public void addErrorCallback(OneSheeldErrorCallback errorCallback){
  if (errorCallback != null && !errorCallbacks.contains(errorCallback))   errorCallbacks.add(errorCallback);
}

public void addErrorCallback ( OneSheeldErrorCallback errorCallback ) { if ( errorCallback != null && ! errorCallbacks . contains ( errorCallback ) ) errorCallbacks . add ( errorCallback ) ; }
add an error callback .
