I'm having trouble hiding SearchBar on NavigationItem.

I'm having trouble hiding SearchBar on NavigationItem.

ex: https://www.dropbox.com/s/6is5mukpk1yew8e/bgSearch.gif?dl=0

Using this code:

func scrollViewDidScroll (_ scrollView: UIScrollView) {
    let transp: Float = Float (1 - (self.mTableView.contentOffset.y / (self.mTableView.contentSize.height - self.mTableView.frame.size.height) * 60))
    self.navigationController! .navigationBar.alpha = CGFloat (transp)
    self.mSearchController.searchBar.alpha = CGFloat (transp)
    titleMsgView.alpha = CGFloat (transp)
}

Everything works fine, except for a background that I could not access to hide with the alpha.

I'm using:

viewDidLoad () {
...
        mSearchController = UISearchController (searchResultsController: nil)
        mSearchController.delegate = self as? UISearchControllerDelegate
        mSearchController.searchResultsUpdater = self
        mSearchController.hidesNavigationBarDuringPresentation = false
        //mSearchController.searchBar.sizeToFit ()
     
        navigationItem.searchController = mSearchController
...
}

As the SerchBar search field disappears correctly, you will understand that it is something with the NavigationItem to which it has been added. Could someone help me solve the problem?