/// create a new list of data
final items = list<int>.generate(40, (index) => index);

/// when the reorder completes remove the list entry from its old position
/// and insert it at its new index
void _onreorder(int oldindex, int newindex) {
setstate(() {
    final item = items.removeat(oldindex);
    items.insert(newindex, item);
});
}

@override
widget build(buildcontext context) {
return materialapp(
    home: scaffold(
      body: reorderablegridview.extent(
        maxcrossaxisextent: 250,
        onreorder: _onreorder,
        childaspectratio: 1,
        children: items.map((item) {
          /// map every list entry to a widget and assure every child has a 
          /// unique key
          return card(
            key: valuekey(item),
            child: center(
            child: text(item.tostring()),
            ),
          );
        }).tolist(),
      ),
    ),
);
}