#flutter shop cart

shopCart.dart

import 'package:flutter/material.dart';
// 购物车
class Product {
  const Product({this.name});
  final String name;
}

typedef void CartChangedCallback(Product product, bool inCart);

class ShoppingListItem extends StatelessWidget {
  ShoppingListItem({Product product, this.inCart, this.onCartChanged})
  : product = product,
  super(key: ObjectKey(product));

  final Product product;
  final bool inCart;
  final CartChangedCallback onCartChanged;

  Color _getColor(BuildContext context) {
    return inCart ? Colors.black54 : Theme.of(context).primaryColor;
  }

  TextStyle _getTextStyle(BuildContext context) {
    if (!inCart) return null;

    return TextStyle(
      color: Colors.black54,
      decoration: TextDecoration.lineThrough
    );
  }
  
  @override
  Widget build(BuildContext context) {
    return ListTile(
      onTap: () {
        onCartChanged(product, !inCart);
      },
      leading: CircleAvatar(
        backgroundColor: _getColor(context),
        child: Text(product.name[0]),
      ),
      title: Text(
        product.name,
        style: _getTextStyle(context),
      ),
    );
  }
}

class ShoppingList extends StatefulWidget {
  ShoppingList({Key key, this.products}) : super(key: key);

  final List<Product> products;

  @override
  _ShoppingListState createState() => _ShoppingListState();
}
  
class _ShoppingListState extends State<ShoppingList>{
  Set<Product> _shoppingCart = Set<Product>();

  void _handleCartChanged(Product product, bool inCart) {
    setState(() {
      if (inCart)
        _shoppingCart.add(product);
      else
        _shoppingCart.remove(product);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Shopping List'),
      ),
      body: ListView(
        padding: EdgeInsets.symmetric(vertical: 8.0),
        children: widget.products.map((Product product) {
          return ShoppingListItem(
            product: product,
            inCart: _shoppingCart.contains(product),
            onCartChanged: _handleCartChanged,
          );
        }).toList(),
      )
    );
  }
}



void main() => runApp(MaterialApp(
  title: 'Shopping App',
  home: ShoppingList(
    products: <Product>[
      Product(name: 'Eggs'),
      Product(name: 'Flour'),
      Product(name: 'Chocolate chips'),
    ],
  ),
));

Hi @MaySnow, I'm new to flutter & dart and i'm having a problem understanding the code on lines 11, 12 and 13...could you kindly explain them to me briefly