UISearchBar，我怎样才能保持调暗视图（阴影背景）始终显示

func searchBarTextDidBeginEditing(_ searchBar: UISearchBar) {
    searchBar.setShadowImage(UIImage(), for: .any)
    searchBar.backgroundColor = UIColor.clear
}

func searchBarTextDidBeginEditing(_ searchBar: UISearchBar) {
    searchBar.layer.shadowColor = UIColor.black.cgColor
    searchBar.layer.shadowOpacity = 0.5
    searchBar.layer.shadowOffset = CGSize(width: 0, height: 2)
    searchBar.layer.shadowRadius = 2
}

func searchBarTextDidEndEditing(_ searchBar: UISearchBar) {
    searchBar.setShadowImage(nil, for: .any)
    searchBar.backgroundColor = UIColor.white
}