Flutter 应用里的国际化

Introduction to localizations in Flutter
Flutter 应用本地化介绍
Advanced topics for further customization
定制的进阶操作
How internationalization in Flutter works
Flutter 里的国际化是如何工作的
Alternative internationalization workflows
其他的国际化方法

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 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.

由于大多数应用程序都是以这种方式编写的，因此该页面主要介绍了使用 MaterialApp 和 CupertinoApp 对 Flutter 应用程序进行本地化所需的概念和工作流程。但是，使用较低级别的 WidgetsApp 类编写的应用程序也可以使用相同的类和逻辑进行国际化。

This page covers concepts and workflows necessary to localize a Flutter application using the MaterialApp and CupertinoApp classes, as most apps are written that way. However, applications written using the lower level WidgetsApp class can also be internationalized using the same classes and logic.

国际化的 app 示例

Sample internationalized apps

如果你想通过阅读已经国际化的 Flutter app 代码来开始的话，这里有两个小例子。第一个例子是一个尽可能简单的实现。第二个例子使用了 intl package 提供的 API 和工具。如果你还不熟悉 Dart 的 intl 包，请查看使用 Dart intl 工具。

If you’d like to start out by reading the code for an internationalized Flutter app, here are two small examples. The first one is intended to be as simple as possible, and the second one uses the APIs and tools provided by the intl package. If Dart’s intl package is new to you, see Using the Dart intl tools.

Flutter 应用本地化介绍

Introduction to localizations in Flutter

本节主要介绍如何对 Flutter 应用进行国际化，以及针对目标平台需要设置的其他内容。

This section provides a tutorial on how to internationalize a Flutter application, along with any additional setup that a target platform might require.

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 或者 CupertinoApp 属性并且添加一个名为 flutter_localizations 的 package。截至到 2020 年 11 月份，这个 package 已经支持大约 78 种语言。

By default, Flutter only provides US English localizations. To add support for other languages, an application must specify additional MaterialApp (or CupertinoApp) properties, and include a package called flutter_localizations. As of November 2020, this package supports 78 languages.

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: # Add this line
    sdk: flutter         # Add this line

下一步，先运行 pub get packages，然后引入 flutter_localizations 库，然后为 MaterialApp 指定 localizationsDelegates 和 supportedLocales：

Next, run pub get packages, then import the flutter_localizations library and specify localizationsDelegates and supportedLocales for MaterialApp:

import 'package:flutter_localizations/flutter_localizations.dart';
// TODO: uncomment the line below after codegen
// import 'package:flutter_gen/gen_l10n/app_localizations.dart';

// ...

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

引入 flutter_localizations package 并添加了上面的代码之后， Material 和 Cupertino 包现在应该被正确地本地化为 78 个受支持的语言环境之一。 widget 应当与本地化信息保持同步，并具有正确的从左到右或从右到左的布局。您可以尝试将目标平台的语言环境切换为阿拉伯语（ar），请注意应该对信息进行本地化，并且此时的 widget 应当以从右到左的布局排列。

After introducing the flutter_localizations package and adding the code above, the Material and Cupertino packages should now be correctly localized in one of the 78 supported locales. Widgets should be adapted to the localized messages, along with correct left-to-right and right-to-left layout. Try switching the target platform’s locale to Arabic (ar) and notice that the messages should be localized and widgets are laid out with right-to-left layout in mind.

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.

添加您自己的本地化信息

Adding your own localized messages

引入 flutter_localizations package 后，请按照以下说明将本地化的文本添加到您的应用程序。

Once the flutter_localizations package is added, use the following instructions to add localized text to your application.

将 intl package 添加到 pubspec.yaml 文件中：

Add the intl package to the pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter
  intl: ^0.16.1    # Add this line

另外，在 pubspec.yaml 文件中，启用 generate 标志。该设置项添加在 pubspec 中 Flutter 部分，通常处在 pubspec 文件中后面的部分。
Also, in the pubspec.yaml file, enable the generate flag. This is added to the section of the pubspec that is specific to Flutter, and usually comes later in the pubspec file.
```
# The following section is specific to Flutter.
flutter:
  generate: true    # Add this line
```
在 Flutter 项目的根目录中添加一个新的 yaml 文件，命名为 l10n.yaml，其内容如下：
Add a new yaml file to the root directory of the Flutter project called l10n.yaml with the following content:
```
arb-dir: lib/l10n
template-arb-file: app_en.arb
output-localization-file: app_localizations.dart
```
该文件用于配置本地化工具；在上面的示例中，指定输入文件在 ${FLUTTER_PROJECT}/lib/l10n 中，app_en.arb 文件提供模板，生成的本地化文件在 app_localizations.dart 文件中。
This file configures the localization tool; in this example, the input files are located in ${FLUTTER_PROJECT}/lib/l10n, the app_en.arb file provides the template, and the generated localizations are placed in the app_localizations.dart file.
在 ${FLUTTER_PROJECT}/lib/l10n 中，添加 app_en.arb 模板文件。如下：
In ${FLUTTER_PROJECT}/lib/l10n, add the app_en.arb template file. For example:
```
{
  "helloWorld": "Hello World!",
  "@helloWorld": {
    "description": "The conventional newborn programmer greeting"
  }
}
```
接下来，在同一目录中添加一个 app_es.arb 文件，对同一条信息做西班牙语的翻译：
Next, add an app_es.arb file in the same directory for Spanish translation of the same message:
```
{
  "helloWorld": "Hola Mundo!"
}
```
要测试本地化工具，可以运行您的应用程序。您将在 ${FLUTTER_PROJECT}/.dart_tool/flutter_gen/gen_l10n 中看到生成的文件。
Now, run your app so that codegen takes place. You should see generated files in ${FLUTTER_PROJECT}/.dart_tool/flutter_gen/gen_l10n.

