gistfile1.cs

public interface ILoginViewModel : IRoutableViewModel
{
    string User { get; set; }
    string Password { get; set; }
    ReactiveAsyncCommand Confirm { get; }
}

public class LoginViewModel : ReactiveObject, ILoginViewModel
{
    string _User;
    public string User
    {
        get { return _User; }
        set { this.RaiseAndSetIfChanged(x => x.User, value); }
    }

    string _Password;
    public string Password
    {
        get { return _Password; }
        set { this.RaiseAndSetIfChanged(x => x.Password, value); }
    }

    ObservableAsPropertyHelper<string> _ErrorMessage;
    public string ErrorMessage {
        get { return _ErrorMessage.Value; }
    }

    public string UrlPathSegment
    {
        get { return "login"; }
    }

    public IScreen HostScreen { get; protected set; }

    public ReactiveAsyncCommand Confirm { get; protected set; }

    public LoginViewModel(ReactiveAsyncCommand confirm = null)
    {
        var canConfirm = this.WhenAny(x => x.User, x => x.Password,
            (u, p) => !String.IsNullOrWhiteSpace(u.Value) && !String.IsNullOrWhiteSpace(p.Value));

        Confirm = confirm ?? new ReactiveAsyncCommand(canConfirm);

        var result = Confirm.RegisterAsyncObservable(_ => {
            var client = new RestClient("https://api.github.com");
            client.Authenticator = new HttpBasicAuthenticator(User, Password);

            return client.RequestAsync<GitHubUser>(new RestRequest("user"))
                .Do(res => this.Log().Info("User's URL: {0}", res.Data.url))
                .Select(res => new Tuple<string, string>(User, Password));
        });

        MessageBus.Current.RegisterMessageSource(result, "login");

        Confirm.ThrownExceptions
            .Select(x => x.Message)
            .ToProperty(this, x => x.ErrorMessage);
    }
}

public class LoginViewModelTests : IEnableLogger
{
    [Fact]
    public void SuccessfulLoginShouldActuallyLogIn()
    {
        var fixture = new LoginViewModel();

        fixture.User = "xpaulbettsx";
        fixture.Password = "[REDACTED]";

        fixture.Confirm.CanExecute(null).Should().BeTrue();
        fixture.Confirm.Execute(null);

        var result = MessageBus.Current.Listen<Tuple<string, string>>("login")
            .Timeout(TimeSpan.FromSeconds(10), RxApp.TaskpoolScheduler)
            .First();

        result.Should().NotBeNull();
        result.Item1.Should().Be(fixture.User);
        result.Item2.Should().Be(fixture.Password);
    }

    [Fact]
    public void BlankFieldsMeansNoLogin()
    {
        var fixture = new LoginViewModel();

        fixture.Confirm.CanExecute(null).Should().BeFalse();

        fixture.User = "Foo";
        fixture.Confirm.CanExecute(null).Should().BeFalse();

        fixture.User = "";
        fixture.Password = "Bar";
        fixture.Confirm.CanExecute(null).Should().BeFalse();

        fixture.User = "Foo";
        fixture.Confirm.CanExecute(null).Should().BeTrue();
    }


    [Fact]
    public void BadPasswordMeansErrorMessage()
    {
        var fixture = new LoginViewModel();

        fixture.User = "herpderp";
        fixture.Password = "woefawoeifjwoefijwe";

        fixture.Confirm.CanExecute(null).Should().BeTrue();
        fixture.Confirm.Execute(null);

        var result = fixture.ObservableForProperty(x => x.ErrorMessage)
            .Timeout(TimeSpan.FromSeconds(10), RxApp.TaskpoolScheduler)
            .First();

        this.Log().Info("Error Message: {0}", result.Value);
        result.Value.Should().NotBeNullOrEmpty();
    }
}