一気にpopしたときのdisposeの順番が不定なので後ろの方に積まれた画面でdispose済みのListenableにアクセスしてしまってエラーになることがある問題

main.dart

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

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Home')),
      body: Center(
        child: RaisedButton(
          child: Text('→A'),
          onPressed: () {
            Navigator.of(context)
                .push(MaterialPageRoute(builder: (context) => A()));
          },
        ),
      ),
    );
  }
}

class A extends StatefulWidget {
  @override
  _AState createState() => _AState();
}

class _AState extends State<A> {
  final _value = ValueNotifier<int>(0);

  @override
  void initState() {
    super.initState();
    print('A:initState');
  }

  @override
  void dispose() {
    print('A:dispose');
    _value.dispose();
    // This workaround fixes the problem
//    Future.delayed(Duration(milliseconds: 50)).then((_) {
//      _value.dispose();
//    });
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('A')),
      body: Center(
        child: RaisedButton(
          child: Text('→B'),
          onPressed: () {
            Navigator.of(context).push(
              MaterialPageRoute(
                builder: (context) => B(
                  value: _value,
                ),
              ),
            );
          },
        ),
      ),
    );
  }
}

class B extends StatefulWidget {
  const B({
    Key key,
    @required this.value,
  }) : super(key: key);

  @override
  _BState createState() => _BState();

  final ValueListenable<int> value;
}

class _BState extends State<B> {
  @override
  void initState() {
    super.initState();
    print('B:initState');
  }

  @override
  void dispose() {
    print('B:dispose');
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('B')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            ValueListenableBuilder(
              valueListenable: widget.value,
              builder: (context, value, child) {
                return Text(value.toString());
              },
            ),
            RaisedButton(
              child: Text('popToRoot'),
              onPressed: () {
                Navigator.of(context).popUntil((r) => r.isFirst);
              },
            ),
          ],
        ),
      ),
    );
  }
}