Codeigniter iOS Web App会话未保存

I have set my codeigniter website to be a web app using Apples documentation. https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html

It works well, but none of my sessions are saving when they switch between apps.

  $this->session->set_userdata('logged_in', true);
  $this->session->set_userdata('id', $data['id']);
  $this->session->set_userdata('role_id', $data['role_id']); 

I found a good article about this but don't know how this would look using codeigniter. Maintaining a PHP session on an iPhone web app

Another approach is to use database sessions, please enable it in application/config/config.php by $config['sess_use_database'] = TRUE;

I mostly use database session because of following fact:

Note: Cookies can only hold 4KB of data, so be careful not to exceed the capacity. The encryption process in particular produces a longer data string than the original so keep careful track of how much data you are storing.

Continue; creating table ci_sessions

CREATE TABLE IF NOT EXISTS  `ci_sessions` (
    session_id varchar(40) DEFAULT '0' NOT NULL,
    ip_address varchar(45) DEFAULT '0' NOT NULL,
    user_agent varchar(120) NOT NULL,
    last_activity int(10) unsigned DEFAULT 0 NOT NULL,
    user_data text NOT NULL,
    PRIMARY KEY (session_id),
    KEY `last_activity_idx` (`last_activity`)
);

And you are good to go, use the code you provided in question, session is saved in database.