UIImagePickerController Video countdown timer

I am using the UIImagePickerController to let the user record video. The duration is limited to 10 seconds. I want to display a countdown in the corner while they are recording though so they know how long they have left. I am able to add this using the cameraOverlayView prop开发者_StackOverflow中文版erty. But I can't find anyway to get a call when they hit the record button (so that I can start my timer to update my countdown).

Does anyone know how to do this?

Thanks in advance