This extension gives access to two important sensor readings, the heading of the device and the magnetometer sensor data.
The Compass ANE is designed to give the magnetic-north heading values from the device calculated through a line from the bottom of the device to the top.
On iOS the location is used to calculate the offset from the magnetic heading to the true heading and both values are reported. (This may require the user to accept a permission for your application to access the device location).
The raw magnetic field values can be retrieved using this extension. You can listen for the MagneticFieldEvent for changes in the magnetic field. There are 2 important events, both indicating the magnetic field, one in the calibrated state and the other the raw uncalibrated data.