transition_appbar.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
        appBar: AppBar(
          // Here we take the value from the MyHomePage object that was created by
          // the App.build method, and use it to set our appbar title.
          title: Text(widget.title),
        ),
        body: CustomScrollView(
          slivers: <Widget>[
            SliverPersistentHeader(
              pinned: true,
              delegate: CollapsedAppBar(),
            ),
            SliverFixedExtentList(
              itemExtent: 48,
              delegate: SliverChildBuilderDelegate((context, i) {
                return ListTile(
                  title: Text("Item $i"),
                );
              }),
            )
          ],
        ) // This trailing comma makes auto-formatting nicer for build methods.
        );
  }
}

class CollapsedAppBar extends SliverPersistentHeaderDelegate {
  double _maxHeight = 200.0;
  final _searchBoxTween = SizeTween(begin: Size(300, 56), end: Size(0, 56));
  final _menuAlignTween = AlignmentTween(begin: Alignment.bottomLeft, end: Alignment(-0.4, -0.7));
  final _menuActionSize = Tween(begin: 24.0, end: 24.0);
  final _menuActionOpacityTween = Tween(begin: 1.0, end: 0.0);
  final _menuActionColorTween = ColorTween(begin: Colors.indigo[900], end: Colors.white70);

  @override
  // TODO: implement maxExtent
  double get maxExtent => _maxHeight;

  @override
  // TODO: implement minExtent
  double get minExtent => 100.0;

  @override
  bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) {
    // TODO: implement shouldRebuild
    return true;
  }

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
    double screenWidth = MediaQuery.of(context).size.width;
    double progress = shrinkOffset / _maxHeight;
    final searchBoxSize = _searchBoxTween.lerp(progress);
    final menuAlign = _menuAlignTween.lerp(progress);
    final menuActionSize = _menuActionSize.lerp(progress);
    final menuActionOpacity = _menuActionOpacityTween.lerp(progress);
    final menuActionColor = _menuActionColorTween.lerp(progress);
    final menuActionBoxWidth = Tween(begin: screenWidth, end: screenWidth * 0.8).lerp(progress);

    return Stack(
      fit: StackFit.expand,
      children: <Widget>[
        Container(
          color: Colors.indigo,
        ),
        Positioned(
          top: 8,
          left: 8,
          width: searchBoxSize.width,
          child: SizedBox.fromSize(
            size: searchBoxSize,
            child: Container(
              child: TextField(
                decoration: InputDecoration(
                    filled: true,
                    fillColor: Colors.indigo[800],
                    prefixIcon: Icon(Icons.search, color: Colors.white70)),
              ),
            ),
          ),
        ),
        Align(
          alignment: menuAlign,
          child: Container(
            width: menuActionBoxWidth,
            margin: EdgeInsets.only(bottom: 16),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceAround,
              children: <Widget>[
                Container(
                    padding: EdgeInsets.all(16.0),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8.0),
                      color: Colors.white.withOpacity(menuActionOpacity),
                    ),
                    child: Icon(
                      Icons.map,
                      color: menuActionColor,
                      size: menuActionSize.toDouble(),
                    )),
                Container(
                    padding: EdgeInsets.all(16.0),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8.0),
                      color: Colors.white.withOpacity(menuActionOpacity),
                    ),
                    child: Icon(
                      Icons.play_circle_filled,
                      color: menuActionColor,
                      size: menuActionSize.toDouble(),
                    )),
                Container(
                    padding: EdgeInsets.all(16.0),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8.0),
                      color: Colors.white.withOpacity(menuActionOpacity),
                    ),
                    child: Icon(
                      Icons.business,
                      color: menuActionColor,
                      size: menuActionSize.toDouble(),
                    )),
                Container(
                    padding: EdgeInsets.all(16.0),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(8.0),
                      color: Colors.white.withOpacity(menuActionOpacity),
                    ),
                    child: Icon(
                      Icons.opacity,
                      color: menuActionColor,
                      size: menuActionSize.toDouble(),
                    )),

              ],
            ),
          ),
        )
      ],
    );
  }
}