Flutter Future Builder

class FutureWidget extends StatefulWidget {
  String computeListOfTimestamps(int count) {
    StringBuffer sb = new StringBuffer();

    var random = Random();

    for (int i = 0; i < 2000; i++) {
      sb.writeln("Random Number : ${random.nextInt(2000)}");
      sb.writeln("");
    }
    return sb.toString();
  }

  Future<String> createFutureCalculation(int count) {
    return new Future(() {
      return computeListOfTimestamps(count);
    },);
  }

  @override
  FutureWidgetState createState() => new FutureWidgetState();
}

class FutureWidgetState extends State<FutureWidget> {
  bool _showCalculation = false;
  bool isLoading = false;
  GlobalKey<ScaffoldState>_scaffoldKey=GlobalKey();

  void _onInvokeFuturePressed() {
    setState(() {
      _showCalculation = !_showCalculation;
      isLoading=true;

    });
  }

  @override
  Widget build(BuildContext context) {
    Widget child = _showCalculation
        ? FutureBuilder(
            future: widget.createFutureCalculation(1000),

            builder: (BuildContext context, AsyncSnapshot snapshot) {
              return (snapshot.connectionState == ConnectionState.none &&
                  snapshot.hasData == null)?Container(child: Text("No Data"),):  Expanded(
                  child: SingleChildScrollView(
                      child: (snapshot.data == null)?Center(child: CircularProgressIndicator()):Text(
                          '${snapshot.data == null ? "" : snapshot.data}',
                          style: TextStyle(fontSize: 20.0))));
            })
        : Text('Press Refresh to load Data');
    return new Scaffold(
      key: _scaffoldKey,
      appBar: new AppBar(
        backgroundColor: Colors.pink,
        title: new Text("Future Builder "),
      ),
      body: new Center(
          child: new Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[child])),
      floatingActionButton: new FloatingActionButton(
        backgroundColor: Colors.pink,
        onPressed: _onInvokeFuturePressed,
        tooltip: 'Call Future',
        child: new Icon(Icons.refresh),
      ), // This trailing comma makes auto-formatting nicer for build
    );
  }
}