Flutter国际化插件i18n的使用

简介

i18n 是一个用于Dart和Flutter的简单国际化包。此包支持热重载，并已在最新的Flutter版本上进行了测试。你可以将消息写入YAML文件，让这个包自动生成方便使用的Dart类。

概述

YAML文件到Dart类的转换

示例：YAML文件

lib/messages.i18n.yaml

button:
  save: Save
  load: Load
users:
  welcome(String name): "Hello $name!"
  logout: Logout

生成的Dart类

class Messages {
    const Messages();
    ButtonMessages get button => ButtonExampleMessages(this);
    UsersMessages get users => UsersExampleMessages(this);
}

class ButtonMessages {
    final Messages _parent;
    const ButtonMessages(this._parent);
    String get save => "Save";
    String get load => "Load";
}

class UsersMessages {
    final Messages _parent;
    const UsersMessages(this._parent);
    String get logout => "Logout";
    String welcome(String name) => "Hello $name!";
}

使用示例

Messages m = Messages();
print(m.users.welcome('World'));
// 输出: Hello World!

动机与目标

• 国际化应该在编译时进行检查。
• 消息键应该是方法而不是任意字符串。
• 支持参数化消息。
• 消息应按主题分组。
• 利用Dart的字符串插值功能。
• 支持build_runner和代码生成。

解决方案

写入YAML文件

默认消息

messages.i18n.yaml (默认消息):

generic:
  ok: OK
  done: DONE
invoice:
  create: Create invoice
  delete: Delete invoice

德语翻译

messages_de.i18n.yaml (_de = German translation)

generic:
  ok: OK
  done: ERLEDIGT
invoice:
  create: Rechnung erstellen
  delete: Rechnung löschen

使用生成的类

Messages m = Messages();
print(m.generic.ok); // 输出: OK
print(m.generic.done); // 输出: DONE

m = Messages_de();
print(m.generic.ok); // 输出: OK
print(m.generic.done); // 输出: ERLEDIGT

参数和复数形式

示例：带有参数的消息

invoice:
  create: Create invoice
  delete: Delete invoice
  help: "Use this function
  to generate new invoices and stuff.
  Awesome!"
  count(int cnt): "You have created $cnt ${_plural(cnt, one:'invoice', many:'invoices')}."
apples:
  _apples(int cnt): "${_plural(cnt, one:'apple', many:'apples')}"
  count(int cnt): "You have eaten $cnt ${_apples(cnt)}."

生成的类

class Messages {
    const Messages();
    InvoiceMessages get invoice => InvoiceExampleMessages(this);        
    ApplesMessages get apples => ApplesExampleMessages(this);
}

class InvoiceMessages {
    final Messages _parent;
    const InvoiceMessages(this._parent);
    String get create => "Create invoice";
    String get help => "Use this function to generate new invoices and stuff. Awesome!";
    String get delete => "Delete invoice";
    String count(int cnt) => "You have created $cnt ${_plural(cnt, one:'invoice', many:'invoices')}.";
}

class ApplesMessages {
    final Messages _parent;
    const ApplesMessages(this._parent);
    String _apples(int cnt) => "${_plural(cnt, one:'apple', many:'apples')}";
    String count(int cnt) => "You have eaten $cnt ${_apples(cnt)}.";
}         

复数函数

String _plural(int count, {String zero, String one, String two, String few, String many, String other})

String _cardinal(int count, {String zero, String one, String two, String few, String many, String other})

String _ordinal(int count, {String zero, String one, String two, String few, String many, String other})

如何使用生成的类

选择使用哪种翻译（如 Messages_de 或 Messages_hu）取决于你。该包仅生成消息类。

示例代码

import 'messages.i18n.dart';
import 'messages_de.i18n.dart' as de;

void main() async {
  Messages m = Messages();
  print(m.apples.count(1));
  print(m.apples.count(2));
  print(m.apples.count(5));

  m = de.Messages_de(); // ExampleMessages_de 继承自 Messages
  print(m.apples.count(1));
  print(m.apples.count(2));
  print(m.apples.count(5));    
}

如何在Flutter中使用

创建YAML文件

lib/messages/foo.i18n.yaml

修改 pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  i18n: any
  ...

dev_dependencies:
  build_runner: any
  flutter_test:
    sdk: flutter

运行生成器

flutter packages pub run build_runner watch

导入并使用生成的消息

import 'packages:my_app/messages/foo.i18n.dart'

...

Foo m = Foo();
return Text(m.bar);
...

自定义复数形式

目前，该包仅正确处理英语和捷克语的复数形式。你可以通过实现自己的语言规则来扩展支持。

示例：注册德语复数规则

i18n.registerResolver('de', (int count, i18n.QuantityType type) {
  if (type == i18n.QuantityType.cardinal && count == 1) {
    return i18n.QuantityCategory.one;
  }
  return i18n.QuantityCategory.other;
});

参考链接

• CLDR Plural Rules
• Unicode Language Plural Rules

希望这些信息能帮助你在Flutter项目中顺利实现国际化！如果有任何问题或需要进一步的帮助，请随时提问。