Flutter 应用里的国际化

如果你的 app 会部署给说其他语言的用户使用，那么你就需要对它进行国际化。这就意味着你在编写 app 的时候，需要采用一种容易对它进行本地化的方式进行开发，这种方式让你能够为每一种语言或者 app 所支持的语言环境下的文本和布局等进行本地化。 Flutter 提供了 widgets 和类来帮助开发者进行国际化，当然 Flutter 库本身就是国际化的。

If your app might be deployed to users who speak another language then you’ll need to “internationalize” it. That means you’ll need to write the app in a way that makes it possible to “localize” values like text and layouts for each language or “locale” that the app supports. Flutter provides widgets and classes that help with internationalization and the Flutter libraries themselves are internationalized.

和大多数应用一样，下面的教程主要都是使用 Flutter MaterialApp 类编写。那些使用更底层的 WidgetsApp 类编写的应用也能通过使用相同的类和逻辑来进行国际化。

The tutorial that follows is largely written in terms of the Flutter MaterialApp class, since most applications are written that way. Applications written in terms of the lower level WidgetsApp class can also be internationalized using the same classes and logic.

配置一个国际化的 app：flutter_localizations package

Setting up an internation­alized app: the flutter_localizations package

默认情况下，Flutter 只提供美式英语的本地化。如果想要添加其他语言，你的应用必须指定额外的 MaterialApp 属性并且添加一个单独的 package，叫做 flutter_localizations。截至到 2020 年 2 月份，这个 package 已经支持大约 77 种语言。

By default, Flutter only provides US English localizations. To add support for other languages, an application must specify additional MaterialApp properties, and include a separate package called flutter_localizations. As of February 2020, this package supports 77 languages. If you want your app to work smoothly on iOS, then you have to add the package flutter_cupertino_localizations as well.

想要使用 flutter_localizations 的话，你需要在 pubspec.yaml 文件中添加它作为依赖：

To use flutter_localizations, add the package as a dependency to your pubspec.yaml file:

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

下一步，引入 flutter_localizations 库，然后为 MaterialApp 指定 localizationsDelegates 和 supportedLocales：

Next, import the flutter_localizations library and specify localizationsDelegates and supportedLocales for MaterialApp:

import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp(
 localizationsDelegates: [
   // ... app-specific localization delegate[s] here
   GlobalMaterialLocalizations.delegate,
   GlobalWidgetsLocalizations.delegate,
   GlobalCupertinoLocalizations.delegate,
 ],
 supportedLocales: [
    const Locale('en', ''), // English, no country code
    const Locale('he', ''), // Hebrew, no country code
    const Locale.fromSubtags(languageCode: 'zh'), // Chinese *See Advanced Locales below*
    // ... other locales the app supports
  ],
  // ...
)

基于 WidgetsApp 构建的 app 在添加语言环境时，除了 GlobalMaterialLocalizations.delegate 不需要之外，其他的操作是类似的。

Apps based on WidgetsApp are similar except that the GlobalMaterialLocalizations.delegate isn’t needed.

虽然 语言环境 (Locale) 默认的构造函数是完全没有问题的，但是还是建议大家使用 Locale.fromSubtags 的构造函数，因为它支持设置文字代码。

The full Locale.fromSubtags constructor is preferred as it supports scriptCode, though the Locale default constructor is still fully valid.

localizationDelegates 数组是用于生成本地化值集合的工厂。 GlobalMaterialLocalizations.delegate 为 Material 组件库提供本地化的字符串和一些其他的值。 GlobalWidgetsLocalizations.delegate 为 widgets 库定义了默认的文本排列方向，由左到右或者由右到左。

The elements of the localizationsDelegates list are factories that produce collections of localized values. GlobalMaterialLocalizations.delegate provides localized strings and other values for the Material Components library. GlobalWidgetsLocalizations.delegate defines the default text direction, either left-to-right or right-to-left, for the widgets library.

想知道更多关于这些 app 属性，它们依赖的类型以及那些国际化的 Flutter app 通常是如何组织的，可以继续阅读下面内容。

More information about these app properties, the types they depend on, and how internationalized Flutter apps are typically structured, can be found below.

