setupParentData method
Sets up the parent data for each child sliver.
If the child's parent data is not an instance of _SliverRowParentData,
it assigns a new _SliverRowParentData instance.
Implementation
@override
void setupParentData(RenderSliver child) {
if (child.parentData is! _SliverRowParentData) {
child.parentData = _SliverRowParentData();
}
}