How do I implement a volume shutter?

I want to implement a volume shutter in my iPhone camera app. When the user presses the volume button, I should get an event to take a photo.

I'm looking for an implementation that meets the following requirements:

• It should work even if the volume is currently at the maximum, and the user presses the volume up button.
• There should be no on-screen UI showing that the volume changed.
• There should be no known cases of Apple rejecting an app that used this technique.

Camera apps like ProCamera, ProCam and Camera+ have the volume shutter that satisfies all these conditions, so it's clearly possible.