高级语言环境定义

Advanced locale definition

一些具有着多个变种的语言仅仅用语言代码是不能合适地区分的。

Some languages with multiple variants require more than just a language code to properly differentiate.

例如，要能完全区分具有多个变种的中文需要指定语言代码、文字代码和国家代码。这是因为存在着简体和繁体的文字系统，而且同时使用相同文字系统写的字符又有地域性的差别。

For example, fully differentiating all variants of Chinese requires specifying the language code, script code, and country code. This is due to the existence of simplified and traditional script, as well as regional differences in the way characters are written within the same script type.

为了让 CN，TW 和 HK 三个不同的国家/地区代码能够完整地表达每个变种的中文，你应该包括以下支持的语言环境:

In order to fully express every variant of Chinese for the country codes CN, TW, and HK, the list of supported locales should include:

// Full Chinese support for CN, TW, and HK
supportedLocales: [
  const Locale.fromSubtags(languageCode: 'zh'), // generic Chinese 'zh'
  const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hans'), // generic simplified Chinese 'zh_Hans'
  const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant'), // generic traditional Chinese 'zh_Hant'
  const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hans', countryCode: 'CN'), // 'zh_Hans_CN'
  const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant', countryCode: 'TW'), // 'zh_Hant_TW'
  const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant', countryCode: 'HK'), // 'zh_Hant_HK'
],

这种明确完整的定义可以确保你的 app 能够区分以及提供完全地道的本地内容给这些国家/地区代码的所有组合的用户。如果用户没有指定首选的语言环境，那么我们就会使用最近的匹配，这很可能与用户的期望会有差异。 Flutter 只会解析定义在 supportedLocales 里面的语言环境。对于那些常用语言，Flutter 为本地化内容提供了文字代码级别的区分。查看 Localizations 了解 Flutter 是如何解析支持的语言环境和首选的语言环境的。

This explicit full definition ensures that your app can distinguish between and provide the fully nuanced localized content to all combinations of these country codes. If a user’s preferred locale is not specified, then the closest match is used instead, which will likely contain differences to what the user expects. Flutter only resolves to locales defined in supportedLocales. Flutter provides scriptCode-differentiated localized content for commonly used languages. See Localizations for information on how the supported locales and the preferred locales are resolved.

虽然中文是最主要的一个示例，但是其他语言如法语（FR_fr，FR_ca 等等）也应该为了更细致的本地化而做完全的区分。

Although Chinese is a primary example, other languages like French (fr_FR, fr_CA) should also be fully differentiated for more nuanced localization.

获取语言环境：Locale 类和 Localizations Widget

Tracking the locale: The Locale class and the Localizations widget

Locale 类用来识别用户的语言。移动设备支持为所有的应用设置语言环境，经常是通过系统设置菜单来进行操作。设置完之后，国际化的 app 就会展示成对应特定语言环境的值。例如，如果用户把设备的语言环境从英语切换到法语，显示 “Hello World” 的文本 widget 会使用 “Bonjour le monde” 进行重建。

The Locale class identifies the user’s language. Mobile devices support setting the locale for all applications, usually using a system settings menu. Internationalized apps respond by displaying values that are locale-specific. For example, if the user switches the device’s locale from English to French, then a Text widget that originally displayed “Hello World” would be rebuilt with “Bonjour le monde”.

Localizations widget 定义了它的子节点的语言环境和依赖的本地化的资源。 WidgetsApp 创建了一个本地化的 widget，如果系统的语言环境变化了，它会重建这个 widget。

The Localizations widget defines the locale for its child and the localized resources that the child depends on. The WidgetsApp widget creates a Localizations widget and rebuilds it if the system’s locale changes.

