iOS 11 safe area layout guide backwards compatibility upto iOS 9

SafeAreaTopAnchorSnippet.swift

        let safeAreaTopAnchor:NSLayoutYAxisAnchor?
        if #available(iOS 11.0, *) {
            safeAreaTopAnchor = contentView.safeAreaLayoutGuide.topAnchor
        } else {
            // Fallback on earlier versions
            
            var parentViewController: UIViewController? {
                var parentResponder: UIResponder? = self
                while parentResponder != nil {
                    parentResponder = parentResponder!.next
                    if let viewController = parentResponder as? UIViewController {
                        return viewController
                    }
                }
                return nil
            }
            
            safeAreaTopAnchor = parentViewController?.topLayoutGuide.bottomAnchor
            
        }