UINavigationController.swift

extension UINavigationController {
    // 1
    static private var coordinatorHelperKey = "UINavigationController.TransitionCoordinatorHelper"

    // 2
    var transitionCoordinatorHelper: TransitionCoordinator? {
        return objc_getAssociatedObject(self, &UINavigationController.coordinatorHelperKey) as? TransitionCoordinator
    }

    func addCustomTransitioning() {
        // 3
        var object = objc_getAssociatedObject(self, &UINavigationController.coordinatorHelperKey)

        guard object == nil else {
            return
        }

        object = TransitionCoordinator()
        let nonatomic = objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC
        objc_setAssociatedObject(self, &UINavigationController.coordinatorHelperKey, object, nonatomic)

        // 4
        delegate = object as? TransitionCoordinator


        // 5
        let edgeSwipeGestureRecognizer = UIScreenEdgePanGestureRecognizer(target: self, action: #selector(handleSwipe(_:)))
        edgeSwipeGestureRecognizer.edges = .left
        view.addGestureRecognizer(edgeSwipeGestureRecognizer)
    }

    // 6
    @objc func handleSwipe(_ gestureRecognizer: UIScreenEdgePanGestureRecognizer) {
        guard let gestureRecognizerView = gestureRecognizer.view else {
            transitionCoordinatorHelper?.interactionController = nil
            return
        }

        let percent = gestureRecognizer.translation(in: gestureRecognizerView).x / gestureRecognizerView.bounds.size.width

        if gestureRecognizer.state == .began {
            transitionCoordinatorHelper?.interactionController = UIPercentDrivenInteractiveTransition()
            popViewController(animated: true)
        } else if gestureRecognizer.state == .changed {
            transitionCoordinatorHelper?.interactionController?.update(percent)
        } else if gestureRecognizer.state == .ended {
            if percent > 0.5 && gestureRecognizer.state != .cancelled {
                transitionCoordinatorHelper?.interactionController?.finish()
            } else {
                transitionCoordinatorHelper?.interactionController?.cancel()
            }
            transitionCoordinatorHelper?.interactionController = nil
        }
    }
}