NavigationPopTransitionController.swift

final class NavigationPopTransitionController: UIPercentDrivenInteractiveTransition {

    let navigationController: UINavigationController

    private lazy var interactivePopGestureRecognizer = UIScreenEdgePanGestureRecognizer(target: self, action: #selector(handle))

    var ifInteractive: Self? {
        interactivePopGestureRecognizer.state == .began ? self : nil
    }

    init(navigationController: UINavigationController) {
        self.navigationController = navigationController
        super.init()
        interactivePopGestureRecognizer.edges = .left
        interactivePopGestureRecognizer.delegate = self
        navigationController.view.addGestureRecognizer(interactivePopGestureRecognizer)
    }

}

private extension NavigationPopTransitionController {

    @objc func handle(_ recognizer: UIScreenEdgePanGestureRecognizer) {
        guard let view = recognizer.view else { return }
        let translation = recognizer.translation(in: view)
        let velocity = recognizer.velocity(in: view)
        let width = view.bounds.width
        let progress = translation.x / width
        switch recognizer.state {
        case .began:
            navigationController.popViewController(animated: true)
        case .changed:
            update(min(progress, 0.999))
        case .ended:
            if progress >= 0.5 || velocity.x > 0 {
                completionSpeed = duration * velocity.x / width
                finish()
            } else {
                cancel()
            }
        case .cancelled:
            cancel()
        default:
            break
        }
    }

}

extension NavigationPopTransitionController: UIGestureRecognizerDelegate {

    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
        // Only allowed if no active transition.
        return navigationController.transitionCoordinator == nil
    }

    func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldBeRequiredToFailBy otherGestureRecognizer: UIGestureRecognizer) -> Bool {
        // Don't muck up horizontal scrolling.
        otherGestureRecognizer is UIPanGestureRecognizer && otherGestureRecognizer.view is UIScrollView
    }

}

extension Foo: UINavigationControllerDelegate {
    
    lazy var navigationPopTransition = NavigationPopTransitionController(...)

    func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationController.Operation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        MyAwesomeAnimator(for: operation)
    }

    func navigationController(_ navigationController: UINavigationController, interactionControllerFor animationController: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        navigationPopTransition?.ifInteractive
    }

}