Flutter-国际化

可以参考flutter中文社区的做法：
https://flutterchina.club/tutorials/internationalization/
本文参考，但有自己的修改：https://www.didierboelens.com/2018/04/internationalization---make-an-flutter-application-multi-lingual/
国际化还可以使用IDE插件：https://github.com/long1eu
文章同步至微信公众号：Android部落格

一、添加需要支持的库

在pubspec.yaml中的dependencies:节点下添加依赖：

flutter_localizations:
  sdk: flutter

同时在程序运行的main.dart中添加import：

import 'package:flutter_localizations/flutter_localizations.dart';

二、TranslationsDelegate

添加一个TranslationsDelegate继承自LocalizationsDelegate<Translations>，然后实现这个类的方法：

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) =>
      supportedLanguages.indexOf(locale.languageCode) >= 0;

  @override
  Future<Translations> load(Locale locale) {
    print("load is called");
    return Translations.load(locale);
  }

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

  static const List<String> supportedLanguages = ['en', 'zh'];

  Iterable<Locale> supportedLocales() =>
      supportedLanguages.map<Locale>((lang) => new Locale(lang, ''));
}

再添加一个翻译资源的处理类：

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) {
    print("start apply text");
    return _localizedValues[key] ?? '** $key not found';
  }

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

  get currentLanguage => locale.languageCode;
}

三、在工程根目录下添加资源文件

在项目根目录下新建locale文件夹，并添加两个翻译资源文件：

en.json
{
  "app_title": "My Application Title",
  "main_title": "My Main Title"
}
zh.json
{
  "app_title": "应用程序标题",
  "main_title": "我的主应用"
}

在pubspec.yaml下声明翻译资源文件：

assets:
  - locale/en.json
  - locale/zh.json

四、实现主程序的逻辑，跟踪语言的变化：

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

void main() => runApp(new 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> {
  TranslationsDelegate _localeOverrideDelegate = new TranslationsDelegate();

  onLocaleChange(Locale locale) {
    setState(() {
      _localeOverrideDelegate = new TranslationsDelegate();
    });
  }

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'My Application',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      localizationsDelegates: [
        _localeOverrideDelegate, // 注册一个新的delegate
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: _localeOverrideDelegate.supportedLocales(),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title:
            new Text(Translations.of(context).text('main_title')), 
      ),
      body: new Container(),
    );
  }
}

可以看到比较核心的代码片段是：

localizationsDelegates: [
        _localeOverrideDelegate, // 注册一个新的delegate
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],

注册系统语言变化的回调到代理类，可以重新加载对应预压的资源然后再去刷新界面，加载对应的多语言资源。

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