Android Shake Listener

ShakeListener.java

/*

Usage:

  public class MainActivity extends Activity 
       implements ShakeListener.Callback {

    @Override
    public void shakingStarted() {
      // Code on started here
    }
  
    @Override
    public void shakingStopped() {
      // Code on stopped here
    }
  }

*/
public class ShakeListener {
  private SensorManager mgr = null;
	private long lastShakeTimestamp = 0;
	private double threshold = 1.0d;
	private long gap = 0;
	private ShakeListener.Callback cb = null;

	public ShakeListener(Context ctxt, double threshold, long gap, ShakeListener.Callback cb) {
		this.threshold = threshold * threshold;
		this.threshold = this.threshold * SensorManager.GRAVITY_EARTH
				* SensorManager.GRAVITY_EARTH;
		this.gap = gap;
		this.cb = cb;

		mgr = (SensorManager) ctxt.getSystemService(Context.SENSOR_SERVICE);
		mgr.registerListener(listener,
				mgr.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
				SensorManager.SENSOR_DELAY_UI);
	}

	public void close() {
		mgr.unregisterListener(listener);
	}

	private void isShaking() {
		long now = SystemClock.uptimeMillis();
		try {
			if (lastShakeTimestamp == 0) {
				lastShakeTimestamp = now;

				if (cb != null) {
					cb.shakingStarted();
				}
			} else {
				lastShakeTimestamp = now;
			}
		} catch (NullPointerException e) {

		}
	}

	private void isNotShaking() {
		long now = SystemClock.uptimeMillis();

		if (lastShakeTimestamp > 0) {
			if (now - lastShakeTimestamp > gap) {
				lastShakeTimestamp = 0;

				if (cb != null) {
					cb.shakingStopped();
				}
			}
		}
	}

	public interface Callback {
		void shakingStarted();

		void shakingStopped();
	}

	private final SensorEventListener listener = new SensorEventListener() {
		@Override
		public void onSensorChanged(SensorEvent e) {
			if (e.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
				double netForce = e.values[0] * e.values[0];

				netForce += e.values[1] * e.values[1];
				netForce += e.values[2] * e.values[2];

				if (threshold < netForce) {
					isShaking();
				} else {
					isNotShaking();
				}
			}
		}

		@Override
		public void onAccuracyChanged(Sensor sensor, int accuracy) {
			// unused
		}
	};

}