FeedbackFormViewModel.cs

public sealed class FeedbackViewModel : ReactiveObject, IActivatableViewModel
{
    public ViewModelActivator Activator { get; } = new ViewModelActivator();
    public ReactiveCommand<Unit, Unit> Submit { get; }

    [Reactive] public bool HasErrors { get; private set; }
    [Reactive] public string Elapsed { get; private set; } = string.Empty;

    [Reactive] public string Title { get; set; } = string.Empty;
    [Reactive] public int TitleLength { get; private set; }
    public int TitleLengthMax { get; } = 15;

    [Reactive] public string Message { get; set; } = string.Empty;
    [Reactive] public int MessageLength { get; private set; }
    public int MessageLengthMax { get; } = 30;

    [Reactive] public int Section { get; set; }
    [Reactive] public bool Issue { get; set; } = true;
    [Reactive] public bool Idea { get; set; }

    public FeedbackViewModel(ISender sender, IClock clock)
    {
        this.WhenAnyValue(x => x.Idea)
            .Select(selected => !selected)
            .Subscribe(value => Issue = value);
        this.WhenAnyValue(x => x.Issue)
            .Select(selected => !selected)
            .Subscribe(value => Idea = value);

        this.WhenAnyValue(x => x.Title)
            .Select(title => title.Length)
            .Subscribe(length => TitleLength = length);
        this.WhenAnyValue(x => x.Message)
            .Select(message => message.Length)
            .Subscribe(length => MessageLength = length);

        var valid = this
            .WhenAnyValue(
                x => x.Title, x => x.Message,
                x => x.Issue, x => x.Idea,
                x => x.Section, 
                (title, message, issue, idea, section) =>
                    !string.IsNullOrWhiteSpace(message) &&
                    !string.IsNullOrWhiteSpace(title) &&
                    (idea || issue) && section >= 0)
            .Log(this, "Form validity changed")
            .Publish()
            .RefCount();

        valid.Subscribe(hasErrors => HasErrors = !hasErrors);
        Submit = ReactiveCommand.CreateFromTask(
            () => sender.Send(Title, Message, Section, Issue), 
            valid);

        this.WhenActivated(disposables => 
            clock.Tick.Select(second => $"{second}s")
                .ObserveOn(RxApp.MainThreadScheduler)
                .Subscribe(elapsed => Elapsed = elapsed)
                .DisposeWith(disposables));
    }
}