a.swift

let navigationBar = UINavigationBar()
navigationBar.isTranslucent = false
navigationBar.delegate = self
navigationBar.backgroundColor = .white
navigationBar.items = [standaloneItem]
 
navigationBar.translatesAutoresizingMaskIntoConstraints = false
navigationBar.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
navigationBar.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
 
if #available(iOS 11, *) {
  navigationBar.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
} else {
  navigationBar.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
}


extension BarcodeScannerController: UINavigationBarDelegate {
  public func position(for bar: UIBarPositioning) -> UIBarPosition {
    return .topAttached
  }
}