ViewModel

ViewModelUsage.cs

        private async Task ExecuteSignin()
        {
            this.IsSigningIn = true;
            bool signinFailed = false;
            string failureMesage = string.Empty;

            try
            {
                User user = await this.userService.RetrieveUser(this.Username, this.Password);
                this.navigationService.Navigate("Main", user);
            }
            catch (Exception ex)
            {
                signinFailed = true;
                failureMesage = ex.Message;
            }

            if (signinFailed)
            {
                await this.alertService.ShowAsync(
                    "Signin Failure", 
                    "Invalid username and/or password");
                this.Password = string.Empty;
            }

            this.IsSigningIn = false;
        }