using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
using TBSMobileApplication.Views;
using TBSMobileApplication.Data;

[assembly: XamlCompilation (XamlCompilationOptions.Compile)]
namespace TBSMobileApplication
{
    public partial class App : Application
    {
        static TokenDatabaseController tokenDatabase;
        static UserDatabaseController userDatabase;

        public App ()
        {
            InitializeComponent();

            MainPage = new LoginPage();
        }

        protected override void OnStart ()
        {
            // Handle when your app starts
        }

        protected override void OnSleep ()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume ()
        {
            // Handle when your app resumes
        }

        public static UserDatabaseController UserDatabase
        {
             get
            {
                if(userDatabase == null)
                {
                    userDatabase = new UserDatabaseController();
                }

                return userDatabase;
            }
        }

        public static TokenDatabaseController TokenDatabase
        {
            get
            {
                if (tokenDatabase == null)
                {
                    tokenDatabase = new TokenDatabaseController();
                }

                return tokenDatabase;
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using TBSMobileApplication.Models;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace TBSMobileApplication.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class LoginPage : ContentPage
    {
        public LoginPage ()
        {
            InitializeComponent ();
        }

        void LoginProcedure(object sender, EventArgs e)
        {
            User user = new User(entUser.Text, entPassword.Text);
            if (user.CheckInformation())
            {
                //DisplayAlert("Login Message", "Login Success", "Ok");
                try
                {
                    App.UserDatabase.SaveUser(user);
                    DisplayAlert("Database Message", "User Saved", "Ok");
                }
                catch(Exception ex)
                {
                    DisplayAlert("Message", ex.Message, "Ok");
                }

            }
            else
            {
                DisplayAlert("Login Message", "Login Failed", "Ok");
            }
        }
    }
}

using SQLite;
using System;
using System.Collections.Generic;
using System.Text;

namespace TBSMobileApplication.Data
{
    public interface ISQLite
    {
        SQLiteConnection GetConnection();
    }
}

using SQLite;
using System;
using System.Collections.Generic;
using System.Text;

namespace TBSMobileApplication.Models
{
    public class User
    {
        [PrimaryKey, AutoIncrement]
        public int ContactID { get; set; }
        [Unique]
        public string UserID { get; set; }
        public string UserPassword { get; set; }

        public User() { }
        public User(string Username, string Password)
        {
            this.UserID = Username;
            this.UserPassword = Password;
        }

        public bool CheckInformation()
        {
            if(!this.UserID.Equals("") || !this.UserPassword.Equals(""))
            {
                return true;
            }
            else
            {
                return false;
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using TBSMobileApplication.Data;
using TBSMobileApplication.Droid.Data;
using Xamarin.Forms;

[assembly: Dependency(typeof(SQLite_Android))]

namespace TBSMobileApplication.Droid.Data
{
    public class SQLite_Android : ISQLite
    {
        public SQLite_Android() { }
        public SQLite.SQLiteConnection GetConnection()
        {
            var DBFileName = "backend.db3";
            string DocumentPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            var path = Path.Combine(DocumentPath, DBFileName);
            var conn = new SQLite.SQLiteConnection(path);

            return conn;
        }
    }
}

using SQLite;
using System;
using System.Collections.Generic;
using System.Text;
using TBSMobileApplication.Models;
using Xamarin.Forms;

namespace TBSMobileApplication.Data
{
    public class UserDatabaseController
    {
        static object locker = new object();

        SQLiteConnection database;

        public UserDatabaseController()
        {
            database = DependencyService.Get<ISQLite>().GetConnection();
            database.CreateTable<User>();
        }

        public User GetUser()
        {
            lock (locker)
            {
                if(database.Table<User>().Count() == 0)
                {
                    return null;
                }
                else
                {
                    return database.Table<User>().First();
                }
            }
        }

        public int SaveUser(User user)
        {
            lock (locker)
            {
                if (user.ContactID != 0)
                {
                    database.Update(user);
                    return user.ContactID;
                }
                else
                {
                    return database.Insert(user);
                }
            }
        }

        public int DeleteUser(int contactid)
        {
            lock (locker)
            {
                return database.Delete<User>(contactid);
            }
        }
    }
}