你可以通过调用 Localizations.localeOf() 方法来查看 app 当前的语言环境。

You can always lookup an app’s current locale with Localizations.localeOf():

Locale myLocale = Localizations.localeOf(context);

加载和获取本地化值

Loading and retrieving localized values

我们使用 Localizations widget 来加载和查询那些包含本地化值集合的对象。 app 通过调用 Localizations.of(context,type) 来引用这些对象。如果设备的语言环境变化了，Localizations widget 会自动地加载新的语言环境的值，然后重建那些使用了语言环境的 widget。这是因为 Localizations 像 继承 widget 一样执行。当一个构建过程涉及到继承 widget，对继承 widget 的隐式依赖就创建了。当一个继承 widget 变化了（即 Localizations widget 的语言环境变化），它的依赖上下文就会被重建。

The Localizations widget is used to load and lookup objects that contain collections of localized values. Apps refer to these objects with Localizations.of(context,type). If the device’s locale changes, the Localizations widget automatically loads values for the new locale and then rebuilds widgets that used it. This happens because Localizations works like an InheritedWidget. When a build function refers to an inherited widget, an implicit dependency on the inherited widget is created. When an inherited widget changes (when the Localizations widget’s locale changes), its dependent contexts are rebuilt.

本地化的值是通过使用 Localizations widget 的 LocalizationsDelegate 加载的。每一个 delegate 必须定义一个异步的 load() 方法。这个方法生成了一个封装本地化值的对象，通常这些对象为每个本地化的值定义了一个方法。

Localized values are loaded by the Localizations widget’s list of LocalizationsDelegates. Each delegate must define an asynchronous load() method that produces an object that encapsulates a collection of localized values. Typically these objects define one method per localized value.

在一个大型的 app 中，不同的模块或者 package 需要和它们对应的本地化资源打包在一起。这就是为什么 Localizations widget 管理着对象的一个对应表，每个 LocalizationsDelegate 对应一个对象。为了获得由 LocalizationsDelegate 的 load 方法生成的对象，你需要指定一个构建上下文 (BuildContext) 和对象的类型。

In a large app, different modules or packages might be bundled with their own localizations. That’s why the Localizations widget manages a table of objects, one per LocalizationsDelegate. To retrieve the object produced by one of the LocalizationsDelegate’s load methods, you specify a BuildContext and the object’s type.

例如，Material 组件 widget 的本地化字符串是由 MaterialLocalizations 类定义的。这个类的实例是由 [MaterialApp 类提供的一个 LocalizationDelegate 方法创建的，它们可以通过 Localizations.of 方法获得。

For example, the localized strings for the Material Components widgets are defined by the MaterialLocalizations class. Instances of this class are created by a LocalizationDelegate provided by the MaterialApp class. They can be retrieved with Localizations.of():

Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);

因为这个特定的 Localizations.of() 表达式经常使用，所以 MaterialLocalizations 类提供了一个快捷访问：

This particular Localizations.of() expression is used frequently, so the MaterialLocalizations class provides a convenient shorthand:

static MaterialLocalizations of(BuildContext context) {
  return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);
}

/// References to the localized values defined by MaterialLocalizations
/// are typically written like this:

tooltip: MaterialLocalizations.of(context).backButtonTooltip,

使用内置的 LocalizationsDelegates

Using the bundled Localizations­Delegates

Flutter package 包括的 MaterialLocalizations 和 WidgetsLocalizations 的接口都只提供美式英语的值，这样使得它尽可能小而简单。这些实现的类被分别称为 DefaultMaterialLocalizations 和 DefaultWidgetsLocalizations。它们会被自动地引入程序，除非你在 localizationsDelegates 参数中，相同的基本类型指定了一个不同的 delegate。

To keep things as small and uncomplicated as possible, the flutter package includes implementations of the MaterialLocalizations and WidgetsLocalizations interfaces that only provide US English values. These implementation classes are called DefaultMaterialLocalizations and DefaultWidgetsLocalizations, respectively. They’re included automatically unless a different delegate of the same base type is specified with the app’s localizationsDelegates parameter.

flutter_localizations package 包括了多种语言本地化接口的实现，它们称为 GlobalMaterialLocalizations 和 GlobalWidgetsLocalizations。国际化 app 必须为这些类的指定本地化 delegate，就如在 配置一个国际化的 app 中描述的那样。

