[animations package] weird State object instantiations when using PageTransitionSwitcher and FooTransition widgets included in package. #53550

import 'package:flutter/material.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: HomePage(),
    );
  }
}

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

class _HomePageState extends State<HomePage> {
  bool _isCounterPage = true;

  @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() : StatefulPage(),
      ),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.sync),
          onPressed: () {
            setState(() {
              _isCounterPage = !_isCounterPage;
            });
          }),
    );
  }
}

class CounterPage extends StatefulWidget {
  @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 {
  @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 = '';
              });
            },
          )
        ],
      ),
    );
  }
}

I/flutter (20212): CounterPage initState called.

I/flutter (20212): CounterPage initState called.
I/flutter (20212): StatefulPage initState called.
I/flutter (20212): CounterPage dispose called.
I/flutter (20212): StatefulPage initState called.
I/flutter (20212): StatefulPage dispose called.
I/flutter (20212): CounterPage dispose called.

[√] Flutter (Channel beta, v1.15.17, on Microsoft Windows [Version 10.0.18362.535], locale en-PH)
    • Flutter version 1.15.17 at C:\src\flutter
    • Framework revision 2294d75bfa (3 weeks ago), 2020-03-07 00:28:38 +0900
    • Engine revision 5aff311948
    • Dart version 2.8.0 (build 2.8.0-dev.12.0 9983424a3c)


[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at C:\Users\Jerome\AppData\Local\Android\sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 3.6)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 44.0.2
    • Dart plugin version 192.7761
    • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)

[√] VS Code (version 1.42.1)
    • VS Code at C:\Users\Jerome\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.8.1

[√] Connected device (3 available)
    • AOSP on IA Emulator • emulator-5554 • android-x86    • Android 9 (API 28) (emulator)
    • Chrome              • chrome        • web-javascript • Google Chrome 80.0.3987.149
    • Web Server          • web-server    • web-javascript • Flutter Tools

• No issues found!