How do I pass a variable from detail view controller to master view controller in Master-Detail iOS app?

What I had to do:

- master view contains a list of "pages"

- when selecting, opens in detail

- once detail is completed, I needed to mark the item in the master list as complete.

Is your use case similar ?

I will complement my example with the passing of a value from detailView.

What I did in this case was to use delegation.

Defined a protocol

protocol DetailViewControllerDelegate  {
    func validateSection(num: Int, valid: Bool)     // num being the item in the list to update

    func passValueFromDetail(value: Int)
}

have master conform to it, and declare a var to get the transmitted value

class MasterViewController: UITableViewController, DetailViewControllerDelegate {
     var valueToUse: Int?

Implement the func in Master

    func validateSection(num: Int, valid: Bool) {
  
        // update dataSource with the new value for item num
        tableView.reloadData()
        // Do what else is needed
        }
    }

    func passValueFromDetail(value: Int) {

        valueToUse = value
       // Do what you need with valueToUse: update a field…
        }
    }

Still in Master, define the delegate in prepare

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
     // Do what's needed

      controller.delegate = self
}

In detailViewController, create a delegate

class DetailViewController: UIViewController  { 

    var delegate: DetailViewControllerDelegate?

// And call the delegate where I need to

   delegate?.validateSection(num: someNum, valid: true) 
   delegate?.passValueFromDetail(value: 100)     // 100 as an example or any value you want