flutter/examples/api/lib/material/app_bar/sliver_app_bar.4.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'package:flutter/material.dart';

/// Flutter code sample for [SliverAppBar].

void main() {
  runApp(const StretchableSliverAppBar());
}

class StretchableSliverAppBar extends StatefulWidget {
  const StretchableSliverAppBar({super.key});

  @override
  State<StretchableSliverAppBar> createState() =>
      _StretchableSliverAppBarState();
}

class _StretchableSliverAppBarState extends State<StretchableSliverAppBar> {
  bool _stretch = true;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      body: CustomScrollView(
        physics: const BouncingScrollPhysics(),
        slivers: <Widget>[
          SliverAppBar(
            stretch: _stretch,
            onStretchTrigger: () async {
              // Triggers when stretching
            },
            // [stretchTriggerOffset] describes the amount of overscroll that must occur
            // to trigger [onStretchTrigger]
            //
            // Setting [stretchTriggerOffset] to a value of 300.0 will trigger
            // [onStretchTrigger] when the user has overscrolled by 300.0 pixels.
            stretchTriggerOffset: 300.0,
            expandedHeight: 200.0,
            flexibleSpace: const FlexibleSpaceBar(
              title: Text('SliverAppBar'),
              background: FlutterLogo(),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return Container(
                  color: index.isOdd ? Colors.white : Colors.black12,
                  height: 100.0,
                  child: Center(
                    child: Text('$index', textScaler: const TextScaler.linear(5.0)),
                  ),
                );
              },
              childCount: 20,
            ),
          ),
        ],
      ),
      bottomNavigationBar: BottomAppBar(
        child: Padding(
          padding: const EdgeInsets.all(8),
          child: OverflowBar(
            overflowAlignment: OverflowBarAlignment.center,
            alignment: MainAxisAlignment.center,
            children: <Widget>[
              Row(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  const Text('stretch'),
                  Switch(
                    onChanged: (bool val) {
                      setState(() {
                        _stretch = val;
                      });
                    },
                    value: _stretch,
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    ));
  }
}