import 'package:flutter/material.dart';
 
void main() => runApp(const MyApp());
 
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
 
  static const String _title = 'Flutter Code Sample';
 
  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}
 
class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);
 
  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}
 
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  bool _pinned = true;
  bool _snap = false;
  bool _floating = false;
 
// [SliverAppBar]s are typically used in [CustomScrollView.slivers], which in
// turn can be placed in a [Scaffold.body].
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            pinned: _pinned,
            snap: _snap,
            floating: _floating,
            expandedHeight: 160.0,
            flexibleSpace: const FlexibleSpaceBar(
              title: Text('SliverAppBar'),
              background: FlutterLogo(),
            ),
          ),
          const SliverToBoxAdapter(
            child: SizedBox(
              height: 20,
              child: Center(
                child: Text('Scroll to see the SliverAppBar in effect.'),
              ),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return Container(
                  color: index.isOdd ? Colors.white : Colors.black12,
                  height: 100.0,
                  child: Center(
                    child: Text('$index', textScaleFactor: 5),
                  ),
                );
              },
              childCount: 20,
            ),
          ),
        ],
      ),
      bottomNavigationBar: BottomAppBar(
        child: Padding(
          padding: const EdgeInsets.all(8),
          child: OverflowBar(
            overflowAlignment: OverflowBarAlignment.center,
            children: <Widget>[
              Row(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const Text('pinned'),
                  Switch(
                    onChanged: (bool val) {
                      setState(() {
                        _pinned = val;
                      });
                    },
                    value: _pinned,
                  ),
                ],
              ),
              Row(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const Text('snap'),
                  Switch(
                    onChanged: (bool val) {
                      setState(() {
                        _snap = val;
                        // Snapping only applies when the app bar is floating.
                        _floating = _floating || _snap;
                      });
                    },
                    value: _snap,
                  ),
                ],
              ),
              Row(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const Text('floating'),
                  Switch(
                    onChanged: (bool val) {
                      setState(() {
                        _floating = val;
                        _snap = _snap && _floating;
                      });
                    },
                    value: _floating,
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}