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flutter国际化实现方案

这里提供一份解决方案，和一份可以直接使用的demo。

1.添加依赖库

需要用到flutter_localizations包，在pubspec.yaml文件中添加如下依赖内容：

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter

添加完记得执行获取这个库：

在terminal中运行 flutter packages get, 或者在IntelliJ\Android Studio中点击’Packages get’

2.添加多种语言的文件

请支持原文 http://tryenough.com/flutter-translation

在项目的lib同级目录下创建locale文件夹，并创建几个json文件，用来存储不同语言的翻译文案。如下文件结构：

上图中创建了三种语言的json文件，你可以根据你的需要创建其他的语言json文件。添加翻译内容，如下：
i18n_en.json:

{
  "about_page_slogan":"Health starts here"
}

i18n_zh.json:

{
  "about_page_slogan":"健康从这里开始"
}

记得在pubspec.yaml文件中添加这几个资源文件：

3.创建一个工具类，保存当前支持的语言等信息

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locale_util.dart

import 'package:flutter/material.dart';

typedef void LocaleChangeCallback(Locale locale);
class LocaleUtil {

  // Support languages list
  final List<String> supportedLanguages = ['en','zh', 'ja'];

  // Support Locales list
  Iterable<Locale> supportedLocales() => supportedLanguages.map<Locale>((lang) => new Locale(lang, ''));

  // Callback for manual locale changed
  LocaleChangeCallback onLocaleChanged;

  Locale locale;
  String languageCode;

  static final LocaleUtil _localeUtil = new LocaleUtil._internal();

  factory LocaleUtil() {
    return _localeUtil;
  }

  LocaleUtil._internal();

  /// 获取当前系统语言
  String getLanguageCode() {
    if(languageCode == null) {
      return "en";
    }
    return languageCode;
  }
}

LocaleUtil localeUtil = new LocaleUtil();

3.添加翻译处理类文件

translations.dart

系统提供了LocalizationsDelegate类帮助我们监听系统语言的切换，所以我们可以继承LocalizationsDelegate类监听语言切换，并在切换时加载不同的json文件，来获取不同的语言文案。这里我们的translations工具类就是利用这样的原理来加载不同的语言文案：

import 'package:translation/locale_util.dart';

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

/// Class for Translate
///
/// For example:
///
/// import 'package:workout/translations.dart';
///
/// ```dart
/// For TextField content
/// Translations.of(context).text("home_page_title");
/// ```
///
/// ```dart
/// For speak string
/// Note: Tts will speak english if currentLanguage[# Tts's parameter] can't support
///
/// Translations.of(context).speakText("home_page_title");
/// ```
///
/// "home_page_title" is the key for text value
///
class Translations {
  Translations(Locale locale) {
    this.locale = locale;
    _localizedValues = null;
  }

  Locale locale;
  static Map<dynamic, dynamic> _localizedValues;
  static Map<dynamic, dynamic> _localizedValuesEn; // English map

  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 _localizedValuesEn[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);
    String enJsonContent = await rootBundle.loadString("locale/i18n_en.json");
    _localizedValuesEn = json.decode(enJsonContent);
    return translations;
  }

  get currentLanguage => locale.languageCode;
}

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

  // Support languages
  @override
  bool isSupported(Locale locale) {
    localeUtil.languageCode = locale.languageCode;
    return localeUtil.supportedLanguages.contains(locale.languageCode);
  }

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

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

// Delegate strong init a Translations instance when language was changed
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;
}

上面的代码我们看到有两个继承自LocalizationsDelegate类的代理类，SpecificLocalizationDelegate类是提供一种让我们强制指定一种位置的方式，通过改变传进来的local来达到从新load新json的方式。如果你没有这种需求，可以只关心TranslationsDelegate这种默认方式。

5.在main方法中将代理设置好

请支持原文 http://tryenough.com/flutter-translation

要想监听系统的代理，就需要设置系统位置代理。方法如下：

在main.dart方法中设置：

import 'package:translation/tanslations.dart';
import 'package:translation/locale_util.dart';

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'My Application',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      localizationsDelegates: [
        const TranslationsDelegate(), 
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
      ],
      supportedLocales: localeUtil.supportedLocales(),
      home: 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("about_page_slogan")),
      ),
      body: Center(
          child: Text(Translations.of(context).text("about_page_slogan")),
        ),
    );
  }
}

请支持原文 http://tryenough.com/flutter-translation

demo下载地址：http://tryenough.com/flutter-translation