          padding: const EdgeInsets.all(8.0),
          child: ListView(
            children: const [
              Inventory(),
              ShoppingCart(),
            ],
          ),
        ),
      ),
    );
  }
}

class ShoppingCart extends StatelessWidget {
  const ShoppingCart({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Consumer<ShoppingCartProvider>(
      builder:
          (BuildContext context, ShoppingCartProvider cart, Widget? child) =>
              ListView(
        shrinkWrap: true,
        physics: const ClampingScrollPhysics(),
        children: [
          Text(
            'Shopping Cart',
            style: Theme.of(context).textTheme.displayMedium,
          ),
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              Text('Items in Cart: ${cart.itemsInCart.length}'),
              Text('Total Cost: \$${cart.totalCost.toStringAsFixed(2)}'),
              OutlinedButton.icon(
                label: const Text('Clear cart'),
                icon: const Icon(Icons.remove_shopping_cart),
                onPressed: () =>
                    Provider.of<ShoppingCartProvider>(context, listen: false)
                        .emptyCart(),
              ),
            ],
          ),
          ListView.builder(
            shrinkWrap: true,
            physics: const ClampingScrollPhysics(),
            itemCount: cart.uniqueItems.length,
            itemBuilder: (context, i) => Card(
              child: ListTile(
                leading: Text(
                  '${cart.itemsInCart.where((element) => element == cart.uniqueItems[i]).length} ×',
                ),
                title: Text(
                  cart.itemsInCart
                      .firstWhere((element) => element == cart.uniqueItems[i])
                      .name,
                ),
                subtitle:
                    Text('\$${cart.itemsInCart[i].cost.toStringAsFixed(2)}'),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class ShoppingCartProvider extends ChangeNotifier {
  final List<Item> _itemsInCart = [];

  UnmodifiableListView<Item> get itemsInCart =>
      UnmodifiableListView(_itemsInCart);

  double get totalCost {
    double price = 0;
    for (Item item in _itemsInCart) {
      price += item.cost;
    }

    return price;
  }

  List<Item> get uniqueItems {
    final List<Item> unique = [];
    for (Item item in _itemsInCart) {
      if (!unique.contains(item)) {
        unique.add(item);
      }
    }
    return unique;
  }

  void add(Item item) {
    _itemsInCart.add(item);
    notifyListeners();
  }

  void emptyCart() {
    _itemsInCart.clear();
    notifyListeners();
  }
}