Fallbacks and syntactic sugar for iOS 11 safeArea API

UIView+SafeAnchors.swift

//
//  UIView+SafeAnchors.swift
//

extension UIView {
    
    var safeTopAnchor: NSLayoutYAxisAnchor { return optionalSafeAreaLayoutGuide?.topAnchor ?? topAnchor }
    var safeBottomAnchor: NSLayoutYAxisAnchor { return optionalSafeAreaLayoutGuide?.bottomAnchor ?? bottomAnchor }
    
    var safeLeftAnchor: NSLayoutXAxisAnchor { return optionalSafeAreaLayoutGuide?.leftAnchor ?? leftAnchor }
    var safeRightAnchor: NSLayoutXAxisAnchor { return optionalSafeAreaLayoutGuide?.rightAnchor ?? rightAnchor }
    
    var safeLeadingAnchor: NSLayoutXAxisAnchor { return optionalSafeAreaLayoutGuide?.leadingAnchor ?? leadingAnchor }
    var safeTrailingAnchor: NSLayoutXAxisAnchor { return optionalSafeAreaLayoutGuide?.trailingAnchor ?? trailingAnchor }
    
    var safeCenterXAnchor: NSLayoutXAxisAnchor { return optionalSafeAreaLayoutGuide?.centerXAnchor ?? centerXAnchor }
    var safeCenterYAnchor: NSLayoutYAxisAnchor { return optionalSafeAreaLayoutGuide?.centerYAnchor ?? centerYAnchor }
    
    var safeWidthAnchor: NSLayoutDimension { return optionalSafeAreaLayoutGuide?.widthAnchor ?? widthAnchor }
    var safeHeightAnchor: NSLayoutDimension { return optionalSafeAreaLayoutGuide?.heightAnchor ?? heightAnchor }
    
    private var optionalSafeAreaLayoutGuide: UILayoutGuide? {
        if #available(iOS 11, *) {
            return safeAreaLayoutGuide
        } else {
            return nil
        }
    }
    
}

You could return layoutMarginsGuide in the else part.