I have a Xamarin project in which I have a Cart Page. I'm trying to update the data whenever I click on an add or remove button but it doesn't. Here's the code

public class CartViewModel : BindableObject
    {
.... 

private ObservableCollection<OrderDisplay> _ordersList;
        public ObservableCollection<OrderDisplay> OrdersList
        {
            get => _ordersList;
            set
            {
                _ordersList = value;
                OnPropertyChanged();
            }
        }

And then I have AddTotal where I try to update it. It is called when pressing the add or remove button

 private async void AddTotal(OrderDisplay oCart, int quantity)
        {
            decimal total = 0;
            int index = 0;

            if (oCart != null)
            {
                foreach (OrderDisplay obj in OrdersList)
                {
                    if (obj.Id == oCart.Id)
                    {
                        break;
                    }
                    index  = 1;
                }
                OrdersList[index].Quantity = quantity;
                OrdersList[index].Total = quantity * OrdersList[index].Price;
                //OrdersList = null;
                //OrdersList = tempOrdersList;

                var order = await _apiService.GetOrderById(OrdersList[index].Id, token);
                order.Data.Quantity = OrdersList[index].Quantity;
                var orderUpdated = await _apiService.UpdateOrder(Convert.ToString(order.Data.Id), token, order.Data);

                if (!orderUpdated)
                {
                    await _messageService.ShowMessageAsync("Error", "Ha ocurrido un error.", "Ok", "");
                    return;
                }
            }

            foreach (OrderDisplay order in OrdersList)
            {
                total = order.Total   total;
            }

            LblTotalCart = string.Format("{0:N2}€", total);

        }

For context here is the view

I don't know how to do it. Please help.

EDIT

I tried doing it with INotifyPropertyChanged but gives me NullReference. I don't know if this is correct

public class OrderDisplay : INotifyPropertyChanged
    {
        //public int Id { get; set; }
        //public int Quantity { get; set; }
        //public decimal Price { get; set; }
        //public decimal Total { get; set; }
        //public CrProduct Product { get; set; }

        private int id;
        public int Id
        {
            get { return id; }
            set
            {
                id = value;
                OnPropertyChanged("Id");
            }
        }

        public int quantity;
        public int Quantity
        {
            get { return quantity; }
            set
            {
                quantity = value;
                OnPropertyChanged("Quantity");
            }
        }

        public decimal price;
        public decimal Price
        {
            get { return price; }
            set
            {
                price = value;
                OnPropertyChanged("Price");
            }
        }

        public decimal total;
        public decimal Total
        {
            get { return total; }
            set
            {
                total = value;
                OnPropertyChanged("Total");
            }
        }

        public CrProduct product;
        public CrProduct Product
        {
            get { return product; }
            set
            {
                product = value;
                OnPropertyChanged("Product");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
                handler(this, new PropertyChangedEventArgs(propertyName));
        }

CodePudding user response：

INotifyPropertyChanged is the mechanism that the UI uses to determine when it needs to update a bound property. If you are changing property P on class X, then X needs to implement INotifyPropertyChanged and raise a PropertyChanged event when P is modified.

an ObservableCollection<T> only raises events when items are removed or added from the collection. It does not raise events when individual properties on the class T are modified.