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);
  List<String> listeners = <String>[];
  if (onPointerDown != null)
    listeners.add('down');
  if (onPointerMove != null)
    listeners.add('move');
  if (onPointerUp != null)
    listeners.add('up');
  if (onPointerCancel != null)
    listeners.add('cancel');
  if (listeners.isEmpty)
    listeners.add('<none>');
  description.add('listeners: ${listeners.join(", ")}');
}