public LoginViewModel()
        {
            _userCredentials = new UserCredentials();

            this.LoginCommand = new Command(async() => {
                AppData.Credentials = _userCredentials;
                var authenticated   = await SitecoreHelper.Authenticate();

                if (authenticated)
                {
                    App.Current.MainPage = new NavigationPage(
                        ViewFactory.CreatePageFromViewModel(new WorkboxViewModel()));
                }
                ;
            }, () => !string.IsNullOrEmpty(_userCredentials.InstanceUrl) &&
                                            !string.IsNullOrEmpty(_userCredentials.Username) &&
                                            !string.IsNullOrEmpty(_userCredentials.Password));
        }