public class LoginViewModel : ViewModelBase
{
    private string _username;
    private string _password;

    public RelayCommand LoginButtonCommand { get; private set; }
    public bool CanExecuteLoginCommand { get; set; }

    public LoginViewModel()
    {
        LoginButtonCommand = new RelayCommand(HandleLoginButtonCommand, () => CanExecuteLoginCommand);
        CanExecuteLoginCommand = true;
    }

    public string Username
    {
        get
        {
            return _username;
        }
        set
        {
            _username = value;
            RaisePropertyChanged(() => Username);
        }
    }

    public string Password
    {
        get
        {
            return _password;
        }
        set
        {
            _password = value;
            RaisePropertyChanged(() => Password);
        }
    }

    private void HandleLoginButtonCommand()
    {
        CanExecuteLoginCommand = false;

        //Validate login?

        CanExecuteLoginCommand = true;
    }
}

public partial class LoginViewController : UIViewController
{
    private Binding _usernameTextFieldBinding;
    private Binding _passwordTextFieldBinding;
    private LoginViewModel _viewModel;

    public LoginViewController(IntPtr handle) : base(handle)
    {
    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
        _viewModel = Application.Locator.Login;

        HideKeyboardHandling(UsernameTextField);
        HideKeyboardHandling(PasswordTextField);

        _usernameTextFieldBinding = this.SetBinding(
            () => _viewModel.Username)
            .ObserveSourceEvent("EditingDidEnd")
            .WhenSourceChanges(() => _viewModel.Username = UsernameTextField.Text);
        _passwordTextFieldBinding = this.SetBinding(
            () => _viewModel.Username)
            .ObserveSourceEvent("EditingDidEnd")
            .WhenSourceChanges(() => _viewModel.Password = PasswordTextField.Text);

        Loginbutton.SetCommand("TouchUpInside", _viewModel.LoginButtonCommand);
    }

    public override void DidReceiveMemoryWarning()
    {
        base.DidReceiveMemoryWarning();
        // Release any cached data, images, etc that aren't in use.
    }

    void HideKeyboardHandling(UITextField textField)
    {
        textField.ShouldReturn = TextField =>
        {
            TextField.ResignFirstResponder();
            return true;
        };

        var gesture = new UITapGestureRecognizer(() => View.EndEditing(true));
        gesture.CancelsTouchesInView = false;
        View.AddGestureRecognizer(gesture);
    }
}