The flutter_localizations package includes multi-language implementations of the localizations interfaces called GlobalMaterialLocalizations and GlobalWidgetsLocalizations. International apps must specify localization delegates for these classes as described in Setting up an internationalized app.

import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp(
 localizationsDelegates: [
   // ... app-specific localization delegate[s] here
   GlobalMaterialLocalizations.delegate,
   GlobalWidgetsLocalizations.delegate,
 ],
 supportedLocales: [
    const Locale('en', ''), // English, no country code
    const Locale('he', ''), // Hebrew, no country code
    const Locale('zh', ''), // Chinese, no country code
    // ... other locales the app supports
  ],
  // ...
)

全球本地化 delegate 构建了对应类在特定语言环境下的实例。例如，GlobalMaterialLocalizations.delegate 就是一个本地化 delegate，它用来产生一个 GlobalMaterialLocalizations 的实例。

The global localization delegates construct locale-specific instances of the corresponding classes. For example, GlobalMaterialLocalizations.delegate is a LocalizationsDelegate that produces an instance of GlobalMaterialLocalizations.

截至 2020 年 2 月，这个全球本地化类一共支持 大约 77 种语言。

As of February 2020, the global localization classes support 77 languages.

为 app 的本地化资源定义一个类

Defining a class for the app’s localized resources

综合所有这些在一起，一个需要国际化的 app 经常以一个封装 app 本地化值的类开始的。下面是使用这种类的典型示例。

Putting all of this together for an internationalized app usually starts with the class that encapsulates the app’s localized values. The example that follows is typical of such classes.

此示例 app 的 完整的源码。

Complete source code for the intl_example for this app.

这个示例是基于 intl package 提供的 API 和工具开发的， app 本地化资源的替代方法 里面讲解了一个不依赖于 intl package 的 示例。

This example is based on the APIs and tools provided by the intl package. An alternative class for the app’s localized resources describes an example that doesn’t depend on the intl package.

DemoLocalizations 类包含了 app 语言环境内支持的已经翻译成了本地化语言的字符串（本例子只有一个）。它通过调用由 Dart 的 intl package 生成的 initializeMessages() 方法来加载翻译好的字符串，然后使用 Intl.message() 来查阅它们。

The DemoLocalizations class contains the app’s strings (just one for the example) translated into the locales that the app supports. It uses the initializeMessages() function generated by Dart’s intl package, Intl.message(), to look them up.

class DemoLocalizations {
  DemoLocalizations(this.localeName);

  static Future<DemoLocalizations> load(Locale locale) {
    final String name = locale.countryCode.isEmpty ? locale.languageCode : locale.toString();
    final String localeName = Intl.canonicalizedLocale(name);
    return initializeMessages(localeName).then((_) {
      return DemoLocalizations(localeName);
    });
  }

  static DemoLocalizations of(BuildContext context) {
    return Localizations.of<DemoLocalizations>(context, DemoLocalizations);
  }

  final String localeName;

  String get title {
    return Intl.message(
      'Hello World',
      name: 'title',
      desc: 'Title for the Demo application',
      locale: localeName,
    );
  }
}

基于 intl package 的类引入了一个生成好的信息目录，它提供了 initializeMessage() 方法和 Intl.message() 方法的每个语言环境的备份存储。 intl 工具 通过分析包含 Intl.message() 调用类的源码生成这个信息目录。在当前情况下，就是 DemoLocalizations 的类（包含了 Intl.message() 调用）。

A class based on the intl package imports a generated message catalog that provides the initializeMessages() function and the per-locale backing store for Intl.message(). The message catalog is produced by an intl tool that analyzes the source code for classes that contain Intl.message() calls. In this case that would just be the DemoLocalizations class.

