LoginPressed.swift

    @IBAction func loginPressed(_ sender: UIButton) {
        let error = validateTextField()
        if error != nil {
            popAlert(title: "Error occured", message: error!, alertTitle: "OK") { (action) in
                self.emailTextField.text = ""
                self.passwordTextField.text = ""
            }
        } else {
            if let email = emailTextField.text,
               let password = passwordTextField.text {
                Auth.auth().signIn(withEmail: email, password: password) { authResult, error in
                    // remove [weak self] & guard let strongSelf = self else { return }
                    if let error = error {
                        self.popAlert(title: "Error occured", message: error.localizedDescription, alertTitle: "OK") { (action) in
                            self.emailTextField.text = ""
                            self.passwordTextField.text = ""
                        }
                        print(error.localizedDescription)
                    } else {
                        if let user = Auth.auth().currentUser {
                            self.uid = user.uid
                            self.performSegue(withIdentifier: Constant.Segue.loginSegue, sender: self)
                            print("login success!")
                        }
                    }
                }
            }
        }
    }