Create custom renderer for Picker

Hello,

I need to create a custom renderer for picker for WP8

I tried to do the following

`
[assembly: ExportRenderer(typeof(Picker), typeof(CustomPickerRenderer))]
namespace PCLXamarinForms.WinPhone
{
public class CustomPickerRenderer : PickerRenderer
{

    public CustomPickerRenderer()
    {
    }

}

}
`

but couldn't override the onModalChanged.

How to do that ?