如何使用iOS 11和prefersLargeTitles为UINavigationBar设置背景渐变？

navigationItem.largeTitleDisplayMode = .always

if let navigationBar = navigationController?.navigationBar {
    // 设置导航栏的背景颜色
    navigationBar.barTintColor = UIColor(red: 0.0, green: 0.5, blue: 1.0, alpha: 1.0)
    
    // 创建一个渐变层
    let gradientLayer = CAGradientLayer()
    gradientLayer.frame = CGRect(x: 0, y: 0, width: navigationBar.bounds.width, height: navigationBar.bounds.height)
    gradientLayer.colors = [UIColor(red: 0.0, green: 0.5, blue: 1.0, alpha: 1.0).cgColor, UIColor(red: 0.0, green: 0.0, blue: 1.0, alpha: 1.0).cgColor]
    
    // 将渐变层添加到导航栏的layer中
    navigationBar.layer.addSublayer(gradientLayer)
}