i18n

i18n

Simple internationalization package for Dart and Flutter. This package now supports hot reload and is tested on latest versions of Flutter.

Overview

Write your messages into YAML files, and let this package generate convenient Dart classes from those files.

Turn this YAML file:

lib/messages.i18n.yaml

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

Into these generated Dart classes:

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!";
}

... and use them in your code - plain and simple.

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

Package is an extension (custom builder) for build_runner (Dart standard for source generation) and it can be used with Flutter, AngularDart or any other type of Dart project.

Motivation and goals

• The official Dart/Flutter approach to i18n seems to be ... complicated and kind of ... heavyweight.
• I would like my messages to be checked during compile time. Is that message really there?
• Key to the localized message shouldn't be just some arbitrary String, it should be a getter method!
• And if the message takes some parameters, the method should take those parameters!
• I like to bundle messages into thematic groups, the i18n tool should support that and help me with it.
• Dart has awesome string interpolation, I want to leverage that!
• I like build_runner and code generation.

Solution

Write your messages into a YAML file:

messages.i18n.yaml (default messages):

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

Write your translations into other YAML files:

messages_de.i18n.yaml (_de = German translation)

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

... run the webdev tool, or build_runner directly, and use your messages like this:

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

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

Parameters and pluralization

The implementation is VERY straightforward, which allows you to do all sorts of crazy stuff:

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)}."

Now see the generated classes:

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)}.";
}         

See how you can reuse the pluralization of _apples(int cnt)? (nice!)

There are three functions you can use in your message:

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})

_plural and _cardinal do the same. I just felt that _plural is a little bit less scary name :-)

We need only two forms of the word "apple" in English. "Apple" (one) and "apples" (many). But in some languages like Czech, we need three:

apples:
  _apples(int cnt): "${_plural(cnt, one:'jablko', few: 'jablka', many:'jablek')}"

See also:

• http://cldr.unicode.org/index/cldr-spec/plural-rules
• https://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html

How to use generated classes

How to decide what translation to use (ExampleMessages_de?, ExampleMessages_hu?) is up to you. The package simply generates message classes, that's all.

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(); // see? ExampleMessages_cs extends ExampleMessages
  print(m.apples.count(1));
  print(m.apples.count(2));
  print(m.apples.count(5));    
}    

Where and how to store instances of these message classes - again, up to you. I would consider ScopedModel for Flutter and registering messages instance into dependency injection in AngularDart.

But in this case a singleton would be acceptable also.

How to use with Flutter

Create YAML file with your messages, for example:

lib/messages/foo.i18n.yaml

Add build_runner as a dev_dependency and i18n as a dependency to pubspec.yaml:

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

dev_dependencies:
  build_runner: any
  flutter_test:
    sdk: flutter

Open a terminal and in the root of your Flutter project run:

flutter packages pub run build_runner watch

... and keep it running. Your message classes will appear next to YAML files and will be rebuilt automatically each time you change the source YAML.

For one-time (re)build of your messages run:

flutter packages pub run build_runner build

Import generated messages and use them:

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

...

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

How to use with AngularDart

You are probably using webdev tool already, so you just need to add i18n as a dependency and that's all.

Custom pluralization

The package can correctly decide between 'one', 'few', 'many', etc. only for English and Czech (for now). But you can easily plug your own language, see example/main.dart and Czech and English implementation.

If you implement support for your language, please let me know, I'll gladly embed it into the package.