Login

// MY ViewModel -> LoginModel.cs

    public class LoginModel : INotifyPropertyChanged
    {




        // navigation from ViewModel Handling with delcaration of INavigation
        public Command OpenPageCommand { get; }
        INavigation Navigation;

        // property changed event handler

        public event PropertyChangedEventHandler PropertyChanged;

        void OnPropertyChanged(string name)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }

        // Strings declarations
        public string password;
        //SecureString token;
        public string email;
        string _DisplayMessage;
        bool IsBusy = false;
        bool IsEmailValid = false;
        bool IsPasswordValid = false;

        public string DisplayMessage => _DisplayMessage;

        public string LoginText { get; set; }
        public string SignupText {get; set; }
        public string LearnMore { get; set; }
        public string EmailText { get; set; }
        public string PasswordText { get; set; }
        public ICommand SignupClickedCommand { get; set; }
        public ICommand LoginClickedCommand { get; set; }
        public ICommand LearnMoreClickedCommand { get; set; }
        public int Length { get; }


        public LoginModel(INavigation LoginView)
        {
            Navigation = LoginView;
            OpenPageCommand = new Command(async () => await OpenPage(), () => !IsBusy);

            async Task OpenPage()
            {
                await Navigation.PushAsync(new LoginView());
            }

            LoginText = "Log in";
            SignupText = "Sign up";
            LearnMore = "Learn more";
            EmailText = "Email";
            PasswordText = "Password";
            SignupClickedCommand = new Command(SignupClicked);
            LoginClickedCommand = new Command(LoginClickedAsync);
            LearnMoreClickedCommand = new Command(LearnMoreClicked);
            var emailStr = App.TokenStorage.GetToken("Email");
            var passStr = App.TokenStorage.GetToken("Password");

            if (emailStr != null && passStr != null)
            {
                Email = emailStr;
                Password = passStr;
            }
            OnPropertyChanged(nameof(email));
            OnPropertyChanged(nameof(password));
            OnPropertyChanged(nameof(DisplayMessage));
        }




        public string Email
        {
            get { return email; }
            set
            {
                email = value;

                if (Regex.Match(email, @"^([\w\.\-]+)@([\w\-]+)((\.(\w){2,3})+)$").Success)
                {

                    IsEmailValid = true;
                    _DisplayMessage = "";
                }
                else
                {
                    // not to forget to set value to false as the value is dynamic
                    IsEmailValid = false;
                    _DisplayMessage = "Wrong email field!";
                }

                //OnPropertyChanged(nameof(Email));
                //OnPropertyChanged(nameof(DisplayMessage));
            }
        }


        public string Password
        {
            get { return password; }
            set
            {
                password = value;

                if (Regex.Match(password, @"^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,}$").Success)
                {

                    IsPasswordValid = true;
                    _DisplayMessage = "";
                }
                else
                {
                    IsPasswordValid = false;
                    _DisplayMessage = "A-z | 0-1 | 8 chars min.";

                }

                //OnPropertyChanged(nameof(Password));
                //OnPropertyChanged(nameof(DisplayMessage));
            }
        }


        public void LoginClickedAsync()
        {
            if (IsEmailValid && IsPasswordValid)
            {
                // delete username & password from SecureStorage
                App.TokenStorage.DeleteToken("Email");
                App.TokenStorage.DeleteToken("Password");

                // creating new user object and saving it within local DB

                User user = new User(email, password);
                //var Login(user);
                //var result = await App.RestService.Login(user);

                //App.UserDatabase.SaveUser(user);

                RestService restService = new RestService();


                // initiate asynchronous call from within syncronous method and getting its value
                var loginTask = Task.Run(() => restService.LoginAsync(user)).Result;

                if(loginTask)
                {


                    Navigation.PushAsync(new WelcomeView());
                }
            }



        }

        public void SignupClicked()
        {
            Navigation.PushAsync(new SignupView());
        }

        public void LearnMoreClicked()
        {
            Navigation.PushAsync(new WelcomeView());
        }
    }
}




// My ModelView LoginView.xaml


    <ContentPage.Content>
        <StackLayout Padding="10" Spacing="10" >
            <StackLayout VerticalOptions="CenterAndExpand" >
                <Image Source="icon.png"/>
            </StackLayout>


            <StackLayout HorizontalOptions="CenterAndExpand"
                         VerticalOptions="FillAndExpand" >

                <StackLayout VerticalOptions="StartAndExpand">
                    <StackLayout>
                        <!--<StackLayout.Padding>
                            <OnIdiom x:TypeArguments="Thickness">
                                <OnIdiom.Phone>40,0,40,0</OnIdiom.Phone>
                                <OnIdiom.Tablet>140,0,140,0</OnIdiom.Tablet>
                            </OnIdiom>
                        </StackLayout.Padding>-->

                        <Entry  Text="{Binding Email}"
                                Placeholder="{Binding EmailText}"
                                WidthRequest="250"/>

                        <Entry  Text="{Binding Password}"
                                Placeholder="{Binding PasswordText}"
                                IsPassword="True"
                                WidthRequest="250"/>

                        <Label  Text="{Binding DisplayMessage}"
                                HorizontalTextAlignment="Center" />
                    </StackLayout>


                    <StackLayout>
                        <Button Text="{Binding LoginText}"
                                BackgroundColor="LightGreen"
                                Command="{Binding LoginClickedCommand}"/>
                    </StackLayout>

                    <StackLayout Orientation="Horizontal"
                                 HorizontalOptions="FillAndExpand">

                        <StackLayout Orientation="Vertical"
                                     HorizontalOptions="FillAndExpand">

                            <Button Text="{Binding SignupText}"
                                    Command="{Binding SignupClickedCommand}"
                                    BackgroundColor="Transparent"/>

                        </StackLayout>

                        <StackLayout Orientation="Vertical"
                                     HorizontalOptions="FillAndExpand">

                            <Button Text="{Binding LearnMore}"
                                    BackgroundColor="Transparent"
                                    Command="{Binding LearnMoreClickedCommand}"/>

                        </StackLayout>
                    </StackLayout>
                </StackLayout>
            </StackLayout>
        </StackLayout>
    </ContentPage.Content>