CatEffectiOS.cs

using System;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;
using SampleApp.iOS.Effects;

[assembly: ResolutionGroupName("SuperAwesome")]
[assembly: ExportEffect(typeof(CatEffect), "CatEffect")]
namespace SampleApp.iOS.Effects
{
    public class CatEffect : PlatformEffect
    {
        protected override void OnAttached()
        {
            try
            {
                if (Control is UITextField)
                {
                    // Set the border style to none to show background image.
                    (Control as UITextField).BorderStyle = UITextBorderStyle.None;

                    // Create an image and set it as background pattern.
                    var image = UIImage.FromBundle("cat.jpg").CreateResizableImage(new UIEdgeInsets(0, 10, 0, 10), UIImageResizingMode.Stretch);
                    Control.BackgroundColor = UIColor.FromPatternImage(image);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Cannot set property on attached control. Error: ", ex.Message);
            }
        }

        protected override void OnDetached()
        {
            // Back to white.
            Control.BackgroundColor = UIColor.White;
        }
    }
}