code: void onColumnFunctionChanged(@Observes GroupFunctionChangedEvent event) {
          GroupFunction gf = event.getGroupFunction();
          String newColumnId = lookupConstraints.buildUniqueColumnId(dataSetLookup,
                                                                     gf);
          gf.setColumnId(newColumnId);
          changeEvent.fire(new DataSetLookupChangedEvent(dataSetLookup));
      }
docs: ColumnFunctionEditor's events