public void addDataCallback(OneSheeldDataCallback dataCallback){
  if (dataCallback != null && !dataCallbacks.contains(dataCallback))   dataCallbacks.add(dataCallback);
}

public void addDataCallback ( OneSheeldDataCallback dataCallback ) { if ( dataCallback != null && ! dataCallbacks . contains ( dataCallback ) ) dataCallbacks . add ( dataCallback ) ; }
add a data callback .
