Dismissible & DismissalDelegate Protocols

Dismissible.swift

import UIKit

protocol DismissalDelegate : class {
    func finishedShowing(viewController: UIViewController)
}

protocol Dismissible : class {
    weak var dismissalDelegate : DismissalDelegate? { get set }
}

extension DismissalDelegate where Self: UIViewController {
    func finishedShowing(viewController: UIViewController) {
        guard
            viewController.isBeingPresented() &&
            viewController.presentingViewController == self
            else {
                navigationController?.popViewControllerAnimated(true)
                return
        }
        dismissViewControllerAnimated(true, completion: nil)
    }
}

class PresentedController: UIViewController, Dismissible {
    weak var dismissalDelegate: DismissalDelegate?
    @IBAction func done() {
        dismissalDelegate?.finishedShowing(self)
    }
}

class PresentingViewController: UIViewController, DismissalDelegate {
    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if let vc = segue.destinationViewController as? Dismissible {
            vc.dismissalDelegate = self
        }
    }
}