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

Add additional information to the given description for use by toStringDeep.

Source 

@override
void debugFillDescription(List<String> description) {
  super.debugFillDescription(description);
  description.add('shader: $shader');
  description.add('maskRect: $maskRect');
  description.add('transferMode: $transferMode');
}