floating.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:math' as math;

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: CustomScrollView(
        slivers: <Widget>[
//           SliverAppBar(
//             floating: true,
//             snap: true,
//             titleSpacing: 0.0,
//             title: Container(
//               color: Colors.blue,
//             ),
//             elevation: 0,
//             backgroundColor: Theme.of(context).scaffoldBackgroundColor,
//           ),
          SliverPersistentHeader(
            floating: true,
            delegate: SliverAppBarDelegate(
              minHeight: 60,
              maxHeight: 60,
              child: Container(
                color: Colors.red,
                child: Center(
                  child: Text(
                      'I want this to appear only after some scroll offset occured'),
                ),
              ),
            ),
          ),
          SliverPersistentHeader(
            pinned: true,
            delegate: SliverAppBarDelegate(
              minHeight: 60.0,
              maxHeight: 60.0,
              child: Container(
                color: Theme.of(context).scaffoldBackgroundColor,
                child: Column(
                  children: <Widget>[
                    Padding(
                      padding: const EdgeInsets.symmetric(
                        horizontal: 16.0,
                        vertical: 8.0,
                      ),
                      child: Text('Some large text',
                          style: TextStyle(fontSize: 20)),
                    ),
                    Divider(),
                  ],
                ),
              ),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate((context, index) {
              return ListTile(
                title: Text('List Tile $index'),
              );
            }, childCount: 100),
          ),
        ],
      ),
    );
  }
}

class SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  SliverAppBarDelegate({
    @required this.minHeight,
    @required this.maxHeight,
    @required this.child,
  });
  final double minHeight;
  final double maxHeight;
  final Widget child;

  @override
  double get minExtent => minHeight;
  @override
  double get maxExtent => math.max(maxHeight, minHeight);
  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return SizedBox.expand(child: child);
  }

  @override
  bool shouldRebuild(SliverAppBarDelegate oldDelegate) {
    return maxHeight != oldDelegate.maxHeight ||
        minHeight != oldDelegate.minHeight ||
        child != oldDelegate.child;
  }
}