Flutter docs are very confusing, what's the use of SliverOverlapAbsorber and SliverOverlapInjector? - flutter

Question

I am using this example from flutter official docs. Here is the minimal code:
List<String> _tabs = ["Tab1", "Tab2"];
#override
Widget build(BuildContext context) {
return DefaultTabController(
length: _tabs.length,
child: Scaffold(
body: NestedScrollView(
headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
return <Widget>[
// 1/2 remove this widget and only use SliverAppBar
SliverOverlapAbsorber(
handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
child: SliverAppBar(
title: const Text('Books'),
pinned: true,
expandedHeight: 150.0,
bottom: TabBar(tabs: _tabs.map((name) => Tab(text: name)).toList()),
),
),
];
},
body: TabBarView(
children: _tabs.map((String name) {
return Builder(
builder: (BuildContext context) {
return CustomScrollView(
key: PageStorageKey<String>(name),
slivers: <Widget>[
// 2/2 remove this widget
SliverOverlapInjector(handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context)),
SliverFixedExtentList(
itemExtent: 48.0,
delegate: SliverChildBuilderDelegate(
(_, i) => ListTile(title: Text('Item $i')),
childCount: 30,
),
)
],
);
},
);
}).toList(),
),
),
),
);
}
Problem:
As you can see if I remove SliverOverlapAbsorber along with SliverOverlapInjector in above code, I don't see any change in the output, what's the use of having it? Docs say
SliverOverlapInjector: This widget takes the overlapping behavior of the SliverAppBar, and redirects it to the SliverOverlapInjector below. If it is missing, then it is possible for the nested "inner" scroll view below to end up under the SliverAppBar even when the inner scroll view thinks it has not been scrolled. This is not necessary if the "headerSliverBuilder" only builds widgets that do not overlap the next sliver.
Can anyone explain what does it mean and what's the use of having SliverOverlapInjector and SliverOverlapInjector

Answer 1

I use it in NestedScrollView
NestedScrollView(
headerSliverBuilder: (context, innerBoxIsScrolled) => [
SliverOverlapAbsorber(
handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
sliver: SliverAppBar()
],
body: ...
)
Without it everything in your body will go under appbar.
Check out https://api.flutter.dev/flutter/widgets/NestedScrollView-class.html
It's said there
SliverOverlapAbsorber(
// This widget takes the overlapping behavior of the SliverAppBar,
// and redirects it to the SliverOverlapInjector below. If it is
// missing, then it is possible for the nested "inner" scroll view
// below to end up under the SliverAppBar even when the inner
// scroll view thinks it has not been scrolled.
// This is not necessary if the "headerSliverBuilder" only builds
// widgets that do not overlap the next sliver.
handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),