Flutter app crashes building the AppBar with multi language using i18n Jetbrains plugin's boilerplate

Question

I'm using Flutter i18n plugin for Android Studio following the official documentation.

I added the few configuration line needed to a minimal app that displays a String:

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [S.delegate],
      supportedLocales: S.delegate.supportedLocales,
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
//      appBar: AppBar(
//        title: Text("Hard-coded English string"),
//      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(S.of(context).string_that_should_be_internationalized),
          ],
        ),
      ),
    );
  }
}

This is working (the string is actually translated to my phone system language). But a warning is thrown at every call to build:

Warning: This application's locale, it_, is not supported by all of its
localization delegates.
> A MaterialLocalizations delegate that supports the it_ locale was not found.

Despite S.delegate.supportedLocales being [en_, it_] and all arb file being correctly configured.

I can make the Scaffold body arbitrarily complicated and the behavior remains the same (i18n works with warning). But if I add a simple AppBar (even if the widged does not require translation) to the Scaffold the whole application crashes. (E.g. uncommenting in the snippet above)

Abstract of the error:

I/flutter (11411): No MaterialLocalizations found.
I/flutter (11411): AppBar widgets require MaterialLocalizations to be provided by a Localizations widget ancestor.
I/flutter (11411): Localizations are used to generate many different messages, labels,and abbreviations which are used
I/flutter (11411): by the material library.
I/flutter (11411): To introduce a MaterialLocalizations, either use a  MaterialApp at the root of your application to
I/flutter (11411): include them automatically, or add a Localization widget with a MaterialLocalizations delegate.
I/flutter (11411): The specific widget that could not find a MaterialLocalizations ancestor was:
I/flutter (11411):   AppBar

The framework is complaining about my AppBar not having a Localizations widget ancestor. But it actually have because it's inside a MaterialApp. No clue on how to solve this mystery.

Full log

EDIT
Another weird thing i just discovered: running the same exact app in --release mode gets rid of all the errors and warnings; everything is correctly translated including the AppBar.
Still wondering why it crashes in --debug mode.

Solution

The problem is not about the boilerplate code or configuration.

The meaning of the error is that there is no support for the language "it_" for the "default system strings". (In particular it crashes due to some default tooltip of the AppBar -> that's why it is crashing if an AppBar is displayed).

As reported in this Q&A you can solve the issue by implementing your own LocalizationsDelegate.

Full fix

class MaterialLocalizationItDelegate extends LocalizationsDelegate<MaterialLocalizations> {
  /// Here list supported country and language codes
  @override
  bool isSupported(Locale locale) => locale.languageCode == "it";
  /// Here create an instance of your [MaterialLocalizations] subclass
  @override
  Future<MaterialLocalizations> load(Locale locale) async => MaterialLocalizationIt();
  @override
  bool shouldReload(_) => false;
}

class MaterialLocalizationIt extends MaterialLocalizations {
  // alt-enter on intellij and implement many overrides (somthing like 57)
}

And then append your delegate to the list of delegates:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [S.delegate, MaterialLocalizationItDelegate()],  // <- append here
      supportedLocales: S.delegate.supportedLocales,
      home: MyHomePage(),
    );
  }
}

Quick fix

An hasty way to check if this will work for you before implementing the 57 overrides.
Copy exactly this code:

class FallbackLocalizationDelegate extends LocalizationsDelegate<MaterialLocalizations> {
  @override
  bool isSupported(Locale locale) => true;
  @override
  Future<MaterialLocalizations> load(Locale locale) async => DefaultMaterialLocalizations();
  @override
  bool shouldReload(_) => false;
}

The above class claim to support all locales (isSupported(Locale locale) => true) and simply returns the default en_US locale.
Finally add this to your list of delegates.

localizationsDelegates: [S.delegate, FallbackLocalizationDelegate()],  // <- append here

This quick fix will set all the default system strings to English, but your "custom" localization should work.

Comments

[Javeed Ishaq]

thanks a lot Pado, you saved my day thanks thanks