从构造函数调用函数时，Codeigniter视图执行两次

I followed a tutorial on how to set up a login system for php apps in codeigniter. The logic when the user has session data is working great, but I have a problem when the user isn't logged in (f.ex. refreshes the page after a while). The view of "not_logged_in" gets executed twice when I were to call for the functin from constructor. The following code works, but it means I gotta add the code for every function I add after.

class App extends CI_Controller {

 function __construct()
 {
      parent::__construct();



 }
 private function logged_in()
 {

      $is_logged_in = $this->session->userdata('is_logged_in');
      if (isset($is_logged_in) OR $is_logged_in)
      {
           return TRUE;
      }
      else
      {
           $data['title'] = 'Chyba přihlášení';
           $data['main_content'] = 'not_logged_in';
           $this->load->view('includes/template', $data);
           return FALSE;
      }
 }

 function index()
 {
      if($this->logged_in())
      {
           $data['title'] = 'APLIKACE';
           $data['main_content'] = 'app_view';
           $data['userid'] = $this->session->userdata('userid'); //get userid from session
           $this->session->unset_userdata('userid'); //destroy the data
           $this->load->view('includes/template' , $data);
      }
 }


 function logout()
 {
     $this->session->sess_destroy();
     redirect('login');
 }


}

Now the real question, how would I go about putting the whole logic into a constructor without having to check for it in every function?

Make APPPATH.'core/MY_Controller.php' file and put authentication logic in constructor there. Than extend that class from every controller (you need auth logic).

class MY_Controller extends CI_Controller
{
    public function __construct();
    {
        parent::__construct();
        $this->check_login();
    }

    protected function check_login()
    {
        $is_logged_in = $this->session->userdata('is_logged_in');

        //here should be *AND* instead *OR* logic
        if (isset($is_logged_in) && !empty($is_logged_in))
        {
            return TRUE;
        }
        else
        {
            redirect('login/index');
            exit();
        }
    }
}

Login.php controller:

class Login extends CI_Controller//NOT extending MY_Controller to avoid infinite loop
{
    public function __construct();
    {
        parent::__construct();
    }

    public function index()
    {
        //here is login view
        //and logic of preserving session
        //with redirect to 'app/index' after successful login
    }

    public function logout()
    {
        $this->session->sess_destroy();
        redirect('login');
    }
}

App.php controller:

class App extends MY_Controller//extending MY_Controller to check login status
{
    public function __construct();
    {
        parent::__construct();
    }

    public function index()
    {
        //here is app dashboard view
    }

    public function statistics()
    {
        //here is some other method that requires logged in user
    }
}

I also would recommend you to check Ion_auth authentication system to see if suitable for you.