public void addOnCentralPositionChangedListener(OnCentralPositionChangedListener listener){
  mOnCentralPositionChangedListeners.add(listener);
}

public void addOnCentralPositionChangedListener ( OnCentralPositionChangedListener listener ) { mOnCentralPositionChangedListeners . add ( listener ) ; }
adds a listener that will be called when the central item of the list changes .
