Backwards compatible Safe Area Layout Guide API for iOS 10 and iOS 9

UIViewController+SafeAreaLayoutGuide.swift

import UIKit

extension UIViewController {
    
    var ft_safeAreaLayoutGuide: UILayoutGuide {
        if #available(iOS 11.0, *) {
            return view.safeAreaLayoutGuide
        } else {
            let id = "ft_safeAreaLayoutGuide"
            
            if let layoutGuide = view.layoutGuides.filter({ $0.identifier == id }).first {
                return layoutGuide
            } else {
                let layoutGuide = UILayoutGuide()
                layoutGuide.identifier = id
                view.addLayoutGuide(layoutGuide)
                layoutGuide.topAnchor.constraint(equalTo: topLayoutGuide.bottomAnchor).isActive = true
                layoutGuide.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
                layoutGuide.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
                layoutGuide.bottomAnchor.constraint(equalTo: bottomLayoutGuide.topAnchor).isActive = true
                return layoutGuide
            }
        }
    }
    
}