Flutter: How to show AppBar only after scrolling?

That's what you read. I don't want to hide AppBar when scrolling, there's a lot of info on that.

What I want is the exact opposite. I want my homepage to open with no AppBar and then, when the user starts scrolling, the appbar will be displayed.

This website does exactly what I want to reproduce: https://www.kirschnerbrasil.cc/ (in the desktop version).

I guess I need do use the SliverAppBar, but I haven't manage to do so yet. Can anyone help?

Thanks!

In that case, you will have to make your custom widget the appbar. Please have a look into below code, it will help you to understand the procedure:

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  // Height of your Container
  static final _containerHeight = 100.0;

  // You don't need to change any of these variables
  var _fromTop = -_containerHeight;
  var _controller = ScrollController();
  var _allowReverse = true, _allowForward = true;
  var _prevOffset = 0.0;
  var _prevForwardOffset = -_containerHeight;
  var _prevReverseOffset = 0.0;

  @override
  void initState() {
    super.initState();
    _controller.addListener(_listener);
  }

  // entire logic is inside this listener for ListView
  void _listener() {
    double offset = _controller.offset;
    var direction = _controller.position.userScrollDirection;

    if (direction == ScrollDirection.reverse) {
      _allowForward = true;
      if (_allowReverse) {
        _allowReverse = false;
        _prevOffset = offset;
        _prevForwardOffset = _fromTop;
      }

      var difference = offset - _prevOffset;
      _fromTop = _prevForwardOffset + difference;
      if (_fromTop > 0) _fromTop = 0;
    } else if (direction == ScrollDirection.forward) {
      _allowReverse = true;
      if (_allowForward) {
        _allowForward = false;
        _prevOffset = offset;
        _prevReverseOffset = _fromTop;
      }

      var difference = offset - _prevOffset;
      _fromTop = _prevReverseOffset + difference;
      if (_fromTop < -_containerHeight) _fromTop = -_containerHeight;
    }
    setState(() {}); // for simplicity I'm calling setState here, you can put bool values to only call setState when there is a genuine change in _fromTop
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("ListView")),
      body: Stack(
        children: <Widget>[
          _yourListView(),
          Positioned(
            top: _fromTop,
            left: 0,
            right: 0,
            child: _yourContainer(),
          )
        ],
      ),
    );
  }

  Widget _yourListView() {
    return ListView.builder(
      itemCount: 100,
      controller: _controller,
      itemBuilder: (_, index) => ListTile(title: Text("Item $index")),
    );
  }

  Widget _yourContainer() {
    return Opacity(
      opacity: 1 - (-_fromTop / _containerHeight),
      child: Container(
        height: _containerHeight,
        color: Colors.red,
        alignment: Alignment.center,
        child: Text("Your Container", style: TextStyle(fontSize: 32, fontWeight: FontWeight.bold, color: Colors.white)),
      ),
    );
  }
}