Question

I have a situation in a new WinRT app where I need to manage an ID property on a collection of objects. Essentially I’m holding the unique ID for each object which I need to increment for each new object added. This is because I’ll be serializing to XML to save the data so need to manage this ID myself. If I was using SQL it would be an auto incrementing field.

The best way I could come up with was to set this using a method called from the constructor and then have a collection changed handler help me to update the value each time.

Here is the view model class:

using MM.Models;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Linq;

namespace MM.ViewModels
{
    public class VehiclesViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged = delegate { };

        public VehiclesViewModel()
        {
            Vehicles = new ObservableCollection<Vehicle>();
            NewVehicle = new Vehicle();
            NextVehicleID = CalculateHighestID(Vehicles.AsQueryable()) + 1;
            Vehicles.CollectionChanged += new NotifyCollectionChangedEventHandler(VehicleCollectionChanged);
        }

        private ObservableCollection<Vehicle> _vehicles;
        public ObservableCollection<Vehicle> Vehicles
        {
            get 
            { 
                return _vehicles; 
            }
            set
            {
                if (_vehicles != value)
                {
                    _vehicles = value;
                    PropertyChanged(this, new PropertyChangedEventArgs("Vehicles"));
                }
            }
        }

        void VehicleCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.Action == NotifyCollectionChangedAction.Add)
            {
                NextVehicleID += 1;
            }
        }

        public Vehicle NewVehicle { get; set; }

        private int _nextVehicleID;
        public int NextVehicleID 
        { 
            get 
            { 
                return _nextVehicleID; 
            }
            private set
            {
                _nextVehicleID = value;
                PropertyChanged(this, new PropertyChangedEventArgs("NextVehicleID"));
            }
        }

        private int CalculateHighestID(IQueryable<Vehicle> vehicles)
        {
            var query = vehicles.OrderByDescending(v => v.VehicleID).FirstOrDefault();
            if (query != null)
            {
                return query.VehicleID;
            }
            else
            {
                return 1;
            }
        }

    }
}

and here is a text button click method I added on the xaml page to add an item.

private void Button_Click_1(object sender, Windows.UI.Xaml.RoutedEventArgs e)
{
    vm.Vehicles.Add(new Vehicle { VehicleID = vm.NextVehicleID });
}

However, the VehicleCollectionChanged is never called. As a test I used the same code to add a vehicle from the constructor method and that did work.

Can anyone explain why the method would not be called with adding a vehicle from the xaml button click?

Also, is there a better overall approach to keep track of an ID value for the next record?

Answer 1

Does your ID have to be an int (or incremental for that matter)? Could it be a GUID? At least then you could leave the creation up to the Vehicle class. As for why the event isn’t being called, are you ever re-assigning the “Vehicles” property on your viewmodel? Is there a reason you have a public “set” for that property? You could potentially set a new ObservableCollection to “Vehicles” and not 1) unhook from the old event and 2) hook-up the CollectionChanged event to the new collection.