вы забыли установить рамку вида и метку, сделайте как
let newView = UIView()
newView.frame = CGRect(x:0,y:UIApplication.shared.statusBarFrame.height, width: view.frame.size.width, height: self.navigationController!.navigationBar.frame.height)
newView.backgroundColor = UIColor.red
let label = UILabel()
label.frame = newView.bounds
label.text = "header"
newView.addSubview(label)
UIApplication.shared.keyWindow?.addSubview(newView)
вы получаете ОП как
Вариант 2
согласно вашему требованию
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// Do any additional setup after loading the view, typically from a nib.
let newView = UIView()
newView.frame = CGRect(x:0,y:UIApplication.shared.statusBarFrame.height, width: view.frame.size.width, height: 30)
newView.backgroundColor = UIColor.red
let label = UILabel()
label.frame = newView.bounds
label.text = "header"
newView.addSubview(label)
UIApplication.shared.keyWindow?.addSubview(newView)
let bounds = self.navigationController!.navigationBar.bounds
self.navigationController?.navigationBar.frame = CGRect(x: 0, y: newView.frame.size.height + newView.frame.origin.y, width: bounds.width, height: bounds.height )
}