必须初始化不可为空的实例字段“_localizedStrings”答案

【问题标题】：Non-nullable instance field '_localizedStrings' must be initialized必须初始化不可为空的实例字段“_localizedStrings”
【发布时间】：2021-10-11 12:03:43
【问题描述】：

说实话，我以前没有遇到过这种错误。我正在关注您可以在此处看到的教程： https://resocoder.com/2019/06/01/flutter-localization-the-easy-way-internationalization-with-json/

这是我的代码： MAIN.dart：

import 'app_localizations.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
 @override
Widget build(BuildContext context) {
  return MaterialApp(
    title: 'Flutter Demo',
     theme: ThemeData(
       primarySwatch: Colors.blue,
     ),
     supportedLocales: [Locale('en', 'US'), Locale('vi', 'VN')],
     localizationsDelegates: [
    // THIS CLASS WILL BE ADDED LATER
    // A class which loads the translations from JSON files
    AppLocalizations.delegate,
    // Built-in localization of basic text for Material widgets
    GlobalMaterialLocalizations.delegate,
    // Built-in localization for text direction LTR/RTL
    GlobalWidgetsLocalizations.delegate,
    ],
   localeResolutionCallback: (locale, supportedLocales) {
    for (var supportedLocale in supportedLocales) {
      if (supportedLocale.languageCode == locale!.languageCode &&
          supportedLocale.countryCode == locale.countryCode) {
        return supportedLocale;
      }
    }

    return supportedLocales.first;
  },
  home: MyHomePage(),
);
}
}

APP_LOCALIZATIONS.dart：

import 'dart:async';
import 'dart:convert';

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

class AppLocalizations {
  final Locale locale;

  //error right here


  AppLocalizations(this.locale);

  
  //error right here

  //modify
   static AppLocalizations? of(BuildContext context) {
  return Localizations.of<AppLocalizations>(context, AppLocalizations);
 }
 //modify

  static const LocalizationsDelegate<AppLocalizations> delegate =
  _AppLocalizationsDelegate();

 Map<String, String> _localizedStrings;

 Future<bool> load() async {
   String jsonString =
    await rootBundle.loadString('lang/${locale.languageCode}.json');
 Map<String, dynamic> jsonMap = json.decode(jsonString);

  _localizedStrings = jsonMap.map((key, value) {
    return MapEntry(key, value.toString());
  });
  return true;
 }


 //modify
 String? translate(String key) {
   return _localizedStrings[key];
 }
 }
 //modify

class _AppLocalizationsDelegate
   extends LocalizationsDelegate<AppLocalizations> {
  const _AppLocalizationsDelegate();

 @override
  bool isSupported(Locale locale) {
    return ['en', 'vi'].contains(locale.languageCode);
  }

  @override
  Future<AppLocalizations> load(Locale locale) async {
   AppLocalizations localizations = new AppLocalizations(locale);
    await localizations.load();
    return localizations;
  }

 @override
   bool shouldReload(_AppLocalizationsDelegate old) => false;
 }

这里的一些代码由于一些错误而被修改，所以它与教程有点不同（修复错误。）。所以也许这就是原因。我在上面对那个修改发表了一些评论。

P/S：我尝试寻找关于此的其他答案，但我无法理解，所以请为我解释！

【问题讨论】：

标签： flutter dart

【解决方案1】：

只需将_localizedStrings 初始化为一个空地图。

 Map<String, String> _localizedStrings = {};

这样即使还没有数据，它也永远不会为空。

另一种选择是在声明之前添加late 修饰符，告诉编译器稍后将对其进行初始化。

late Map<String, String> _localizedStrings;

【讨论】：