1. override
void detach()

Mark this node as detached.

Typically called only from the parent's detach, and by the owner to mark the root of a tree as detached.

Subclasses with children should detach all their children whenever this method is called.

Source 

@override
void detach() {
  super.detach();
  ChildType child = _firstChild;
  while (child != null) {
    child.detach();
    final ParentDataType childParentData = child.parentData;
    child = childParentData.nextSibling;
  }
}