flutter : The getter 'languageCode' was called on null

Question

我正在尝试在我的应用程序中实现本地化。
我的应用程序仅支持英语和印地语两种语言。但用户可以选择从设备设置中选择英语或印地语以外的任何语言。如果用户选择了任何其他语言，我想将应用程序区域设置设置为印地语。为此，我使用 localeResolutionCallback ，如下面的代码所示。但我收到下面提到的错误消息。

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following NoSuchMethodError was thrown building ScrollConfiguration(behavior:
flutter: _MaterialScrollBehavior):
flutter: The getter 'languageCode' was called on null.
flutter: Receiver: null
flutter: Tried calling: languageCode
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
flutter: #1      Demo.build.<anonymous closure> (package:simple/main.dart:49:54)

调试后我发现 localeResolutionCallback 被调用了两次，第一次区域设置的值为空，第二次它给出了一些值。

flutter :区域设置---->null。

flutter :区域设置---->en_US。

1. 为什么 localeResolutionCallback 被调用两次且值为空
2. 这是将设备的默认区域设置覆盖为应用特定区域设置的正确方法

class Demo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      onGenerateTitle: (BuildContext context) => AppLocalizations.of(context).title,
      localizationsDelegates: [
        AppLocalizationsDelegate(),
        GlobalWidgetsLocalizations.delegate,
        GlobalMaterialLocalizations.delegate,
        GlobalCupertinoLocalizations.delegate,
      ],
      supportedLocales: [
        const Locale('en', ''), // English
        const Locale('hi', ''), // hindi
      ],
      localeResolutionCallback: (locale, supportedLocales) {
        for (var supportedLocale in supportedLocales) {
          if (supportedLocale.languageCode == locale.languageCode &&
              supportedLocale.countryCode == locale.countryCode) {
            return supportedLocale;
          }
        }
        return supportedLocales.last;
      },
      home: DemoApp(),
    );
  }
}

Answer 1

我有一个多语言支持的工作代码。请将您的代码与此代码进行比较。不过，如果您无法弄清楚，请与我联系以获取完整的源代码。我有一个关于这个东西的小演示项目。

class MyApp extends StatefulWidget {

  static void setLocale(BuildContext context, Locale newLocale) {
    var state = context.findAncestorStateOfType<_MyAppState>();
    state.setLocale(newLocale);
  }

  @override
  State<StatefulWidget> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {


  Locale _locale;

  void setLocale(Locale locale) {
    setState(() {
      _locale = locale;
    });
  }

  @override
  void didChangeDependencies() async {
    getLocale().then((locale) {
      setState(() {
        _locale = locale;
      });
    });
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      builder: (context, child) {
        return MediaQuery(
          child: child,
          data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
        );
      },
      title: 'Multi Language',
      debugShowCheckedModeBanner: false,
      locale: _locale,
      home: Home(),
      supportedLocales: [
        Locale('en', ''),
        Locale('ar', ''),
        Locale('hi', '')
      ],
      localizationsDelegates: [
        AppLocalizationsDelegate(),
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
        GlobalCupertinoLocalizations.delegate,
      ],
      localeResolutionCallback: (locale, supportedLocales) {
        for (var supportedLocale in supportedLocales) {
          if (supportedLocale?.languageCode == locale?.languageCode &&
              supportedLocale?.countryCode == locale?.countryCode) {
            return supportedLocale;
          }
        }
        return supportedLocales?.first;
      },
    );
  }
}