Always up button on scroll view

scrolly_button.dart

//Hello Gonçalo!
class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  ValueNotifier<double> _notifier;
  ScrollController _controller;
  double _previousOffset;

  @override
  void dispose() {
    _notifier?.dispose();
    super.dispose();
  }

  @override
  void initState() {
    _notifier = ValueNotifier<double>(0);
    _controller = ScrollController()..addListener(_onScroll);
    _previousOffset = 0;
    super.initState();
  }

  void _onScroll(){
    if (_controller.offset == 0 || _controller.offset < _previousOffset) {
      _notifier.value = -(_controller.position.maxScrollExtent - _controller.offset);
    }else{
      _notifier.value = 0;
    }
    _previousOffset = _controller.offset;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Potato"),),
      body: SingleChildScrollView(
        controller: _controller,
        child: Column(
          children: <Widget>[
            Text("potato " * 900),
            ValueListenableBuilder(
              valueListenable: _notifier,
              child: RaisedButton(
                child: Text("POTATO!"),
                onPressed: () {},
                color: Theme.of(context).accentColor,
              ),
              builder: (context, offset, child) {
                return Transform.translate(
                  offset: Offset(0, offset),
                  child: child,
                );

              },
            )
          ],
        ),
      ),
    );
  }
}