Flutter: How to show AppBar only after scrolling? - flutter

Question

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!

Answer 1

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)),
),
);
}
}

Answer 2

Heres the code i edit from #Gourango Sutradhar answer, if you want the top to disappear only when it reach the top
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;
_prevReverseOffset = _fromTop;
}
var difference = offset - _prevOffset;
if (offset > 100.0) {
_prevOffset = offset;
}
if (offset < 100.0) {
_fromTop = _prevReverseOffset + difference;
if (_fromTop < -_containerHeight) _fromTop = -_containerHeight;
}
}
setState(() {});
}
#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,
child: Container(
height: _containerHeight,
color: Colors.red,
alignment: Alignment.center,
child: Text("Your Container", style: TextStyle(fontSize: 32, fontWeight: FontWeight.bold, color: Colors.white)),
),
);
}
}