navigator_scroll_fade_demo.dart

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

class Destination {
  const Destination(this.index, this.title, this.icon, this.color);
  final int index;
  final String title;
  final IconData icon;
  final MaterialColor color;
}

const List<Destination> allDestinations = <Destination>[
  Destination(0, 'Home', Icons.home, Colors.teal),
  Destination(1, 'Business', Icons.business, Colors.cyan),
  Destination(2, 'School', Icons.school, Colors.orange),
  Destination(3, 'Flight', Icons.flight, Colors.blue)
];

class RootPage extends StatelessWidget {
  const RootPage({ Key key, this.destination }) : super(key: key);

  final Destination destination;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(destination.title),
        backgroundColor: destination.color,
      ),
      backgroundColor: destination.color[50],
      body: SizedBox.expand(
        child: InkWell(
          onTap: () {
            Navigator.pushNamed(context, "/list");
          },
          child: Center(
            child: Text('tap here'),
          ),
        ),
      ),
    );
  }
}

class ListPage extends StatelessWidget {
  const ListPage({ Key key, this.destination }) : super(key: key);

  final Destination destination;

  @override
  Widget build(BuildContext context) {
    const List<int> shades = <int>[50, 100, 200, 300, 400, 500, 600, 700, 800, 900];

    return Scaffold(
      appBar: AppBar(
        title: Text(destination.title),
        backgroundColor: destination.color,
      ),
      backgroundColor: destination.color[50],
      body: SizedBox.expand(
        child: ListView.builder(
          itemCount: shades.length,
          itemBuilder: (BuildContext context, int index) {
            return SizedBox(
              height: 128,
              child: Card(
                color: destination.color[shades[index]].withOpacity(0.25),
                child: InkWell(
                  onTap: () {
                    Navigator.pushNamed(context, "/text");
                  },
                  child: Center(
                    child: Text('Item $index', style: Theme.of(context).primaryTextTheme.display1),
                  ),
                ),
              ),
            );
          },
        ),
      ),
    );
  }
}

class TextPage extends StatefulWidget {
  const TextPage({ Key key, this.destination }) : super(key: key);

  final Destination destination;

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

class _TextPageState extends State<TextPage> {
  TextEditingController _textController;

  @override
  void initState() {
    super.initState();
    _textController = TextEditingController(
      text: 'sample text: ${widget.destination.title}',
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.destination.title),
        backgroundColor: widget.destination.color,
      ),
      backgroundColor: widget.destination.color[50],
      body: Container(
        padding: const EdgeInsets.all(32.0),
        alignment: Alignment.center,
        child: TextField(controller: _textController),
      ),
    );
  }

  @override
  void dispose() {
    _textController.dispose();
    super.dispose();
  }
}


class ViewNavigatorObserver extends NavigatorObserver {
  ViewNavigatorObserver(this.onNavigation);

  final VoidCallback onNavigation;

  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
    onNavigation();
  }
  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
    onNavigation();
  }
}

class DestinationView extends StatefulWidget {
  const DestinationView({ Key key, this.destination, this.onNavigation }) : super(key: key);

  final Destination destination;
  final VoidCallback onNavigation;

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

class _DestinationViewState extends State<DestinationView> {
  @override
  Widget build(BuildContext context) {
    return Navigator(
      observers: <NavigatorObserver>[
        ViewNavigatorObserver(widget.onNavigation),
      ],
      onGenerateRoute: (RouteSettings settings) {
        return MaterialPageRoute(
          settings: settings,
          builder: (BuildContext context) {
            switch(settings.name) {
              case '/':
                return RootPage(destination: widget.destination);
              case '/list':
                return ListPage(destination: widget.destination);
              case '/text':
                return TextPage(destination: widget.destination);
            }
          },
        );
      },
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with TickerProviderStateMixin<HomePage> {
  List<Key> _destinationKeys;
  List<AnimationController> _faders;
  AnimationController _hide;
  int _currentIndex = 0;

  @override
  void initState() {
    super.initState();

    _faders = allDestinations.map<AnimationController>((Destination destination) {
      return AnimationController(vsync: this, duration: Duration(milliseconds: 200));
    }).toList();
    _faders[_currentIndex].value = 1.0;
    _destinationKeys = List<Key>.generate(allDestinations.length, (int index) => GlobalKey()).toList();
    _hide = AnimationController(vsync: this, duration: kThemeAnimationDuration);
  }

  @override
  void dispose() {
    for (AnimationController controller in _faders)
      controller.dispose();
    _hide.dispose();
    super.dispose();
  }

  bool _handleScrollNotification(ScrollNotification notification) {
    if (notification.depth == 0) {
      if (notification is UserScrollNotification) {
        final UserScrollNotification userScroll = notification;
        switch (userScroll.direction) {
          case ScrollDirection.forward:
            _hide.forward();
            break;
          case ScrollDirection.reverse:
            _hide.reverse();
            break;
          case ScrollDirection.idle:
            break;
        }
      }
    }
    return false;
  }

  @override
  Widget build(BuildContext context) {
    return NotificationListener<ScrollNotification>(
      onNotification: _handleScrollNotification,
      child: Scaffold(
        body: SafeArea(
          top: false,
          child: Stack(
            fit: StackFit.expand,
            children: allDestinations.map((Destination destination) {
              final Widget view = FadeTransition(
                opacity: _faders[destination.index].drive(CurveTween(curve: Curves.fastOutSlowIn)),
                child: KeyedSubtree(
                  key: _destinationKeys[destination.index],
                  child: DestinationView(
                    destination: destination,
                    onNavigation: () {
                      _hide.forward();
                    },
                  ),
                ),
              );
              if (destination.index == _currentIndex) {
                _faders[destination.index].forward();
                return view;
              } else {
                _faders[destination.index].reverse();
                if (_faders[destination.index].isAnimating) {
                  return IgnorePointer(child: view);
                }
                return Offstage(child: view);
              }
            }).toList(),
          ),
        ),
        bottomNavigationBar: ClipRect(
          child: SizeTransition(
            sizeFactor: _hide,
            axisAlignment: -1.0,
            child: BottomNavigationBar(
              currentIndex: _currentIndex,
              onTap: (int index) {
                setState(() {
                  _currentIndex = index;
                });
              },
              items: allDestinations.map((Destination destination) {
                return BottomNavigationBarItem(
                  icon: Icon(destination.icon),
                  backgroundColor: destination.color,
                  title: Text(destination.title)
                );
              }).toList(),
            ),
          ),
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(home: HomePage()));
}