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项目地址：
https://github.com/bettersun/helloflutterintl.git

APP内可实时切换语言，同时保存切换后语言。
默认启动是简体中文，下次启动为保存后的语言。

该APP设置了四种语言： 简体中文、繁体中文、英语、日语。

先上图

简体中文

001.png

繁体中文

002.png

英语

003.png

日语

004.png

1. 添加依赖

pubspec.yaml

  # 国际化
  flutter_localizations:
    sdk: flutter
  flutter_cupertino_localizations:
  intl:
  intl_translation:
  # 本地存储
  shared_preferences:

2. 手动添加多语言类并生成多语言文件

手动添加多语言类 message.dart
参照链接(该链接内容很详细)：
https://www.jianshu.com/p/82c6656462b8

3. 基于message.dart生成arb文件

flutter pub pub run intl_translation:extract_to_arb --output-dir=lib/localization lib/localization/message.dart

4. 生成多语言dart文件

生成的arb文件重命名为各语言对应arb(例：message_zh_CN.arb)
然后执行命令生成对应的dart文件

例：
简体中文：

flutter pub pub run intl_translation:generate_from_arb --output-dir=lib/localization    --no-use-deferred-loading lib/localization/message.dart lib/localization/message_zh_CN.arb

英文：

flutter pub pub run intl_translation:generate_from_arb --output-dir=lib/localization    --no-use-deferred-loading lib/localization/message.dart lib/localization/message_en.arb

5. 定义全局语言切换Controller(locale_controller.dart)

import 'package:flutter/material.dart';

typedef ChangeLocaleCallback = void Function(Locale locale);

/// 多语言切换
class LocaleController {
  /// 语言切换回调方法
  ChangeLocaleCallback onLocaleChanged;

  /// Internals
  static final LocaleController _localeController = LocaleController._internal();

  factory LocaleController() {
    return _localeController;
  }

  LocaleController._internal();
}

/// 语言切换控制器实例
LocaleController localeController = LocaleController();

6. 定义多语言委托(locale_delegate.dart)

BsMessageLocalDelegate
BsCupertinoLocalDelegate
BsMaterialLocalizationsDelegate

7. main方法中读取本地存储里的Locale，并设置给启动APP

取不到默认使用简体中文

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  final SharedPreferences preferences = await SharedPreferences.getInstance();
  // 从本地存储中取得已保存的语言
  String localeName = preferences.getString(PreferenceKey.localeName);
  // 取不到默认设置为简体中文
  localeName = localeName ?? LocaleConst.zhCN.toString();

  // 启动程序
  runApp(BsApp(localeName: localeName));
}

8. 启动App里初始化多语言委托,并绑定全局事件用来更改App的State

class _BsAppState extends State<BsApp> {
  // 语言委托
  // App用
  BsMessageLocalDelegate _bsMessageLocalDelegate;
  // iOS用
  BsCupertinoLocalDelegate _bsCupertinoLocalDelegate;
  // 系统共通用（复制、粘贴等）
  BsMaterialLocalizationsDelegate _bsMaterialLocalizationsDelegate;

  @override
  void initState() {
    super.initState();

    final Locale locale = toLocale(widget.localeName);

    // 语言委托初始化
    _bsMessageLocalDelegate = BsMessageLocalDelegate(locale);
    _bsCupertinoLocalDelegate = BsCupertinoLocalDelegate(locale);
    _bsMaterialLocalizationsDelegate = BsMaterialLocalizationsDelegate(locale);
    // 语言切换回调方法
    // 绑定全局事件用来更改App的State
    localeController.onLocaleChanged = onLocaleChange;
  }

  /// 语言地区字符串转语言地区
  Locale toLocale(String localName) {
    assert(localName != null);

    final List<String> localeCode = localName.split('_');

    if (localeCode.length > 1) {
      return Locale(localeCode[0], localeCode[1]);
    } else if (localeCode.isNotEmpty) {
      return Locale(localeCode[0]);
    }
  }

  /// 语言切换
  void onLocaleChange(Locale newLocale) {
    setState(() {
      _bsMessageLocalDelegate = BsMessageLocalDelegate(newLocale);
      _bsCupertinoLocalDelegate = BsCupertinoLocalDelegate(newLocale);
      _bsMaterialLocalizationsDelegate = BsMaterialLocalizationsDelegate(newLocale);
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: ' ', // 这个地方多语言Message还未加载,无法使用Message响应多语言
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      // 语言委托
      localizationsDelegates: [
        _bsMessageLocalDelegate,
        _bsCupertinoLocalDelegate,
        _bsMaterialLocalizationsDelegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      // 支持的语言
      supportedLocales: const <Locale>[
        LocaleConst.zhCN,
        LocaleConst.zhHK,
        LocaleConst.en,
        LocaleConst.en,
      ],
      home: const BsHome(),
    );
  }
}

9. 切换语言事件里调用全局事件触发启动App里的setState来实时改变App语言，并保存切换后语言

  /// 切换语言
  Future<void> _changeLocale() async {
    _localeIndex++;

    Locale locale = LocaleConst.zhCN;
    // 切换语言
    if (_localeIndex % _localeTotal == 0) {
      locale = LocaleConst.zhCN;
    }
    if (_localeIndex % _localeTotal == 1) {
      locale = LocaleConst.zhHK;
    }
    if (_localeIndex % _localeTotal == 2) {
      locale = LocaleConst.en;
    }
    if (_localeIndex % _localeTotal == 3) {
      locale = LocaleConst.ja;
    }

    // 切换语言
    // 调用全局事件触发启动App里的setState
    localeController.onLocaleChanged(locale);

    final SharedPreferences preferences = await SharedPreferences.getInstance();
    // 切换后的语言保存到本地存储
    preferences.setString(PreferenceKey.localeName, locale.toString());
  }