Flutter:How do I set the SliverAppBar collapsed by default - flutter

Question

I'm new guy using the flutter and when I code my app I met a problem that how could I set SliverAppBar collapsed status by default. To solve my confuse, I check the flutter's doc, unfortunately I couldn't find the solution, so I'm here.
Below is my code, if you know how to solve it, help me plz. Thanks guys.
class CollapsingBarLayoutState extends State<CollapsingBarLayout>
with TickerProviderStateMixin {
final List<ListItem> listData = [];
#override
Widget build(BuildContext context) {
for (int i = 0; i < 20; i++) {
listData.add(new ListItem("$i", Icons.cake));
}
return Scaffold(
body: NestedScrollView(
headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
return <Widget>[
SliverAppBar(
expandedHeight: 200.0,
pinned: true,
centerTitle: false,
snap: true,
floating: true,
titleSpacing: 0,
backgroundColor: Colors.green,
flexibleSpace: FlexibleSpaceBar(
centerTitle: true,
title: Center(
child: Row(
mainAxisAlignment: MainAxisAlignment.center,
children: _buildBriefCalendarChildren(),
),
),
background: GridPage(
children: _buildCalendarPageChildren(),
column: 7,
row: 5,
)),
),
];
},
body: Center(
child: new ListView.builder(
itemBuilder: (BuildContext context, int index) {
return new ListItemWidget(listData[index]);
},
itemCount: listData.length,
),
),
),
);
}
}

Answer 1

There is a good plugin https://pub.dev/packages/after_layout I used in a situation like this.
Just add a mixin and then the only method you would need to implement would look something like this:
void afterFirstLayout(BuildContext context) {
_scrollController //scroll controller of your list view under sliver
.animateTo((expandedHeight) - collapsedHeight,
duration: new Duration(milliseconds: 1),//0 dont work as duration
curve: Curves.linear)
.then((_) {
setState(() {
expandedPercentage = 0; //just my custom tracking of expansion percentage
});
});
}
Edit - better solution:
Initialise a new ScrollController(initialScrollOffset: expandedHeight - collapsedHeight); and pass it as a controller to your NestedScrollView.