final class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        view.safeArea.insetsDidChange = { insets in
            // If safeAreaInsets of `self.view`, this closure called.
            print("view.safeAreaInsets = \(insets)")
        }
    }
}

extension UIView {
    // UIView Extension about Safe Area
    var safeArea: SafeAreaExtension { get }
}

class SafeAreaExtension {
    // Safe Area Insets handler
    var insetsDidChange: ((UIEdgeInsets) -> ())? { get set }
}