SumNodeViewModel.cs

using System.Reactive.Linq;
using DynamicData;
using NodeNetwork.Toolkit.ValueNode;
using NodeNetwork.ViewModels;
using NodeNetwork.Views;
using ReactiveUI;

namespace ExampleCalculatorApp.ViewModels.Nodes
{
    public class SumNodeViewModel : NodeViewModel
    {
        static SumNodeViewModel()
        {
            Splat.Locator.CurrentMutable.Register(() => new NodeView(), typeof(IViewFor<SumNodeViewModel>));
        }

        public ValueNodeInputViewModel<int?> Input1 { get; }
        public ValueNodeInputViewModel<int?> Input2 { get; }
        public ValueNodeOutputViewModel<int?> Output { get; }

        public SumNodeViewModel()
        {
            Name = "Sum";

            Input1 = new ValueNodeInputViewModel<int?>
            {
                Name = "A",
                Editor = new IntegerValueEditorViewModel()
            };
            EditableInputs().Add(Input1);

            Input2 = new ValueNodeInputViewModel<int?>
            {
                Name = "B",
                Editor = new IntegerValueEditorViewModel()
            };
			EditableInputs().Add(Input2);
            
            var sum = this.WhenAnyValue(vm => vm.Input1.Value, vm => vm.Input2.Value)
                .Select(_ => Input1.Value != null && Input2.Value != null ? Input1.Value + Input2.Value : null);

            Output = new ValueNodeOutputViewModel<int?>
            {
                Name = "A + B",
                Value = sum
            };
            EditableOutputs().Add(Output);
        }
    }
}