Xamarin objc_getAssociatedObject / objc_setAssociatedObject snippet

ParallaxHeaderExtension.cs

    public class ParallaxHeader : UIView
    {
        public UIScrollView ScrollView { get; set; }
    }

    public static class ParallaxHeaderExtension
    {
        private static readonly NSString DescriptiveName = new NSString("Vapolia."  + nameof(ParallaxHeader));

        enum AssociationPolicy {
            //ASSIGN = 0,
            RETAIN_NONATOMIC = 1,
            //COPY_NONATOMIC = 3,
            //RETAIN = 01401,
            //COPY = 01403,
        }

        [DllImport("/usr/lib/libobjc.dylib")]
        static extern void objc_setAssociatedObject(IntPtr obj, IntPtr key, IntPtr value, AssociationPolicy policy);

        [DllImport ("/usr/lib/libobjc.dylib")]
        static extern IntPtr objc_getAssociatedObject (IntPtr obj, IntPtr key);

        public static ParallaxHeader GetParallaxHeader(this UIScrollView scrollView)
        {
            var handle = objc_getAssociatedObject(scrollView.Handle, DescriptiveName.Handle);
            if(handle != IntPtr.Zero)
            {
                var value = ObjCRuntime.Runtime.GetNSObject(handle);
                if (value != null)
                    return (ParallaxHeader)value;
            }

            var ph = new ParallaxHeader();
            scrollView.SetParallaxHeader(ph);
            return ph;
        }

        public static void SetParallaxHeader(this UIScrollView scrollView, ParallaxHeader parallaxHeader)
        {
            parallaxHeader.ScrollView = scrollView;
            objc_setAssociatedObject(scrollView.Handle, DescriptiveName.Handle, parallaxHeader.Handle, AssociationPolicy.RETAIN_NONATOMIC);
        }
    }