indexOf method
Returns the index of the given child, as given by the MultiSliverMultiBoxAdaptorParentData.index field of the child's parentData.
Implementation
int indexOf(RenderBox child) {
final childParentData =
child.parentData! as MultiSliverMultiBoxAdaptorParentData;
assert(childParentData.index != null);
return childParentData.index!;
}