sortByKeys method
Passes a multi-column sort on, highest priority first.
The default forwards the primary key to sortBy, so a source written before multi-column sorting existed keeps working and simply sorts by the first column. Override it to honour the rest.
Implementation
void sortByKeys(List<FitGridSortKey> keys) => keys.isEmpty
? sortBy(null, FitGridSortDirection.none)
: sortBy(keys.first.columnId, keys.first.direction);