【Flutter】通知を制御！ScaffoldMessengerとScaffoldMessengerState

ScaffoldMessenger.of(context).showSnackBar(
  SnackBar(content: Text('アクションが成功しました'))
);

final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();

// ...

ScaffoldMessenger(
  key: scaffoldMessengerKey,
  child: Scaffold(
    // ...
  ),
);

// ...

// 画面遷移後にSnackBarを表示
SnackBar snackBar = SnackBar(
  content: Text('画面遷移中'),
);
scaffoldMessengerKey.currentState?.showSnackBar(snackBar);

 [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Looking up a deactivated widget's ancestor is unsafe.
E/flutter ( 5027): At this point the state of the widget's element tree is no longer stable.
E/flutter ( 5027): To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling dependOnInheritedWidgetOfExactType() in the widget's didChangeDependencies() method.

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // GlobalKeyを作成します。これを使用してScaffoldMessengerにアクセスします。
  final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey =
      GlobalKey<ScaffoldMessengerState>();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      scaffoldMessengerKey:
          scaffoldMessengerKey, // MaterialAppにGlobalKeyを設定します。
      theme: ThemeData(useMaterial3: true),
      home: MyHomePage(scaffoldMessengerKey: scaffoldMessengerKey),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey;

  MyHomePage({required this.scaffoldMessengerKey});

  void _navigateToNextPage(BuildContext context) {
    Navigator.of(context).push(MaterialPageRoute(
        builder: (context) => NextPage(scaffoldMessengerKey)));
  }

  final _snackBar = SnackBar(content: Text('次の画面に遷移中'));

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('ScaffoldMessenger Demo'),
      ),
      body: Center(
        child: Column(
          children: [
            FilledButton(
              onPressed: () {
                ScaffoldMessenger.of(context).showSnackBar(_snackBar);
                Future.delayed(
                    Duration(seconds: 1), () => _navigateToNextPage(context));
              },
              child: Text('ScaffoldMessenger'),
            ),
            FilledButton(
              onPressed: () {
                scaffoldMessengerKey.currentState?.showSnackBar(_snackBar);
                Future.delayed(
                    Duration(seconds: 1), () => _navigateToNextPage(context));
              },
              child: Text('ScaffoldMessengerState'),
            ),
          ],
        ),
      ),
    );
  }
}

class NextPage extends StatelessWidget {
  final _snackBar = SnackBar(content: Text('前の画面に遷移中'));
  final GlobalKey<ScaffoldMessengerState> scaffoldMessengerKey;

  NextPage(this.scaffoldMessengerKey);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Next Page'),
      ),
      body: Center(
        child: Column(
          children: [
            Text('Welcome to the next page!'),
            FilledButton(
                onPressed: () {
                  Navigator.of(context).pop();
                  Future.delayed(
                      Duration(seconds: 1),
                      // 例外が発生する
                      () => ScaffoldMessenger.of(context)
                          .showSnackBar(_snackBar));
                },
                child: Text('back with ScaffoldMessenger(エラー発生)')),
            FilledButton(
                onPressed: () {
                  Navigator.of(context).pop();
                  Future.delayed(
                      Duration(seconds: 1),
                      () => scaffoldMessengerKey.currentState
                          ?.showSnackBar(_snackBar));
                },
                child: Text('back with scaffoldMessengerKey')),
          ],
        ),
      ),
    );
  }
}