Notify when property of nested object changes using MVVM Toolkit in WPF

Question

In my WPF application, I want to build a graph where I sum up a "measured" current and display this for every graph element.

I tried using the MVVM Toolkit for this and set up 2 classes:

    public partial class CurrentFlow:ObservableObject
    {
        [ObservableProperty]
        private int _id;
        [ObservableProperty]
        private float _current;
    }

    public partial class Connection:CurrentFlow
    {
        [ObservableProperty]
        private CurrentFlow? _inflow1;

        [ObservableProperty]
        private CurrentFlow? _inflow2;

        
        partial void OnInflow1Changed(CurrentFlow? value)
        {
            Current = value?.Current ?? 0.0f + Inflow2?.Current ?? 0.0f;
        }

        partial void OnInflow2Changed(CurrentFlow? value)
        {
            Current = value?.Current ?? 0.0f + Inflow1?.Current ?? 0.0f;
        }
    }

I then create the graph like this, where Powerstages is an ObservableCollection<CurrentFlow> and TerminalConnection is a Connection:

        private void CreateConnections()
        {
            var lastConnection = TerminalConnection;
            foreach (var ps in Powerstages)
            {
                lastConnection.Inflow1 = ps;

                if (ps != Powerstages.LastOrDefault())
                {
                    var con = new Connection { Id = ps.Id + 100000 };
                    lastConnection.Inflow2 = con;
                    lastConnection = con;
                }
            }
        }

My problem is, that the OnInflow1Changed and OnInflow2Changed methods don't get triggered when I change the Powerstages[i].Current property.

Has anyone an idea what I am doing wrong and where I can improve?

I suspect the PropertyChanged event does not propagate, but I thought this would be the exact usecase for the [ObservableProperty]attribute.

I could manually subscribe to the currents change events, but Im hoping there is a neat way using the MVVM Toolkit.

So Current is basically calculated as Inflow1 + Inflow2. Why is it then even writeable? What would you do it it is changed from outside? — Klaus Gütter
– Klaus Gütter, Commented Apr 15 at 15:33
Good point. I need it to be writable on the CurrentFlow class, but not in the Connection class anymore. — Freakwave
– Freakwave, Commented Apr 15 at 16:26
Powerstages[i] is a CurrentFlow but not a Connection. Why do you expect changing the property of the former calls the methods of the latter? — emoacht
– emoacht, Commented Apr 15 at 20:36

EldHasp · Accepted Answer · 2025-04-26 07:38:06Z

I'm not entirely sure if I understood you correctly.
But take a look at this solution. Maybe it's what you need.

    public partial class CurrentFlow : ObservableObject
    {
        [ObservableProperty]
        private int _id;
        [ObservableProperty]
        private float _current;
    }

    public partial class Connection : ObservableObject /*: CurrentFlow*/
    {
        [ObservableProperty]
        private CurrentFlow? _inflow;

        [ObservableProperty]
        private Connection? _previous;


        public float Total { get; private set; }
        private static readonly PropertyChangedEventArgs totalChangedEventArgs = new PropertyChangedEventArgs(nameof(Total));

        partial void OnInflowChanged(CurrentFlow? oldValue, CurrentFlow? newValue)
            => OnPropChanged(oldValue, newValue);
        partial void OnPreviousChanged(Connection? oldValue, Connection? newValue)
            => OnPropChanged(oldValue, newValue);

        private void OnPropChanged(INotifyPropertyChanged? oldValue, INotifyPropertyChanged? newValue)
        {
            if (oldValue is not null)
            {
                oldValue.PropertyChanged -= OnCurrentChanged;
            }
            if (newValue is not null)
            {
                newValue.PropertyChanged += OnCurrentChanged;
            }
            OnCurrentChanged(null, null!);
        }

        private void OnCurrentChanged(object? sender, PropertyChangedEventArgs e)
        {
            Total = Inflow?.Current ?? 0.0f + Previous?.Total ?? 0.0f;
            OnPropertyChanged(totalChangedEventArgs);
        }

    }

        private void CreateConnections()
        {
            var lastConnection = TerminalConnection;
            for (var i = 0; i < Powerstages.Count; i++)
            {
                lastConnection.Inflow = Powerstages[i];
                lastConnection = new Connection() { Previous = lastConnection};
            }
        }

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