具体说明 app 支持的语言环境参数

Specifying the app’s supported­Locales parameter

虽然 Flutter 的 flutter_localizations 库能够支持大约 52 种语言，但是默认只支持英语翻译。这是因为应该由开发者决定到底要支持哪一种语言，让工具库默认去支持和 app 不一样的语言环境是完全没有意义的。

Although Flutter’s flutter_localizations library includes support for 77 languages, only English language translations are available by default. It’s up to the developer to decide exactly which languages to support, since it wouldn’t make sense for the toolkit libraries to support a different set of locales than the app does.

MaterialApp 的 supportedLocales 参数限制了语言环境的变化范围。当用户在他们的设备切换语言环境的时候，只有当新语言环境是 supportedLocales 列表项中之一时， app 的 Localizations widget 才会跟着一起变。如果这个设备的语言环境不能被精确匹配， languageCode 相同的第一个支持的语言环境会被使用。如果这个也失败了，那就会使用 supportedLocales 的第一个语言环境。

The MaterialApp supportedLocales parameter limits locale changes. When the user changes the locale setting on their device, the app’s Localizations widget only follows suit if the new locale is a member of the this list. If an exact match for the device locale isn’t found, then the first supported locale with a matching languageCode is used. If that fails, then the first element of the supportedLocales list is used.

以上面那个 DemoApp 例子来说，这个 app 仅接受美式英语或者加拿大法语的语言环境。对于其他任何语言环境都是使用美式英语作为替代（因为它是列表当中的第一个）。

In terms of the previous DemoApp example, the app only accepts the US English or French Canadian locales, and it substitutes US English (the first locale in the list) for anything else.

如果一个 app 想要使用不同的语言环境解析方案，它可以提供一个 localeResolutionCallback，例如，让你的 app 无条件的接受用户选择的任何语言环境：

An app that wants to use a different “locale resolution” method can provide a localeResolutionCallback. For example, to have your app unconditionally accept whatever locale the user selects:

class DemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
       localeResolutionCallback: (Locale locale, Iterable<Locale> supportedLocales) {
         return locale;
       }
       // ...
    );
  }
}

app 本地化资源的替代方法

An alternative class for the app’s localized resources

之前的那个 DemoApp 示例是使用 Dart intl package 进行开发的。为了更简便，或者和其他不同的 i18n 框架集成，开发者可以选择他们自己的方法来管理本地化的值。

The previous DemoApp example was defined in terms of the Dart intl package. Developers can choose their own approach for managing localized values for the sake of simplicity or perhaps to integrate with a different i18n framework.

这个示例 APP 的 完整代码。

Complete source code for the minimal app.

在这个版本的 DemoApp 中，这个类包含了 app 的 localizations 和 DemoLocalizations，并且直接将它所有的翻译放在每个语言的映射当中。

In this version of DemoApp the class that contains the app’s localizations, DemoLocalizations, includes all of its translations directly in per language Maps.

class DemoLocalizations {
  DemoLocalizations(this.locale);

  final Locale locale;

  static DemoLocalizations of(BuildContext context) {
    return Localizations.of<DemoLocalizations>(context, DemoLocalizations);
  }

  static Map<String, Map<String, String>> _localizedValues = {
    'en': {
      'title': 'Hello World',
    },
    'es': {
      'title': 'Hola Mundo',
    },
  };

  String get title {
    return _localizedValues[locale.languageCode]['title'];
  }
}

在这个最小实现的 app 当中，DemoLocalizationDelegate 有一点不一样。它的 load 方法返回了一个 SynchronousFuture，因为不需要进行异步的加载。

In the minimal app the DemoLocalizationsDelegate is slightly different. Its load method returns a SynchronousFuture because no asynchronous loading needs to take place.

class DemoLocalizationsDelegate extends LocalizationsDelegate<DemoLocalizations> {
  const DemoLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) => ['en', 'es'].contains(locale.languageCode);

  @override
  Future<DemoLocalizations> load(Locale locale) {
    return SynchronousFuture<DemoLocalizations>(DemoLocalizations(locale));
  }

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

添加支持新的语言

Adding support for a new language

如果你要开发一个 app 需要支持的语言不在 GlobalMaterialLocalizations 当中，那就需要做一些额外的工作：它必须提供大概 70 个字和词的翻译（本地化）。

An app that needs to support a language that’s not included in GlobalMaterialLocalizations has to do some extra work: it must provide about 70 translations (“localizations”) for words or phrases.

