Reset ReactiveUI ObservableAsPropertyHelper Value

Question

I am building a simple DateTime calculator using a ReactiveUI ReactiveObject viewmodel.

It has a calculate command that calculates a value and correctly updates the result backed by an ObservableAsPropertyHelper<DateTime?> field.

I also have a ReactiveUICommand that is supposed to reset all of the UI values.

The problem that I am running into is how to reset the readonly "Result" value.

public class VM : ReactiveObject
{
   private readonly ObservableAsPropertyHelper<DateTime?> _result;

   [Reactive]
   public DateTime Start { get; set; } = DateTime.Now;

   [Reactive]
   public int Days { get; set; } = 1;

   public DateTime? Result => _result.Value;

   public ReactiveCommand<Unit, DateTime?> CalculateCommand { get; }

   public ReactiveCommand<Unit, Unit> ResetCommand { get; }

   public VM()
   {
      CalculateCommand = ReactiveCommand
        .CreateFromTask<Unit, DateTime?>((_, cancellationToken) => CalculateAsync(cancellationToken));

      _result = CalculateCommand.ToProperty(this, nameof(Result));

      // everything above this works.

      ResetCommand = ReactiveCommand.Create(() =>
      {
          Start = DateTime.Now;
          Days = 1;
          // how do I reset "_result" or "Result" back to null???
      });
   }
 
   private Task<DateTime?> CalculateAsync(CancellationToken _)
   {
      // to be replaced by an API call later.
      return Task.FromResult<DateTime?>(Start.AddDays(Days));
   }

}

How do I reset a value backed by an instance of ObservableAsPropertyHelper?

Answer 1

Figured this out myself:

namespace WpfApp4;
using System.Reactive.Linq;
using System.Threading.Tasks;

using ReactiveUI;

using System;
using System.Reactive;

public class MainViewModel : ReactiveObject
{

    public ReactiveCommand<Unit, DateTime?> StartCommand { get; }

    public ReactiveCommand<Unit, DateTime?> ResetCommand { get; }

    private DateTime _start = DateTime.Now;

    private int _processingDays = 1;

    private readonly ObservableAsPropertyHelper<DateTime?> _result;

    public DateTime? Result => _result.Value;

    public DateTime Start
    {
        get => _start;
        set => this.RaiseAndSetIfChanged(ref _start, value);
    }

    public int ProcessingDays
    {
        get => _processingDays;
        set => this.RaiseAndSetIfChanged(ref _processingDays, value);
    }

    public MainViewModel()
    {
        StartCommand = ReactiveCommand.CreateFromTask<DateTime?>(() => Task.FromResult<DateTime?>(Start.AddDays(ProcessingDays)));

        ResetCommand = ReactiveCommand.Create<DateTime?>(execute: () => null);

        _ = ResetCommand.Subscribe(onNext: (_) =>
          {
              Start = DateTime.Now;

              ProcessingDays = 1;
          });

        _result = StartCommand.Merge(ResetCommand).ToProperty(this, nameof(Result));
    }
}