main.dart

import 'package:flutter/material.dart';
import 'package:sliver_appbar_example/sliver_items.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: [
          SliverPersistentHeader(
            pinned: true,
            delegate: const MyHeaderDelegate(),
          ),
          const SliverItems(),
        ],
      ),
    );
  }
}

class MyHeaderDelegate extends SliverPersistentHeaderDelegate {
  const MyHeaderDelegate();

  @override
  Widget build(
    BuildContext context,
    double shrinkOffset,
    bool overlapsContent,
  ) {
    final progress = shrinkOffset / maxExtent;

    return Material(
      child: Stack(
        fit: StackFit.expand,
        children: [
          AnimatedOpacity(
            duration: const Duration(milliseconds: 150),
            opacity: progress,
            child: ColoredBox(
              color: const Color(0xBE7A81FF),
            ),
          ),
          AnimatedOpacity(
            duration: const Duration(milliseconds: 150),
            opacity: 1 - progress,
            child: Image.asset(
              'assets/images/mountains.jpg',
              fit: BoxFit.cover,
            ),
          ),
          AnimatedContainer(
            duration: const Duration(milliseconds: 100),
            padding: EdgeInsets.lerp(
              EdgeInsets.symmetric(horizontal: 16, vertical: 16),
              EdgeInsets.only(bottom: 16),
              progress,
            ),
            alignment: Alignment.lerp(
              Alignment.bottomLeft,
              Alignment.bottomCenter,
              progress,
            ),
            child: Text(
              'Mountains',
              style: TextStyle.lerp(
                Theme.of(context)
                    .textTheme
                    .headline4
                    .copyWith(color: Colors.white),
                Theme.of(context)
                    .textTheme
                    .headline6
                    .copyWith(color: Colors.white),
                progress,
              ),
            ),
          ),
          AppBar(
            backgroundColor: Colors.transparent,
            leading: BackButton(),
            actions: [
              IconButton(
                icon: const Icon(Icons.search),
                onPressed: () {},
              ),
              IconButton(
                icon: const Icon(Icons.more_vert),
                onPressed: () {},
              ),
            ],
            elevation: 0,
          ),
        ],
      ),
    );
  }

  @override
  double get maxExtent => 264;

  @override
  double get minExtent => 84;

  @override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) =>
      true;
}