Initialization

private String TAG = "SensorCompass";

// Sensors & SensorManager
private Sensor accelerometer;
private Sensor magnetometer;
private SensorManager mSensorManager;

// Storage for Sensor readings
private float[] mGravity = null;
private float[] mGeomagnetic = null;

// Rotation around the Z axis
private double mRotationInDegress;

OnCreate

protected void onCreate(Bundle savedInstanceState) {

		// ...

		// Get a reference to the SensorManager
		mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);

		// Get a reference to the accelerometer
		accelerometer = mSensorManager
				.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

		// Get a reference to the magnetometer
		magnetometer = mSensorManager
				.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);

		// Exit unless both sensors are available
		if (null == accelerometer || null == magnetometer)
			finish();

	}

Register this class as a listener for the accelerometer events and for magnetometer events:

@Override
	protected void onResume() {
		super.onResume();


		// Register for sensor updates

		mSensorManager.registerListener(this, accelerometer,
				SensorManager.SENSOR_DELAY_NORMAL);

		mSensorManager.registerListener(this, magnetometer,
				SensorManager.SENSOR_DELAY_NORMAL);
	}

The onPause method unregister this class as a listener for all sesnors

@Override
protected void onPause() {
	super.onPause();

	// Unregister all sensors
	mSensorManager.unregisterListener(this);
}

The onSensorChanged method process the incoming sensors’ events

@Override
public void onSensorChanged(SensorEvent event) {

	// Acquire accelerometer event data
	
	if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {

		mGravity = new float[3];
		System.arraycopy(event.values, 0, mGravity, 0, 3);

	} 
	
	// Acquire magnetometer event data
	
	else if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD) {

		mGeomagnetic = new float[3];
		System.arraycopy(event.values, 0, mGeomagnetic, 0, 3);

	}

	// If we have readings from both sensors then
	// use the readings to compute the device's orientation
	// and then update the display.

	if (mGravity != null && mGeomagnetic != null) {

		float rotationMatrix[] = new float[9];

		// Users the accelerometer and magnetometer readings
		// to compute the device's rotation with respect to
		// a real world coordinate system

		boolean success = SensorManager.getRotationMatrix(rotationMatrix,
				null, mGravity, mGeomagnetic);

		if (success) {

			float orientationMatrix[] = new float[3];

			// Returns the device's orientation given
			// the rotationMatrix

			SensorManager.getOrientation(rotationMatrix, orientationMatrix);

			// Get the rotation, measured in radians, around the Z-axis
			// Note: This assumes the device is held flat and parallel
			// to the ground

			float rotationInRadians = orientationMatrix[0];

			// Convert from radians to degrees
			mRotationInDegress = Math.toDegrees(rotationInRadians);

			// Request redraw
			mCompassArrow.invalidate();

			// Reset sensor event data arrays
			mGravity = mGeomagnetic = null;

		}
	}

}