1. override
void debugFillDescription(List<String> description)

Accumulates a list of strings describing the current node's fields, one field per string. Subclasses should override this to have their information included in toStringDeep.

Source 

@override
void debugFillDescription(List<String> description) {
  super.debugFillDescription(description);
  description.add('widthFactor: ${_widthFactor ?? "expand"}');
  description.add('heightFactor: ${_heightFactor ?? "expand"}');
}