Flutter之国际化多语言

在说flutter国际化前，不得不提到，在uni-app中支持的Vue-i18n，两者有相似之处，也有差异的地方。
本篇借鉴了三篇热门帖子
Flutter中的国际化：如何写一个多语言的App
Flutter国际化完整例子
Internationalization - Make an Flutter application multi-lingual
只记录根据手机系统语言自动转换app语言

1.引入依赖

引入后，保存会自动加入安装此依赖

//pubspec.yaml
dependencies:
  flutter:
    sdk: flutter

  flutter_localizations: #多语言
    sdk: flutter
//这里是Http网络请求需要加入的
  dio: ^2.1.13  #Http 请求Dio
  http: ^0.12.0 #http
  json_serializable: ^3.1.0 #json 序列化
  json_annotation: ^2.4.0 #json 注释

2.创建翻译json文件和添加配置

我们新建一个和"/lib"同级别的文件夹"/locale"，然后在这个文件夹中新建两个文件，分别为"i18n_en.json" 和" i18n_zh.json"。再在"/lib"文件夹下创建和"main.dart"同级的"translation.dart"和"application.dart"。
  文件夹树现在是这个样子的：
MyApplication
  |
  +- android
  +- build
  +- images
  +- ios
  +- lib
   |
   +-main.dart
   +-translation.dart
   +-application.dart
  +- locale
   |
   +- i18n_en.json
   +- i18n_zh.json
  +- test

//i18n_zh.json      or  i18n_en.json
{ "hello" : "你好！" , "hello" : "Hello~" }

在pubspec.yaml继续加入json，引入静态资源

 assets:
  - locale/i18n_zh.json
  - locale/i18n_en.json 
  //图片也是需要手动加入到assets中的
  - images/lake.jpg
  - images/number.png

3.main.dart中加入配置

//导入flutter的包
import 'package:flutter_localizations/flutter_localizations.dart';
import 'translation.dart';
import 'application.dart';

void main(){
  debugPaintSizeEnabled = !true;
  runApp(MyApp());  
}
/// 将App设置为Stateful，这让它可以响应刷新事件，调用应用的SetState()
class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {
  SpecificLocalizationDelegate _localeOverrideDelegate;
   
  @override
  void initState(){
    super.initState();
    /// 初始化一个新的Localization Delegate，有了它，当用户选择一种新的工作语言时，可以强制初始化一个新的Translations
    _localeOverrideDelegate = new SpecificLocalizationDelegate(null);

    /// 保存这个方法的指针，当用户改变语言时，我们可以调用applic.onLocaleChanged(new Locale('en',''));，通过SetState()我们可以强制App整个刷新
    applic.onLocaleChanged = onLocaleChange;
  }

/// 改变语言时的应用刷新核心，每次选择一种新的语言时，都会创造一个新的SpecificLocalizationDelegate实例，强制Translations类刷新。
  onLocaleChange(Locale locale){
    setState((){
      _localeOverrideDelegate = new SpecificLocalizationDelegate(locale);
    });
  }


  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: [
        _localeOverrideDelegate,  // 注册一个新的delegate
        const TranslationsDelegate(), // 指向默认的处理翻译逻辑的库
        // 本地化的代理类
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: applic.supportedLocales(), 
   
      title: 'Flutter  Router',
      debugShowCheckedModeBanner: false, //关掉模拟器右上角debug图标
      theme: ThemeData(
        primarySwatch: Colors.orange,
      ),
      home: Homes(),
      //路由表设置
      routes: <String, WidgetBuilder>{
        '/Second':(BuildContext context) => new SecondPage(),
        '/Http':(BuildContext context) => new HttpPage(),
      },
    );
  }
}
......
//Homes()省略了

4.给translation.dart和application.dart添加内容

//application.dart
import 'package:flutter/material.dart';
typedef void LocaleChangeCallback(Locale locale);
class APPLIC {
    // List of supported languages
    final List<String> supportedLanguages = ['en','zh'];
    // Returns the list of supported Locales
    Iterable<Locale> supportedLocales() => supportedLanguages.map<Locale>((lang) => new Locale(lang, ''));
    // Function to be invoked when changing the working language
    LocaleChangeCallback onLocaleChanged;
    ///
    /// Internals
    ///
    static final APPLIC _applic = new APPLIC._internal();
    factory APPLIC(){
        return _applic;
    }
    APPLIC._internal();
}
APPLIC applic = new APPLIC();

//translation.dart
import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart' show  rootBundle;
import 'application.dart';

class Translations {
  Translations(Locale locale) {
    this.locale = locale;
    _localizedValues = null;
  }

  Locale locale;
  static Map<dynamic, dynamic> _localizedValues;
  
  static Translations of(BuildContext context){
    return Localizations.of<Translations>(context, Translations);
  }

    String text(String key) {
    try {
      String value = _localizedValues[key];
      if(value == null || value.isEmpty) {
        return englishText(key);
      } else {
        return value;
      }
    } catch (e) {
      return englishText(key);
    }
  }

  String englishText(String key) {
    return _localizedValues[key] ?? '** $key not found';
  }

  static Future<Translations> load(Locale locale) async {
    Translations translations = new Translations(locale);
    String jsonContent = await rootBundle.loadString("locale/i18n_${locale.languageCode}.json");
    _localizedValues = json.decode(jsonContent);
    return translations;
  }

  get currentLanguage => locale.languageCode;
}

class TranslationsDelegate extends LocalizationsDelegate<Translations> {
  const TranslationsDelegate();

/// 改这里是为了不硬编码支持的语言
  @override
  bool isSupported(Locale locale) => applic.supportedLanguages.contains(locale.languageCode);

  @override
  Future<Translations> load(Locale locale) => Translations.load(locale);

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

/// Delegate类的实现，每次选择一种新的语言时，强制初始化一个新的Translations类
class SpecificLocalizationDelegate extends LocalizationsDelegate<Translations> {
  final Locale overriddenLocale;

  const SpecificLocalizationDelegate(this.overriddenLocale);

  @override
  bool isSupported(Locale locale) => overriddenLocale != null;

  @override
  Future<Translations> load(Locale locale) => Translations.load(overriddenLocale);

  @override
  bool shouldReload(LocalizationsDelegate<Translations> old) => true;
}

5.获取翻译

......
 @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("JSON"),
      ), 
      body:Center(
        child: Column(
          children: <Widget>[
            Text(Translations.of(context).text('hello')) //这里
          ],
        )
      ),......

补充：如果需要强制转换的话(小编自己没有用到)

//为了强制切换另一种工作语言，只需要一行代码，可以在App的任何地方调用：
applic.onLocaleChanged(new Locale('fr',''));