tab_instantiate_controller.swift

  var currentViewController: UIViewController?
    lazy var firstChildTabVC: UIViewController? = {
        let firstChildTabVC = self.storyboard?.instantiateViewController(withIdentifier: "FirstViewControllerId")
        return firstChildTabVC
    }()
    lazy var secondChildTabVC : UIViewController? = {
        let secondChildTabVC = self.storyboard?.instantiateViewController(withIdentifier: "SecondViewControllerId")
        
        return secondChildTabVC
    }()  

  func viewControllerForSelectedSegmentIndex(_ index: Int) -> UIViewController? {
        var vc: UIViewController?
        switch index {
        case TabIndex.firstChildTab.rawValue :
            vc = firstChildTabVC
        case TabIndex.secondChildTab.rawValue :
            vc = secondChildTabVC
        default:
        return nil
        }
    
        return vc
    }