Introduction to Flutter Workshop

We will create a set of classes responsible for managing the data of our app, a DataManager class in a new file DataManager.dart. It will include two repositories in lists and then some functions for the cart management.

import 'dart:convert';
import 'package:http/http.dart' as http;
import 'datamodel.dart';

class DataManager {
  List<Category>? _menu;
  List<ItemInCart> cart = [];

  cartAdd(Product p) {
    bool found = false;
    for (var item in cart) {
      if (item.product.id == p.id) {
        item.quantity++;
        found = true;
      }
    }
    if (!found) {
      cart.add(ItemInCart(product: p, quantity: 1));
    }
  }

  cartDelete(Product p) {
    cart.removeWhere((element) => element.product.id == p.id);
  }

  cartClear() {
    cart.clear();
  }

  double cartTotal() {
    var sum = 0.0;
    for (var item in cart) {
      sum += item.quantity * item.product.price;
    }
    return sum;
  }
}

Using the Data Manager

We will create an instance of the DataManager at the App level in HomePage and pass it to final properties to the MenuPage and OrderPage that will need it.

Your HomePage widget will start with:

class _MyHomePageState extends State<MyHomePage> {
  int _currentIndex = 0;
  DataManager dataManager = DataManager();

  @override
  Widget build(BuildContext context) {
    late Widget currentPage;
    switch (_currentIndex) {
      case 0:
        currentPage = MenuPage(dataManager: dataManager);
        break;
      case 1:
        currentPage = const OffersPage();
        break;
      case 2:
        currentPage = OrderPage(dataManager: dataManager);
        break;
    }
    // continues ....

At MenuPage and OrderPage we will add the final variables and also receive it as a required named argument in the constructor:

class MenuPage extends StatelessWidget {
  final DataManager dataManager;

  const MenuPage({Key? key, required this.dataManager}) : super(key: key);
    // continues ....

Now MenuPage is ready to use the data.