举个例子，我们将给大家展示如何支持白俄罗斯语。

See the following for an example of how to add support for the Belarusian language.

我们需要定义一个新的 GlobalMaterialLocalizations 子类，它定义了 Material 库依赖的本地化资源。同时，我们也必须定义一个新的 LocalizationsDelegate 子类，它是给 GlobalMaterialLocalizations 子类作为一个工厂使用的。

A new GlobalMaterialLocalizations subclass defines the localizations that the Material library depends on. A new LocalizationsDelegate subclass, which serves as factory for the GlobalMaterialLocalizations subclass, must also be defined.

这是支持添加一种新语言的一个完整例子的源码，相对实际上要翻译的白俄罗斯语数量，我们只翻译了部分。

Here’s the source code for the complete add_language example, minus the actual Belarusian translations.

这个特定语言环境的 GlobalMaterialLocalizations 子类被称为 BeMaterialLocalizations， LocalizationsDelegate 子类被称为 _BeMaterialLocalizationsDelegate。 BeMaterialLocalizations.delegate 是 delegate 的一个实例，这就是 app 使用这些本地化所需要的全部。

The locale-specific GlobalMaterialLocalizations subclass is called BeMaterialLocalizations, and the LocalizationsDelegate subclass is _BeMaterialLocalizationsDelegate. The value of BeMaterialLocalizations.delegate is an instance of the delegate, and is all that’s needed by an app that uses these localizations.

delegate 类包括基本的日期和数字格式的本地化。其他所有的本地化是由 BeMaterialLocalizations 里面的字符串值属性的 getters 所定义的，像下面这样：

The delegate class includes basic date and number format localizations. All of the other localizations are defined by String valued property getters in BeMaterialLocalizations, like this:

@override
String get backButtonTooltip => r'Back';

@override
String get cancelButtonLabel => r'CANCEL';

@override
String get closeButtonLabel => r'CLOSE';

// etc..

These are the English translations, of course. To complete the job you need to change the return value of each getter to an appropriate Belarusian string.

当然，这些都是英语翻译。为了完成本地化操作，你需要把每一个 getter 的返回值翻译成合适的白俄罗斯语字符。

These are the English translations of course. To complete the job you need to change the return value of each getter to an appropriate Belarusian string.

像 r'About $applicationName' 一样，这些带 r 前缀的 getters 返回的是原始的字符串，因为有一些时候这些字符串会包含一些带有 $ 前缀的变量。通过调用带参数的本地化方法，这些变量会被替换：

The getters return “raw” Dart strings that have an r prefix, like r'About $applicationName', because sometimes the strings contain variables with a $ prefix. The variables are expanded by parameterized localization methods:

@override
String get aboutListTileTitleRaw => r'About $applicationName';

@override
String aboutListTileTitle(String applicationName) {
  final String text = aboutListTileTitleRaw;
  return text.replaceFirst(r'$applicationName', applicationName);
}

需要了解更多关于本地化字符串的内容，可以查看 flutter_localizations README。

For more information about localization strings, see the flutter_localizations README.

一旦你实现了指定语言的 GlobalMaterialLocalizations 和 LocalizationsDelegate 的子类，你只需要给你的 app 添加此语言以及一个 delegate 的实例。这里有一些代码展示了如何设置 app 的语言为白俄罗斯语以及如何给 app 的 localizationsDelegates 列表添加 BeMaterialLocalizations delegate 实例。

Once you’ve implemented your language-specific subclasses of GlobalMaterialLocalizations and LocalizationsDelegate, you just need to add the language and a delegate instance to your app. Here’s some code that sets the app’s language to Belarusian and adds the BeMaterialLocalizations delegate instance to the app’s localizationsDelegates list:

MaterialApp(
  localizationsDelegates: [
    GlobalWidgetsLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    BeMaterialLocalizations.delegate,
  ],
  supportedLocales: [
    const Locale('be', 'BY')
  ],
  home: ...
)

附录：使用 Dart intl 工具

Appendix: Using the Dart intl tools

在你使用 Dart intl package 进行构建 API 之前，你应该想要了解一下 intl package 的文档。

Before building an API using the Dart intl package you’ll want to review the intl package’s documentation. Here’s a summary of the process for localizing an app that depends on the intl package.

