TabBar Pressing Button Brings it Back to Top of ScrollView

TabBarBackToTop.swift

class tapBarController: UITabBarController, UITabBarControllerDelegate {

    /// Determines whether the scrolling capability's enabled.
    var scrollEnabled: Bool = true

    private var previousIndex = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        delegate = self
    }

    /*
     Always call "super" if you're overriding this method in your subclass.
     */
    func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
        guard scrollEnabled else {
            return
        }

        guard let index = viewControllers?.indexOf(viewController) else {
            return
        }

        if index == previousIndex {
            var scrollViews = [UIScrollView]()

            dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), { [weak self] () in

                self?.iterateThroughSubviews(self?.view) { (scrollView) in
                    scrollViews.append(scrollView)
                }

                guard let scrollView = scrollViews.first else {
                    return
                }

                dispatch_async(dispatch_get_main_queue(), {
                    scrollView.setContentOffset(CGPointZero, animated: true)
                })
                })
        }

        previousIndex = index
    }

    /*
     Iterates through the view hierarchy in an attempt to locate a UIScrollView with "scrollsToTop" enabled.
     Since the functionality relies on "scrollsToTop", it plugs easily into existing architectures - you can
     control the behaviour by modifying "scrollsToTop" on your UIScrollViews.
     */
    private func iterateThroughSubviews(parentView: UIView?, onRecognition: (UIScrollView) -> Void) {
        guard let view = parentView else {
            return
        }

        for subview in view.subviews {
            if let scrollView = subview as? UIScrollView where scrollView.scrollsToTop == true {
                onRecognition(scrollView)
            }

            iterateThroughSubviews(subview, onRecognition: onRecognition)
        }
    }
}