SliverAppBarのVariation1

main.dart

// https://stackoverflow.com/questions/50909791/flutter-change-text-when-flexiblespacebar-is-collapsed/63407889#63407889

import 'dart:math' as math;

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: SafeArea(
          child: MyHomePage(),
        ),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  ScrollController controller = ScrollController();
  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      physics: ClampingScrollPhysics(),
      controller: controller,
      slivers: [
        SliverAppBar(
          expandedHeight: 220.0,
          floating: true,
          pinned: true,
          elevation: 50,
          backgroundColor: Colors.pink,
          leading: IconButton(
            icon: Icon(Icons.menu),
            onPressed: () {},
          ),
          flexibleSpace: _MyAppSpace(),
        ),
        SliverList(
          delegate: SliverChildListDelegate(
            List.generate(
              200,
              (index) => Card(
                child: Padding(
                  padding: EdgeInsets.all(10),
                  child: Text('text $index'),
                ),
              ),
            ),
          ),
        )
      ],
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, c) {
        final settings = context
            .dependOnInheritedWidgetOfExactType<FlexibleSpaceBarSettings>();
        final deltaExtent = settings.maxExtent - settings.minExtent;
        final t =
            (1.0 - (settings.currentExtent - settings.minExtent) / deltaExtent)
                .clamp(0.0, 1.0) as double;
        final fadeStart = math.max(0.0, 1.0 - kToolbarHeight / deltaExtent);
        const fadeEnd = 1.0;
        final opacity = 1.0 - Interval(fadeStart, fadeEnd).transform(t);

        return Stack(
          children: [
            Center(
              child: Opacity(
                  opacity: 1 - opacity,
                  child: getTitle(
                    'Collapsed Title',
                  )),
            ),
            Opacity(
              opacity: opacity,
              child: Stack(
                alignment: Alignment.bottomCenter,
                children: [
                  getImage(),
                  getTitle(
                    'Expended Title',
                  )
                ],
              ),
            ),
          ],
        );
      },
    );
  }

  Widget getImage() {
    return Container(
      width: double.infinity,
      child: Image.network(
        'https://source.unsplash.com/daily?code',
        fit: BoxFit.cover,
      ),
    );
  }

  Widget getTitle(String text) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Text(
        text,
        textAlign: TextAlign.center,
        style: TextStyle(
          color: Colors.white,
          fontSize: 26.0,
          fontWeight: FontWeight.bold,
        ),
      ),
    );
  }
}