测试应用中生成的本地化文件内容如下：

Remove the comment for the import statement on app_localizations.dart and AppLocations.delegate in your call to the constructor for MaterialApp. Test the generated localizations in your app as follows:

import 'package:flutter_localizations/flutter_localizations.dart';
// import 'package:flutter_gen/gen_l10n/app_localizations.dart'; // remove the comment for this line

// ...
   
MaterialApp(
  localizationsDelegates: [
    // ... app-specific localization delegate[s] here
    // AppLocalizations.delegate, // remove the comment for this line
    GlobalMaterialLocalizations.delegate,
    // ...
);
// ...

// In your Material/Widget/CupertinoApp:
@override
Widget build(BuildContext context) {
  return MaterialApp(
    localizationsDelegates: AppLocalizations.localizationsDelegates, // Add this line
    supportedLocales: AppLocalizations.supportedLocales, // Add this line
    home: // ...
  );
}

// ...

// Use AppLocalizations anywhere in your app.
// Here, the translated message is used in a Text widget.
Widget build(BuildContext context) {
  // ...
  return Text(AppLocalizations.of(context).helloWorld);
}

如果目标设备的语言环境设置为英语，此代码生成的 Text widget 会展示「Hello World!」。如果目标设备的语言环境设置为西班牙语，则展示「Hola Mundo!」，在 arb 文件中，每个条目的键值都被用作 getter 的方法名称，而该条目的值则表示本地化的信息。

This code generates a Text widget that displays “Hello World!” if the target device’s locale is set to English, and “Hola Mundo!” if the target device’s locale is set to Spanish. In the arb files, the key of each entry is used as the method name of the getter, while the value of that entry contains the localized message.

要查看使用该工具的示例 Flutter 应用，请参阅 gen_l10n_example。

To see a sample Flutter app using this tool, please see gen_l10n_example.

如需本地化设备应用描述，你可以将本地化后的字符串传递给 MaterialApp.onGenerateTitle:

To localize your device app description, you can pass in the localized string into MaterialApp.onGenerateTitle:

  // In your Material/Widget/CupertinoApp:
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      onGenerateTitle: (BuildContext context) => AppLocalizations.of(context).appTitle,
      // ...
    );
  }

有关本地化工具的更多信息，例如处理 DateTime 和复数，请参见国际化用户指南。

For more information about the localization tool, such as dealing with DateTime and handling plurals, see the Internationalization User’s Guide.

iOS 本地化：更新 iOS app bundle

Localizing for iOS: Updating the iOS app bundle

iOS 应用在内置于应用程序包中的 Info.plist 文件中定义了关键的应用程序元数据，其中包括了受支持的语言环境，要配置您的应用支持的语言环境，请按照以下步骤进行操作：

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, use the following instructions:

打开项目的 ios/Runner.xcworkspace Xcode 文件。
Open your project’s ios/Runner.xcworkspace Xcode file.
在 Project Navigator 中，打开 Runner 项目的 Runner 文件夹下的 Info.plist 文件。
In the Project Navigator, open the Info.plist file under the Runner project’s Runner folder.
选择 Information Property List 项。然后从 Editor 菜单中选择 Add Item，接着从弹出菜单中选择 Localizations。
Select the Information Property List item. Then select Add Item from the Editor menu, and select Localizations from the pop-up menu.
选择并展开新创建的 Localizations 项。对于您的应用程序支持的每种语言环境，请添加一个新项，然后从 Value 字段中的弹出菜单中选择要添加的语言环境。该列表应需要与 supportedLocales 参数中列出的语言一致。
Select and expand the newly-created Localizations item. 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.
添加所有受支持的语言环境后，保存文件。
Once all supported locales have been added, save the file.

定制的进阶操作

Advanced topics for further customization

本节介绍自定义本地 Flutter 应用程序的其他方法。

This section covers additional ways to customize a localized Flutter application.

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 likely contains 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.

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);

Specifying the app’s supportedLocales parameter

Although the flutter_localizations library currently supports 78 languages and language variants, only English language translations are available by default. It’s up to the developer to decide exactly which languages to support.