这个 demo app 依赖于一个生成的源文件，叫做 l10n/messages_all.dart，这个文件定义了 app 使用的所有本地化的字符串。

The demo app depends on a generated source file called l10n/messages_all.dart, which defines all of the localizable strings used by the app.

重建 l10n/messages_all.dart 需要 2 步。

Rebuilding l10n/messages_all.dart requires two steps.

1. 在 app 的根目录，使用 lib/main.dart 生成 l10n/intl_messages.arb：

   With the app’s root directory as the current directory, generate l10n/intl_messages.arb from lib/main.dart:

   $ flutter pub run intl_translation:extract_to_arb --output-dir=lib/l10n lib/main.dart
   

   intl_messages.arb 是一个 JSON 格式的文件，每一个入口代表定义在 main.dart 里面的 Intl.message() 方法。 intl_en.arb 和 intl_es.arb 分别作为英语和西班牙语翻译的模板。这些翻译是由你（开发者）来创建的。

   The intl_messages.arb file is a JSON format map with one entry for each Intl.message() function defined in main.dart. This file serves as a template for the English and Spanish translations, intl_en.arb and intl_es.arb. These translations are created by you, the developer.

2. 在 app 的根目录，生成每个 intl_<locale>.arb 文件对应的 intl_messages_<locale>.dart 文件，以及 intl_messages_all.dart 文件，它引入了所有的信息文件。

   With the app’s root directory as the current directory, generate intl_messages_<locale>.dart for each intl_<locale>.arb file and intl_messages_all.dart, which imports all of the messages files:

   $ flutter pub run intl_translation:generate_from_arb \
       --output-dir=lib/l10n --no-use-deferred-loading \
       lib/main.dart lib/l10n/intl_*.arb
   

   DemoLocalizations 类使用生成的 initializeMessages() 方法（该方法定义在 intl_messages_all.dart 文件）来加载本地化的信息，然后使用 Intl.message() 来查阅这些本地化的信息。

   The DemoLocalizations class uses the generated initializeMessages() function (defined in intl_messages_all.dart) to load the localized messages and Intl.message() to look them up.

附录：更新 iOS app 包

Appendix: Updating the iOS app bundle

iOS 应用在 Info.plist 文件当中定义了很多关键应用元数据，其中就包括支持的语言环境，而这个文件是会被打包进应用包里面的。为了配置 app 支持的语言环境，你需要编辑这个文件。

iOS applications define key application metadata, including supported locales, in an Info.plist file that is built into the application bundle. To configure the locales supported by your app, you’ll need to edit this file.

首先，打开你项目的 Xcode 工作区文件 ios/Runner.xcworkspace，在项目导航栏中，打开运行项目的对应运行文件夹下的 Info.plist 文件。

First, open your project’s ios/Runner.xcworkspace Xcode workspace file then, in the Project Navigator, open the Info.plist file under the Runner project’s Runner folder.

下一步，选择 Information Property List 项，从 Editor 菜单中选择 Add Item，然后从弹出菜单中选择 Localizations。

Next, select the Information Property List item, select Add Item from the Editor menu, then select Localizations from the pop-up menu.

选择和展开新创建的 Localizations 项，对于应用需要支持的每个语言环境，你需要添加一个新的项。然后点击 Value 域，从弹出菜单当中选择你想要的语言环境。这个列表应该和 supportedLocales 参数当中的语言列表保持一致。

Select and expand the newly-created Localizations item then, for each locale your application supports, add a new item and select the locale you wish to add from the pop-up menu in the Value field. This list should be consistent with the languages listed in the supportedLocales parameter.

添加完所有支持的语言环境后，保存这个文件。

Once all supported locales have been added, save the file.