UIButton - rounded corners in highlight state

In the image below the button is being pressed. The shadow is square. How do I make the shadow have rou开发者_运维知识库nded edges?

Here's how I did it:

1) Subclass the UIButton

2) Change the Layer like this:

public class GalleryButton: UIButton
    {

        public GalleryButton (RectangleF frame) : base(frame)
        {
            this.Layer.CornerRadius = 12f;
            this.Layer.MasksToBounds = true;
            this.Layer.BorderWidth = 0f;
        }

I think if you have a transparent png, it works as it is.