code: public void zoomChanged(double newZoom) {
              final long roundedZoom = Math.round(newZoom);
              if (logger.isTraceEnabled()) {
                  logger.trace("JS reports zoom value {}", roundedZoom);
              }
              lastZoomFromMap.set(roundedZoom);
              setZoom(roundedZoom);
          }
docs: called when the user changed the zoom with the controls in the map.
  
  @param newZoom
  new zoom value