https://github.com/vicradon/shopping-cart-app (flutter Shopping-Cart-App)

main.dart

///https://dartpad.dev/?id=c5cd30bc8f043796078ce0da5d0263f9
///https://github.com/vicradon/shopping-cart-app
import 'package:flutter/material.dart';

import 'dart:async';

class CartItemsBloc {
  /// The [cartStreamController] is an object of the StreamController class
  /// .broadcast enables the stream to be read in multiple screens of our app
  final cartStreamController = StreamController.broadcast();

  /// The [getStream] getter would be used to expose our stream to other classes
  Stream get getStream => cartStreamController.stream;

  final Map allItems = {
    'shop_items': [
      {'name': '(chau-test)App dev kit', 'price': 20, 'id': 1},
      {'name': 'App consultation', 'price': 100, 'id': 2},
      {'name': 'Logo Design', 'price': 10, 'id': 3},
      {'name': 'Code review', 'price': 90, 'id': 4},
    ],
    'cart_items': []
  };
void getCart(){
  return allItems['shop_items'];
}

  void addToCart(item) {
    allItems['shop_items'].remove(item);
    allItems['cart_items'].add(item);
    cartStreamController.sink.add(allItems);
  }

  void removeFromCart(item) {
    allItems['cart_items'].remove(item);
    allItems['shop_items'].add(item);
    cartStreamController.sink.add(allItems);
  }

  /// The [dispose] method is used 
  /// to automatically close the stream when the widget is removed from the widget tree
  void dispose() {
    cartStreamController.close(); // close our StreamController
  }
}

final bloc = CartItemsBloc();


class ShopItems extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Shopping Cart App'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.shopping_cart),
            onPressed: () => Navigator.pushNamed(context, '/checkout'),
          )
        ],
      ),
      body: ShopItemsWidget(),
    );
  }
}

class ShopItemsWidget extends StatelessWidget {
  Widget build(BuildContext context) {
    return StreamBuilder(
      initialData: bloc.allItems,
      stream: bloc.getStream,
      builder: (context, snapshot) {
        // print(snapshot);
        // return ShopItems();
        return bloc.allItems["shop_items"].length >0
            ? shopItemsListBuilder(snapshot)
            : Center(child: Text("All items in shop have been taken"));
      },
    );
  }
}

Widget shopItemsListBuilder(snapshot) {
  return ListView.builder(
    itemCount: snapshot.data["shop_items"].length,
    itemBuilder: (BuildContext context, i) {
      final shopList = snapshot.data["shop_items"];
      return ListTile(
        title: Text(shopList[i]['name']),
        subtitle: Text("\$${shopList[i]['price']}"),
        trailing: IconButton(
          icon: Icon(Icons.add_shopping_cart),
          onPressed: () {
            bloc.addToCart(shopList[i]);
          },
        ),
        onTap: () {},
      );
    },
  );
}

class Checkout extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Checkout')),
      body: StreamBuilder(
        stream: bloc.getStream,
        initialData: bloc.allItems,
        builder: (context, snapshot) {
          // return build(context);
          print("bloc.allItems");
          print(bloc.allItems['cart_items'].length);
          var vLang = bloc.allItems['cart_items'].length;
          //var vLang = 1;
          return vLang > 0
              ? Column(
                  children: <Widget>[
                    /// The [checkoutListBuilder] has to be fixed
                    /// in an expanded widget to ensure it
                    /// doesn't occupy the whole screen and leaves
                    /// room for the the RaisedButton
                    Expanded(child: checkoutListBuilder(snapshot)),
                    RaisedButton(
                      onPressed: () {},
                      child: Text("Checkout"),
                      color: Theme.of(context).primaryColor,
                    ),
                    SizedBox(height: 40)
                  ],
                )
              : Center(child: Text("You haven't taken any item yet"));
        },
      ),
    );
  }
}

Widget checkoutListBuilder(snapshot) {
  return ListView.builder(
    itemCount: snapshot.data["cart_items"].length,
    itemBuilder: (BuildContext context, i) {
      final cartList = snapshot.data["cart_items"];
      return ListTile(
        title: Text(cartList[i]['name']),
        subtitle: Text("\$${cartList[i]['price']}"),
        trailing: IconButton(
          icon: Icon(Icons.remove_shopping_cart),
          onPressed: () {
            bloc.removeFromCart(cartList[i]);
          },
        ),
        onTap: () {},
      );
    },
  );
}


void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    Map<int, Color> color = {
      50: Color.fromRGBO(255, 144, 0, .1),
      100: Color.fromRGBO(255, 144, 0, .2),
      200: Color.fromRGBO(255, 144, 0, .3),
      300: Color.fromRGBO(255, 144, 0, .4),
      400: Color.fromRGBO(255, 144, 0, .5),
      500: Color.fromRGBO(255, 144, 0, .6),
      600: Color.fromRGBO(255, 144, 0, .7),
      700: Color.fromRGBO(255, 144, 0, .8),
      800: Color.fromRGBO(255, 144, 0, .9),
      900: Color.fromRGBO(255, 144, 0, 1),
    };
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: MaterialColor(0xFFFF7000, color),
      ),
      initialRoute: '/',
      routes: {
        '/': (BuildContext context) => ShopItems(),
        '/checkout': (BuildContext context) => Checkout()
      },
    );
  }
}