1. @override
Widget buildChildLayout(BuildContext context)

Subclasses should override this method to build the layout model.

Source

@override
Widget buildChildLayout(BuildContext context) {
  if (itemExtent != null) {
    return new SliverFixedExtentList(
      delegate: childrenDelegate,
      itemExtent: itemExtent,
    );
  }
  return new SliverList(delegate: childrenDelegate);
}