full.dart

import 'dart:collection';

void main() {
  
  Cart cart = Cart();
  
  // add item
  cart.addItem(Item(1,'iPhone X', 1, 1000)); // will add the item
  cart.addItem(Item(1,'iPhone X', 1, 1000)); // won't add the item since it's already added
  cart.addItem(Item(2,'Samsung S20', 2, 2000)); // will add the item
  
  // get item total price
  cart.totalPrice
  
  // get list of items
  cart.items
  
  // remove one item of id 2
  cart.removeOneItem(2);
  
  // remove all items
  cart.removeAllItem();
  
}

class Item {
  int id;
  String name; 
  int quantity; 
  int price;
  
  Item(this.id, this.name, this.quantity, this.price);
  
}

class Cart {
  final List<Item> _items = [];
  
  UnmodifiableListView<Item> get items => UnmodifiableListView(_items);
  
  int get totalPrice => _totalPrice();
  
  int _totalPrice () {
    int total = 0;
    _items.forEach((Item item) => {
      total += item.price * item.quantity
    });
    return total;
  }
  
  bool _checkItem (Item item){
    bool isExist = false;
    _items.forEach((Item i) => {
      if(i.id == item.id){
        isExist = true
      }
    });
    return isExist;
  }
  
  void addItem(Item item){
    if(!_checkItem(item)){
      _items.add(item);
    } else {
      print('item added already');
    }
  }

  void removeOneItem (int id){
    _items.removeWhere((Item i) => i.id == id);
  }
  
  void removeAllItem () {
    _items.clear();
  }
  
}

Thank you for this example

You are welcome

Hi Verdotte thank you for this,
This took me 4days to get i was stuck creating a global list of selecteditems and finalitems, considering i had dynamic items with dynamic textinput fields.. Thank you for this !!!