How to set NavigationBar's background color to gradient color?

Use the following code I only get black opaque background color for NavigationBar

[assembly: ExportRenderer (typeof (NavigationPage), typeof (TabbedNavigationRenderer))]
namespace DutyFreeCollection.iOS
{
    public class TabbedNavigationRenderer : NavigationRenderer
    {
        public TabbedNavigationRenderer ()
        {
            NavigationBar.BarStyle = UIBarStyle.Black;
            NavigationBar.Translucent = true;
            NavigationBar.BarTintColor = UIColor.FromRGBA (0f, 0f, 0f, 1f);
        }

        public override UIStatusBarStyle PreferredStatusBarStyle () {
            // Here is never reached
            return AppDelegate.Theme.StatusBarStyle;
        }
    }
}

Thank you.