void PlatformStop()
        {
            if (listener == null)
            {
                return;
            }

            Platform.SensorManager.UnregisterListener(listener, barometer);
            listener.Dispose();
            listener  = null;
            barometer = null;
        }
 void PlatformStart(SensorSpeed sensorSpeed)
 {
     listener  = new BarometerListener(RaiseReadingChanged);
     barometer = DefaultBarometer;
     Platform.SensorManager.RegisterListener(listener, barometer, sensorSpeed.ToPlatform());
 }