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 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.

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;
       }
       // ...
    );
  }
}

Flutter 里的国际化是如何工作的

How internationalization in Flutter works

本节涵盖了 Flutter 中本地化工作的技术细节，如果你计划使用自定的一套本地化消息，下面的内容会很有帮助。反之则可以跳过本节。

This section covers the technical details of how localizations work in Flutter. If you’re planning on supporting your own set of localized messages, the following content would be helpful. Otherwise, you can skip this section.

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,

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 together an internationalized Flutter 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.

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 个字和词还有日期以及符号的翻译（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 and the date patterns and symbols for the locale.

举个例子，我们将给大家展示如何支持挪威尼诺斯克语。

See the following for an example of how to add support for the Norwegian Nynorsk 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尼诺斯克语数量，我们只翻译了一小部分。

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

这个特定语言环境的 GlobalMaterialLocalizations 子类被称为 NnMaterialLocalizations， LocalizationsDelegate 子类被称为 _NnMaterialLocalizationsDelegate。 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 NnMaterialLocalizations, and the LocalizationsDelegate subclass is _NnMaterialLocalizationsDelegate. The value of NnMaterialLocalizations.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 NnMaterialLocalizations, 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 Nynorsk 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尼诺斯克语字符。

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);
}

语言对应的日期格式和符号需要一并指定。在源码中，它们会以下列形式进行定义：

The date patterns and symbols of the locale will also need to be specified. In the source code, the date patterns and symbols are defined like this:

const nnLocaleDatePatterns = {
  'd': 'd.',
  'E': 'ccc',
  //...
}

const nnDateSymbols = {
  'NAME': 'nn',
  'ERAS': <dynamic>[
    'f.Kr.',
    'e.Kr.',
  ],
  // ...
}

上列内容需要修改以匹配语言的正确日期格式。可惜的是，intl 并不具备数字格式的灵活性，以至于 _NnMaterialLocalizationsDelegate 需要使用现有的语言的格式作为替代方法：

These will need to be modified for the locale to use the correct date formatting. Unfortunately, since the intl library does not share the same flexibility for number formatting, the formatting for an existing locale will have to be used as a substitute in _NnMaterialLocalizationsDelegate:

class _NnMaterialLocalizationsDelegate extends LocalizationsDelegate<MaterialLocalizations> {
  // ...
  @override
  Future<MaterialLocalizations> load(Locale locale) async {
    return SynchronousFuture<MaterialLocalizations>(
      NnMaterialLocalizations(
        localeName: localeName,
        // The `intl` library's NumberFormat class is generated from CLDR data
        // (see https://github.com/dart-lang/intl/blob/master/lib/number_symbols_data.dart).
        // Unfortunately, there is no way to use a locale that isn't defined in
        // this map and the only way to work around this is to use a listed
        // locale's NumberFormat symbols. So, here we use the number formats
        // for 'en_US' instead.
        decimalFormat: intl.NumberFormat('#,##0.###', 'en_US'),
        twoDigitZeroPaddedFormat: intl.NumberFormat('00', 'en_US'),
        //...
      ),
    );
  }
}

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 的语言为尼诺斯克语以及如何给 app 的 localizationsDelegates 列表添加 NnMaterialLocalizations 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 Nynorsk and adds the NnMaterialLocalizations delegate instance to the app’s localizationsDelegates list:

MaterialApp(
  localizationsDelegates: [
    GlobalWidgetsLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    NnMaterialLocalizations.delegate,
  ],
  supportedLocales: [
    const Locale('nn')
  ],
  home: ...
)

其他的国际化方法

Alternative internationalization workflows

本节主要介绍国际化 Flutter 应用的不同方法。

This section describes different approaches to internationalize your Flutter application.

应用程序本地化资源的替代类

An alternative class for the app’s localized resources

之前的 DemoApp 示例应用主要根据 Dart intl package 定义，为了简单起见，或者可能想要与不同的 i18n 框架集成，开发者也可以选择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.

点击查看 minimal 应用的完整源代码。

Complete source code for the minimal app.

在该版本的 DemoApp 中，包含应用程序本地化版本的类 DemoLocalizations 直接在每种语言的 Map 中包括了所有的翻译。

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 minimal 应用中，DemoLocalizationsDelegate 略有不同，它的 load 方法返回一个 SynchronousFuture，因为不需要进行异步加载。

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;
}

附录：使用 Dart intl 工具

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.

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.
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
```
Windows 系统不支持文件名通配符。列出的 .arb 文件是由 intl_translation:extract_to_arb 命令生成的。
Windows does not support file name wildcarding. Instead, list the .arb files that were generated by the intl_translation:extract_to_arb command.
```
$ flutter pub run intl_translation:generate_from_arb \
    --output-dir=lib/l10n --no-use-deferred-loading \
    lib/main.dart \
    lib/l10n/intl_en.arb lib/l10n/intl_fr.arb lib/l10n/intl_messages.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.