import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var btnRegister: UIButton!
    @IBOutlet weak var tbUsername: UITextField!
    @IBOutlet weak var tbPassword: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    @IBAction func unwindToMenu(segue: UIStoryboardSegue){
        tbUsername.text = ""
        tbPassword.text = ""
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?){
        let dest = segue.destination as? ApplicationViewController
        dest?.username = tbUsername.text!

    }
    //checks if a user already exists
    func existsUsername(_ key : String) -> Bool{
        return UserDefaults.standard.object(forKey: key) != nil
    }



    @IBAction func login(_ sender: Any) {
        let user = tbUsername.text
        let password = tbPassword.text
        let userDefaults = Foundation.UserDefaults.standard

        if !existsUsername(user!) {

            //username does not exists Alert
            let alertController = UIAlertController(title: "Error", message: "this username does not exist", preferredStyle: .actionSheet)
            let okAction = UIAlertAction(title: "ok", style: .cancel, handler: { (action:UIAlertAction!) in
                print("ok")
            })
            alertController.addAction(okAction)
            self.present(alertController,animated : true, completion:nil)
            return
        }

        if  password == userDefaults.string(forKey: user!) {


            //succesful login
            performSegue(withIdentifier: "toApplicationSegue", sender: self)

        }
        else{

            //wrong password Alert
            let alertController = UIAlertController(title: "Error", message: "wrong password", preferredStyle: .actionSheet)
            let okAction = UIAlertAction(title: "ok", style: .default, handler: { (action:UIAlertAction!) in
                print("ok")
            })
            let changePasswordAction = UIAlertAction(title: "change password", style: .destructive, handler: { (action:UIAlertAction!) in
                self.btnRegister.sendActions(for: UIControlEvents.touchUpInside)

            })
            alertController.addAction(okAction)
            alertController.addAction(changePasswordAction)
            self.present(alertController,animated : true, completion:nil)

        }
    }
}



import UIKit

class ApplicationViewController: UIViewController {

    @IBOutlet weak var lbWelcome: UILabel!

    var username = ""

    override func viewDidLoad() {
        super.viewDidLoad()
        lbWelcome.text = "welcome " + username + "!"

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}



import UIKit

class RegistrierenViewController: UIViewController {


    @IBOutlet weak var tbUsername: UITextField!
    @IBOutlet weak var tbPassword: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func register(_ sender: Any) {
        let key = tbUsername.text
        let value = tbPassword.text
        let userDefaults = Foundation.UserDefaults.standard

        if userDefaults.string(forKey: key!) == value {

            //Alert
            let alertController = UIAlertController(title: "Error", message: "this user already exists", preferredStyle: .actionSheet)
            let okAction = UIAlertAction(title: "ok", style: .cancel, handler: { (action:UIAlertAction!) in
                print("ok")
            })
            alertController.addAction(okAction)
            self.present(alertController,animated : true, completion:nil)
            return
        }
        else{
            //create new user
            userDefaults.set(value, forKey: key!)

            //go back to Login
            performSegue(withIdentifier: "unwindBack", sender: self)

        }
    }
}