animations package, state of child transitioning fixed using GlobalKeys

main.dart

import 'package:flutter/material.dart';
import 'package:animations/animations.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(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: HomePage(),
    );
  }
}

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

class _HomePageState extends State<HomePage> {
  bool _isCounterPage = true;
  final GlobalKey _counterKey = GlobalKey();
  final GlobalKey _statefulKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home Page'),
      ),
      body: PageTransitionSwitcher(
        duration: const Duration(milliseconds: 500),
        transitionBuilder: (child, animation, secondaryAnimation) {
          return FadeThroughTransition(
            child: child,
            animation: animation,
            secondaryAnimation: secondaryAnimation,
          );
        },
        child: _isCounterPage
            ? CounterPage(
                key: _counterKey,
              )
            : StatefulPage(
                key: _statefulKey,
              ),
      ),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.sync),
          onPressed: () {
            setState(() {
              _isCounterPage = !_isCounterPage;
            });
          }),
    );
  }
}

class CounterPage extends StatefulWidget {
  const CounterPage({Key key}) : super(key: key);

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

class _CounterPageState extends State<CounterPage> {
  int _counter = 0;

  @override
  void initState() {
    super.initState();
    print('CounterPage initState called.');
  }

  @override
  void dispose() {
    print('CounterPage dispose called.');
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Text('Counter value: $_counter'),
          RaisedButton(
            child: Text('Increment'),
            onPressed: () {
              setState(() {
                _counter++;
              });
            },
          ),
        ],
      ),
    );
  }
}

class StatefulPage extends StatefulWidget {
  const StatefulPage({Key key}) : super(key: key);

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

class _StatefulPageState extends State<StatefulPage> {
  String _text;
  TextEditingController _controller;

  @override
  void initState() {
    super.initState();
    _text = '';
    _controller = TextEditingController();
    print('StatefulPage initState called.');
  }

  @override
  void dispose() {
    print('StatefulPage dispose called.');
    _controller?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Text('Saved text: $_text'),
          TextField(
            controller: _controller,
          ),
          RaisedButton(
            child: Text('Save text'),
            onPressed: () {
              setState(() {
                _text = _controller.text;
                _controller.text = '';
              });
            },
          )
        ],
      ),
    );
  }
}