How to Implement Pull To Refresh in Flutter?

class PullToRerfresh extends StatefulWidget {
  @override
  _PullToRerfreshState createState() => _PullToRerfreshState();
}

class _PullToRerfreshState extends State {
   List list;
  var random;

  var refreshKey = GlobalKey();

  @override
  void initState() {
    list = List.generate(3, (i) => "Item $i");
    super.initState();
    random = Random();
    refreshList();
  }

  Future refreshList() async {
    refreshKey.currentState?.show(atTop: false);
    await Future.delayed(Duration(seconds: 2));

    setState(() {

      list.addAll( List.generate(random.nextInt(10), (i) => "Item $i"));
    });

    return null;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Pull to refresh"),
      ),
      body: RefreshIndicator(
        key: refreshKey,
        child: ListView.builder(
          itemCount: list?.length,
          itemBuilder: (context, i) => ListTile(
            title: Text(list[i]),
          ),
        ),
        onRefresh: refreshList,
      ),
    );
  }
}

class CupertinoRefresh extends StatefulWidget{
  @override
  State createState() {
    // TODO: implement createState
    return _PullToRefreshDemoState();
  }

}
class _PullToRefreshDemoState
    extends State {
  static const listCount = 20;
  var randomList = List.generate(listCount, (i) => i + 1);

  void _shuffleList() => randomList.shuffle(Random());

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: CustomScrollView(

          physics: const BouncingScrollPhysics(
            parent: AlwaysScrollableScrollPhysics(),
          ),
          slivers: [
            CupertinoSliverNavigationBar(
              automaticallyImplyLeading: false,
              backgroundColor: Colors.blue,
              leading: InkWell(child: Icon(Icons.arrow_back,color: Colors.white,),onTap: (){
                Navigator.pop(context);
              },),

              largeTitle: Text(
                "Pull To Refresh",
                style: TextStyle(color: Colors.white),
              ),
            ),
            CupertinoSliverRefreshControl(
              onRefresh: () {
                return Future.delayed(const Duration(seconds: 1))
                  ..then((_) {
                    if (mounted) {
                      setState(() => _shuffleList());
                    }
                  });
              },
            ),
            SliverList(
              delegate: SliverChildBuilderDelegate(
                    (context, index) {
                  final String title ="Item ${randomList[index]}";
                  return ListTile(title: Text(title));
                },
                childCount: listCount,
              ),
            ),
          ],
        ),
      ),
    );
  }
}