单例实现只实例化一次，暴露一个全局变量allTranslations

all_translations.dart

import 'dart:async';
import 'dart:convert';
import 'dart:ui';
import 'package:flutter/services.dart';
import 'package:shared_preferences/shared_preferences.dart';

///
/// Preferences related
///
const String _storageKey = "MyApplication_";
const List<String> _supportedLanguages = ['en','fr'];
Future<SharedPreferences> _prefs = SharedPreferences.getInstance();

class GlobalTranslations {
  Locale _locale;
  Map<dynamic, dynamic> _localizedValues;
  VoidCallback _onLocaleChangedCallback;

  ///
  /// Returns the list of supported Locales
  ///
  Iterable<Locale> supportedLocales() => _supportedLanguages.map<Locale>((lang) => new Locale(lang, ''));

  ///
  /// Returns the translation that corresponds to the [key]
  ///
  String text(String key) {
    // Return the requested string
     return (_localizedValues == null || _localizedValues[key] == null) ? '** $key not found' : _localizedValues[key];
  }

  ///
  /// Returns the current language code
  ///
  get currentLanguage => _locale == null ? '' : _locale.languageCode;

  ///
  /// Returns the current Locale
  ///
  get locale => _locale;
  
  ///
  /// One-time initialization
  /// 
  Future<Null> init([String language]) async {
    if (_locale == null){
      await setNewLanguage(language);
    }
    return null;
  }

  /// ----------------------------------------------------------
  /// Method that saves/restores the preferred language
  /// ----------------------------------------------------------
  getPreferredLanguage() async {
    return _getApplicationSavedInformation('language');
  }
  setPreferredLanguage(String lang) async {
    return _setApplicationSavedInformation('language', lang);
  }

  ///
  /// Routine to change the language
  ///
  Future<Null> setNewLanguage([String newLanguage, bool saveInPrefs = false]) async {
    String language = newLanguage;
    if (language == null){
      language = await getPreferredLanguage();
    }

    // Set the locale
    if (language == ""){
      language = "en";
    }
    _locale = Locale(language, "");

    // Load the language strings
    String jsonContent = await rootBundle.loadString("locale/i18n_${_locale.languageCode}.json");
    _localizedValues = json.decode(jsonContent);

    // If we are asked to save the new language in the application preferences
    if (saveInPrefs){
        await setPreferredLanguage(language);
    }

    // If there is a callback to invoke to notify that a language has changed
    if (_onLocaleChangedCallback != null){
        _onLocaleChangedCallback();
    }

    return null;
  }

  ///
  /// Callback to be invoked when the user changes the language
  ///
  set onLocaleChangedCallback(VoidCallback callback){
    _onLocaleChangedCallback = callback;
  }

  ///
  /// Application Preferences related
  ///
  /// ----------------------------------------------------------
  /// Generic routine to fetch an application preference
  /// ----------------------------------------------------------
  Future<String> _getApplicationSavedInformation(String name) async {
    final SharedPreferences prefs = await _prefs;

    return prefs.getString(_storageKey + name) ?? '';
  }

  /// ----------------------------------------------------------
  /// Generic routine to saves an application preference
  /// ----------------------------------------------------------
  Future<bool> _setApplicationSavedInformation(String name, String value) async {
    final SharedPreferences prefs = await _prefs;

    return prefs.setString(_storageKey + name, value);
  }


  ///
  /// Singleton Factory
  /// 
  static final GlobalTranslations _translations = new GlobalTranslations._internal();
  factory GlobalTranslations() {
    return _translations;
  }
  GlobalTranslations._internal();
}

GlobalTranslations allTranslations = new GlobalTranslations();

i18n_en.json

{
    "app_title": "My Application Title",
    "main_title": "My Main Title",
    "main_body": "My Main Body"
}

i18n_fr.json

{
    "app_title": "Le titre de mon application",
    "main_title": "Mon titre principal",
    "main_body":"Mon corps principal"
}

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'all_translations.dart';

void main() async {
    // Initializes the translation module
    await allTranslations.init();

    // then start the application
    runApp(MyApplication(),);
}

class MyApplication extends StatefulWidget {
    @override
    _MyApplicationState createState() => _MyApplicationState();
}

class _MyApplicationState extends State<MyApplication> {
    @override
    void initState(){
        super.initState();

        // Initializes a callback should something need 
        // to be done when the language is changed
        allTranslations.onLocaleChangedCallback = _onLocaleChanged;
    }

    ///
    /// If there is anything special to do when the user changes the language
    ///
    _onLocaleChanged() async {
        // do anything you need to do if the language changes
        print('Language has been changed to: ${allTranslations.currentLanguage}');
    }

    ///
    /// Main initialization
    ///
    @override
    Widget build(BuildContext context){
        return MaterialApp(
            localizationsDelegates: [
                GlobalMaterialLocalizations.delegate,
                GlobalWidgetsLocalizations.delegate,
            ],
            // Tells the system which are the supported languages
            supportedLocales: allTranslations.supportedLocales(),

            home: HomePage(),
        );
    }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    final String language = allTranslations.currentLanguage;
    final String buttonText = language == 'fr' ? '=> English' : '=> Français';

    return Scaffold(
      appBar: AppBar(title: Text(allTranslations.text('main_title'))),
      body: Container(
        width: double.infinity,
        child: Column(
          children: <Widget>[
            RaisedButton(
              child: Text(buttonText),
              onPressed: () async {
                await allTranslations.setNewLanguage(language == 'fr' ? 'en' : 'fr');
                setState((){});
              },
            ),
            Text(allTranslations.text('main_body')),
          ],
        ),
      ),
    );
  }
}