如何通过向下滚动listView来删除FloatingActionButton？

Question

我在扑朔迷离中制作了一个应用程序。屏幕上有SliverAppBar，LiatView和FloatingActionButton。在下方有FloatingActionButton SliverAppBar。滚动ListView之后，SliverAppBar卷曲并且FloatingActionButton消失，但是向上滚动时应该会出现。 我该怎么办？

我的代码：

class AccountScreen extends StatefulWidget {
  @override
  AccountScreenState createState() => AccountScreenState();
}

class AccountScreenState extends State<AccountScreen> {
  double appBarHeight = 190.0;

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Scaffold(
          resizeToAvoidBottomPadding: false,
          backgroundColor: Color(0xff424242),
          body: DefaultTabController(
            length: 2,
            child: NestedScrollView(
                headerSliverBuilder:
                    (BuildContext context, bool innerBoxIsScrolled) {
                  return <Widget>[
                    SliverAppBar(
                      backgroundColor: Colors.grey[900].withOpacity(0.89),
                      expandedHeight: 200.0,
                      floating: false,
                      pinned: true,
                      flexibleSpace: FlexibleSpaceBar(
                          centerTitle: true,
                          title: Column(
                            mainAxisSize: MainAxisSize.min,
                            children: <Widget>[
                              const Text("Batmen",
                                  style: TextStyle(color: Color(0xfffafafa))),
                              const Text("Poland, qwer",
                                  style: TextStyle(
                                      color: Color(0xfffafafa),
                                      fontSize: 10.0)),
                            ],
                          ),
                          background: Image.asset(
                            "images/batmen.jpg",
                            fit: BoxFit.cover,
                          )),
                    )
                  ];
                },
                body: SingleChildScrollView(
                    child: ConstrainedBox(
                        constraints: BoxConstraints(),
                        child: Column(children: <Widget>[
                          ListTile(
                              title: Text('Driving information',
                                  style: TextStyle(
                                      color: Color(0xfffafafa),
                                      fontSize: 20.0))),
                                      ........
                                ],
                              )),
                        ])))),
          ),
        ),
        Positioned(
          child: new FloatingActionButton(
            child: Icon(
              Icons.create,
              color: Color(0xfffafafa),
            ),
            backgroundColor: Color(0xff31803f),
            onPressed: () {
              Navigator.push(context,
                  MaterialPageRoute(builder: (context) => RegisterForm()));
            },
          ),
          right: 10.0,
          top: appBarHeight - 5.0,
        )
      ],
    );
  }
}