Firebase features

//Register User

let emailField = alert.textFields![0]
let passwordField = alert.textFields![1]

FIRAuth.auth()!.createUser(withEmail: emailField.text!,
                           password: passwordField.text!) { user, error in
  if error == nil {
    FIRAuth.auth()!.signIn(withEmail: self.textFieldLoginEmail.text!,
                           password: self.textFieldLoginPassword.text!)
  }
}

//Login for users

@IBAction func loginDidTouch(_ sender: AnyObject) {
    FIRAuth.auth()!.signIn(withEmail: textFieldLoginEmail.text!,
                            password: textFieldLoginPassword.text!)
}

//Observing auth state

override func viewDidLoad() {
  super.viewDidLoad()

  FIRAuth.auth()!.addStateDidChangeListener() { auth, user in

    if user != nil {
      self.performSegue(withIdentifier: self.loginToList, sender: nil)
    }
  }
}

//Reference for User's actions

FIRAuth.auth()!.addStateDidChangeListener { auth, user in
  guard let user = user else { return }
  self.user = User(authData: user)
}