class FirstViewController: UIViewController {

    @IBOutlet weak var lbl: UILabel!
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        if let switchStatus = (self.tabBarController as? TabBarController)?.switchStatus {
            lbl.text = switchStatus ? "Switch is On" : "Switch is off"
        }
    }
}

class SecondViewController: UIViewController {

    @IBOutlet weak var `switch`: UISwitch!

    @IBAction func switchAction(_ sender: UISwitch) {
        if let tabBar = self.tabBarController as? TabBarController {
            tabBar.switchStatus = sender.isOn
        }
    }
}

class TabBarController: UITabBarController {
    var switchStatus = true
}