How to tell if the iPhone is "face down" or "face up"?

Question

I need to detect which direction the iPhone is facing so I can record from the non obstructed camera (Front or Back) when dropped.

This will be used in emergency situation and the phone is dropped, or placed in such a way that the active camera is recording nothing of value.

What API or technique can I use to determine this?

Answer 1

Use the Core Motion accelerometer. It reports the direction of gravity, which tells you which way the device is facing.