code: public void setZoomOnScrollEnabled(boolean zoomOnScrollEnabled) {
  		if (mouseWheelRegistration != null) {
  			mouseWheelRegistration.removeHandler();
  			mouseWheelRegistration = null;
  		}
  		this.zoomOnScrollEnabled = zoomOnScrollEnabled;
  		if (zoomOnScrollEnabled) {
  			mouseWheelRegistration = graphics.addMouseWheelHandler(new ZoomOnScrollController());
  		}
  	}
docs: Determines whether or not the zooming using the mouse wheel should be enabled or not.
  
  @param zoomOnScrollEnabled
  True or false. Enable or disable zooming using the mouse wheel.