Re-enables the interactivePopGestureRecognizer functionality that we lose when we implement our own back button handling.

SwipeNavigationController.swift

/*
 This class re-enables the interactivePopGestureRecognizer 
 functionality that we lose when we implement our own back button handling.
 
 It disables the pop gesture in three scenarios:
 1) when the user is still on the root view controller
 2) when a push animation is in progress
 3) when the user swipes quickly multiple times and animations don't have time to be performed
 */

import UIKit

final class SwipeNavigationController: UINavigationController {
    
    // MARK: - Lifecycle
    
    override init(rootViewController: UIViewController) {
        super.init(rootViewController: rootViewController)
    }
    
    override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
        
        delegate = self
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // This needs to be in here, not in init
        interactivePopGestureRecognizer?.delegate = self
    }
    
    deinit {
        delegate = nil
        interactivePopGestureRecognizer?.delegate = nil
    }
    
    // MARK: - Overrides
    
    override func pushViewController(_ viewController: UIViewController, animated: Bool) {
        duringPushAnimation = true
        
        super.pushViewController(viewController, animated: animated)
    }
    
    // MARK: - Private Properties
    
    fileprivate var duringPushAnimation = false
    
    // MARK: - Unsupported Initializers
    
    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }
    
}

// MARK: - UINavigationControllerDelegate

extension SwipeNavigationController: UINavigationControllerDelegate {
    
    func navigationController(_ navigationController: UINavigationController, didShow viewController: UIViewController, animated: Bool) {
        guard let swipeNavigationController = navigationController as? SwipeNavigationController else { return }
        
        swipeNavigationController.duringPushAnimation = false
    }
    
}

// MARK: - UIGestureRecognizerDelegate

extension SwipeNavigationController: UIGestureRecognizerDelegate {
    
    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
        guard gestureRecognizer == interactivePopGestureRecognizer else {
            return true // default value will disable the gesture
        }
        
        return viewControllers.count > 1 && duringPushAnimation == false
    }
}

add
delegate = self
to
override init(rootViewController: UIViewController)