How to save the state of the checkbox to core data in Swift?

**I'm not a good English speaker. Please forgive me of ray awkward English.

I'm working on a to do list project using checkbox. I cannot find a way to save the state of the checkbox to core data

This is a part of the code I use right now. Tasksis the Entity (class definition) and it has isMarked as a Boolean Attribute. (I cut a lot to make it simple so if you find something strange in the code please write a comment)

import UIKit
import CoreData

var toDolist: [Tasks] = []

class FirstViewController: UIViewController, UITableViewDelegate, UITableViewDataSource{


    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(toDoTable)
    }

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

    @IBOutlet weak var toDoTable: UITableView!

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int{
        return toDolist.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell{
        let cell = tableView.dequeueReusableCell(withIdentifier: "cell") as! CustomTableViewCell
        cell.box.setImage(#imageLiteral(resourceName: "uncheckedbox"), for: .normal)

        let task = toDolist[indexPath.row]

        return cell
    }

    func getData(){
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        do{
            toDolist = try context.fetch(Tasks.fetchRequest())
        }catch{
            print("fetching failed")
        }
    }

    override func viewWillAppear(_ animated: Bool) {
        getData()
        toDoTable.reloadData()
    }

}


class CustomTableViewCell: UITableViewCell {
    override func awakeFromNib() {
        super.awakeFromNib()
    }
    override func setSelected(_ selected: Bool, animated: Bool) {
        super.setSelected(selected, animated: animated)
    }
    @IBOutlet weak var box: CheckBox!
    @IBOutlet weak var taskLbl: UILabel!
}

class CheckBox: UIButton {
    var isChecked: Bool = false{
        didSet{
            if isChecked == true{
                self.setImage(#imageLiteral(resourceName: "checkedbox"), for: .normal)
            }else{
                self.setImage(#imageLiteral(resourceName: "uncheckedbox"), for: .normal)
            }
        }
    }

    override func awakeFromNib(){
        self.addTarget(self, action: #selector(self.buttonClicked(_:)), for: .touchUpInside)
        self.isChecked = false
    }

    func buttonClicked(_ sender: UIButton){
        if sender == self{
            if isChecked == true{
                isChecked = false
            }else{
                isChecked = true
            }
        }
    }
}

How can I solve this by adding some code to it? Or do I have to change all the code above?