The code snippets for the shopping cart app

a.dart

import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      initialRoute: '/',
      routes: {
        '/': (BuildContext context) => ShopItems(),
        '/checkout': (BuildContext context) => Checkout()
      },
    );
  }
}

b.dart

import 'package:flutter/material.dart';
import 'package:shopping_cart_app/pages/checkout.dart';
import 'package:shopping_cart_app/pages/shop_items.dart';

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

...

c.dart

import 'package:flutter/material.dart';

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 Center(child: Text("All items in shop have been taken"));
  }
}

d.dart

import 'package:flutter/material.dart';

class Checkout extends StatelessWidget {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Checkout')),
      body:  Center(child: Text("You haven't taken any item yet"))
    );
  }
}

e.dart

import 'package:shopping_cart_app/pages/checkout.dart';
import 'package:shopping_cart_app/pages/shop_items.dart';
import 'package:flutter/material.dart';

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(
      theme: ThemeData(
        primarySwatch: MaterialColor(0xFFFF7000, color),
      ),
      initialRoute: '/',
      routes: {
        '/': (BuildContext context) => ShopItems(),
        '/checkout': (BuildContext context) => Checkout()
      },
    );
  }
}

f.dart

/// The [dart:async] is necessary for using streams
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;

  /// The [allItems] Map would hold all the data this bloc provides
  final Map 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
  }
}

g.dart

...
Stream get getStream => cartStreamController.stream;

/// The [allItems] Map would hold all the data this bloc provides
final Map allItems = {
  'shop items': [
    {'name': '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': []
};
...

h.dart

 ],
  'cart items': []
};

/// [addToCart] adds items from the shop to the cart
void addToCart(item) {
  allItems['shop items'].remove(item);
  allItems['cart items'].add(item);
  cartStreamController.sink.add(allItems);
}

/// [removeFromCart] removes items from the cart, back to the shop
void removeFromCart(item) {
  allItems['cart items'].remove(item);
  allItems['shop items'].add(item);
  cartStreamController.sink.add(allItems);
}
...

i.dart

...
  /// 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();  // add to the end of the file

j.dart

import 'package:flutter/material.dart';
import 'package:shopping_cart_app/bloc/cart_items_bloc.dart';

...

k.dart

class ShopItemsWidget extends StatelessWidget {
  Widget build(BuildContext context) {
    return StreamBuilder(
      initialData: bloc.allItems, // The bloc was already instantiated.
      stream: bloc.getStream, // The stream we'd be listing to
      builder: (context, snapshot) {
        // snapshot contains the data of the bloc
        return  Center(child: Text("All items in shop have been taken"));
      },
    );
  }
}

l.dart

...

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: () {},
      );
    },
  );
}

m.dart

import 'package:flutter/material.dart';
import 'package:shopping_cart_app/bloc/cart_items_bloc.dart'; // do as before
...

n.dart

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 snapshot.data['cart items'].length > 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"));
        },
      ),
    );
  }
}

o.dart

...
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: () {},
      );
    },
  );
}