¿Cómo puedo llamar a func checkField cuando se toca nextButton? Si ningún campo está vacío en ManagedTableEleventhViewController, me gustaría pasar a TwelvethViewController.

Tengo un UIViewControllerElevethViewController que tiene una vista de contenedor. La vista del contenedor tiene una incrustación segue a un UITableViewControllerManagedTableEleventhViewController. DeElevethViewController hay un Show segue a un UIViewControllerTwelevethViewController

La jerarquía de vista se ve así:

ElevethViewController 
  Container View
     ManagedTableEleventhViewController embedded in Container View with Embed segue
      Show segue "eleventhToTwelveth" to "Tweleveth View Controller"
       Embed segue "myEmbeddedSegue" to "Managed Table"

ManagedTableEleventhViewController contiene 4 celdas estáticas que contienen 1 textField cada una y una celda estática vacía.

 import UIKit
 import Foundation

  protocol DelegateEleventh {
    func checkField(sender:EleventhViewController)
   }

 class EleventhViewController: UIViewController {

      var delegate:DelegateEleventh?
      @IBAction func nextButton(_ sender: Any) {

         //if delegate is not nil, call func checkField
           if let delegateVC = delegate {
        delegateVC.checkField(sender:self) 

    } else{
        print("delegateVC value \(delegate)") //prints nil
     }
   }       
 } //end of class




 import Foundation
 import UIKit

 class ManagedTableEleventhViewController:    
 UITableViewController,UITextFieldDelegate,DelegateEleventh {


@IBOutlet weak var fullName: UITextField!
@IBOutlet weak var flatNumber: UITextField!
@IBOutlet weak var streetAddress: UITextField!
@IBOutlet weak var phoneNumber: UITextField!


override func viewDidLoad() {
    super.viewDidLoad()
}


//When nextButton is touched in EleventhViewController, call this method
func checkField(sender:EleventhViewController){

    for cell in self.tableView.visibleCells {
        for sub in cell.contentView.subviews{

            if sub is UITextField{

let textF = sub as? UITextField
         //if textField is empty, make its border red,else clear
                if textF?.text == "" {
                    self.hasText = false
                    textF?.layer.cornerRadius = 8.0
                    textF?.layer.masksToBounds = true
                    textF?.layer.borderColor = UIColor.red.cgColor
                    textF?.layer.borderWidth = 1.0
                      } else {
                    self.hasText = true

                    //segue to nextViewcontroller
               sender.performSegue(withIdentifier: "elevethToTwelveth", sender: sender)

                }
            }// end of if sub is UITextField

        }//end of for sub in cell.contentView

    } //end of  for cell in tableView
}



       //KEYBOARD DOES NOT RESIGN....
// When tapping outside of the keyboard, close the keyboard down
 override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    self.view.endEditing(true)
 }

 // used to check textField has a string value
 var hasText:Bool! 


  //called by ManagedTableEleventhViewController (namely the delegate) when    
           editing has begun
func textFieldDidBeginEditing(_ textField: UITextField) {

    if textField.text == "" {

        print("false value in textFieldDidBeginEditing")
        self.hasText = false
        textField.layer.cornerRadius = 8.0
        textField.layer.masksToBounds = true
        textField.layer.borderColor = UIColor.red.cgColor
        textField.layer.borderWidth = 1.0

    } else {
        print("true value in textFieldDidBeginEditing")
        textField.layer.borderWidth = 1
        textField.layer.borderColor = UIColor.white.cgColor
        print("call in else")
        self.hasText = true
    }
 }


//called by ManagedTableEleventhViewController (namely the delegate) when    
             // editing stopped
   func textFieldDidEndEditing(_ textField: UITextField, reason: UITextFieldDidEndEditingReason) {

         //if textField is empty make the border red
    if textField.text == "" {
        self.hasText = false
        textField.layer.cornerRadius = 8.0
        textField.layer.masksToBounds = true
        textField.layer.borderColor = UIColor.red.cgColor
        textField.layer.borderWidth = 1.0
        print("false value in textFieldDidEndEditing")
             } else {
        textField.layer.borderWidth = 1
        textField.layer.borderColor = UIColor.white.cgColor
        print("true value in textFieldDidEndEditing")
        self.hasText = true
    }
}

    // Stop Editing on Return Key Tap. 
     func textFieldShouldReturn(_ textField: UITextField) -> Bool {
    textField.resignFirstResponder()
    return true
  }
}//end of class