Intl

This package provides internationalization and localization facilities, including message translation, plurals and genders, date/number formatting and parsing, and bidirectional text.

General

The most important library is intl. It defines the Intl class, with the default locale and methods for accessing most of the internationalization mechanisms. This library also defines the DateFormat, NumberFormat, and BidiFormatter classes.

Current locale

The package has a single current locale, called defaultLocale. Operations will use that locale unless told to do otherwise.

To set the global locale, you can explicitly set it, e.g.

  Intl.defaultLocale = 'pt_BR';

or get it from the browser by

  import "package:intl/intl_browser.dart";
  ...
  findSystemLocale().then(runTheRestOfMyProgram);

To temporarily override the current locale, pass the operation to withLocale.

  Intl.withLocale('fr', () => print(myLocalizedMessage());

To override it for a very specific operation you can create a format object in a specific locale, or pass in the locale as a parameter to methods.

  var format = new DateFormat.yMd("ar");
  var dateString = format.format(new DateTime.now());
  print(myMessage(dateString, locale: 'ar');

Initialization

All the different types of locale data require an async initialization step to make sure the data is available. This reduces the size of the application by only loading the data that is actually required. However, deferred loading does not yet work for multiple libraries, so currently all the code will be included anyay, increasing the code size in the short term.

Each different area of internationalization (messages, dates, numbers) requires a separate initialization process. That way, if the application only needs to format dates, it doesn't need to take the time or space to load up messages, numbers, or other things it may not need.

With messages, there is also a need to import a file that won't exist until the code generation step has been run. This can be awkward, but can be worked around by creating a stub messages_all.dart file, running an empty translation step, or commenting out the import until translations are available. See "Extracting and Using Translated Messages"

Messages

Messages to be localized are written as functions that return the result of an Intl.message call.

  String continueMessage() => Intl.message(
      "Hit any key to continue",
      name: "continueMessage",
      args: [],
      desc: "Explains that we will not proceed further until "
          "the user presses a key");
  print(continueMessage());

This provides, in addition to the basic message string, a name, a description for translators, the arguments used in the message, and examples. The name and args parameters are required, and must match the name and arguments list of the function. In the future we hope to have these provided automatically.

This can be run in the program before any translation has been done, and will just return the message string. It can also be extracted to a file and then be made to return a translated version without modifying the original program. See "Extracting Messages" below for more details.

The purpose of wrapping the message in a function is to allow it to have parameters which can be used in the result. The message string is allowed to use a restricted form of Dart string interpolation, where only the function's parameters can be used, and only in simple expressions. Local variables cannot be used, and neither can expressions with curly braces. Only the message string can have interpolation. The name, desc, args, and examples must be literals and not contain interpolations. Only the args parameter can refer to variables, and it should list exactly the function parameters. If you are passing numbers or dates and you want them formatted, you must do the formatting outside the function and pass the formatted string into the message.

  greetingMessage(name) => Intl.message(
      "Hello $name!",
      name: "greetingMessage",
      args: [name],
      desc: "Greet the user as they first open the application",
      examples: {'name': "Emily"});
  print(greetingMessage('Dan'));

There is one special class of complex expressions allowed in the message string, for plurals and genders.

  remainingEmailsMessage(int howMany, String userName) => 
    Intl.message(
      "${Intl.plural(howMany,
          zero: 'There are no emails left for $userName.',
          one: 'There is one email left for $userName.',
          other: 'There are $howMany emails left for $userName.')}",
    name: "remainingEmailsMessage",
    args: [howMany, userName],
    desc: "How many emails remain after archiving.",
    examples: {'number': 42, 'userName': 'Fred'});

  print(remainingEmailsMessage(1, "Fred"));

However, since the typical usage for a plural or gender is for it to be at the top-level, we can also omit the Intl.message call and provide its parameters to the Intl.plural call instead.

  remainingEmailsMessage(int howMany, String userName) => 
    Intl.plural(
      howMany,
      zero: 'There are no emails left for $userName.',
      one: 'There is one email left for $userName.',
      other: 'There are $howMany emails left for $userName.'),
      name: "remainingEmailsMessage",
      args: [howMany, userName],
      desc: "How many emails remain after archiving.",
      examples: {'number': 42, 'userName': 'Fred'});

Similarly, there is an Intl.gender message, and plurals and genders can be nested.

  notOnlineMessage(String userName, String userGender) => 
    Intl.gender(
      userGender,
      male: '$userName is unavailable because he is not online.',
      female: '$userName is unavailable because she is not online.',
      other: '$userName is unavailable because they are not online'),
      name: "notOnlineMessage",
      args: [userName, userGender],
      desc: "The user is not available to hangout.",
      examples: {{'userGender': 'male', 'userName': 'Fred'},
          {'userGender': 'female', 'userName' : 'Alice'}});

Extracting And Using Translated Messages

When your program contains messages that need translation, these must be extracted from the program source, sent to human translators, and the results need to be incorporated. This is still work in progress, and the extraction is done to a custom JSON format that is not supported by translation tools. We intend to support one or more actual translation file formats.

To extract messages, run the pkg/intl/test/extract_to_json.dart program.

  dart extract_to_json.dart --output-dir=target/directory
      my_program.dart more_of_my_program.dart

This will produce a file intl_messages.json with the messages from all of these programs. This is in a simple JSON format with a map from message names to message strings.

The reverse step expects to receive a series of files, one per locale. These consist of a map with the entry for "_locale" indicating the locale, and with the function name mapped to the translated string. However, plurals and genders are currently represented in an opaque form, by serializing the internal objects that represent them. You can see the generation of this code in the make_hardcoded_translation.dart test file.

If you manage to create such a set of input files, then you can run

  dart generate_from_json.dart --generated_file_prefix=<prefix> 
      <my dart files> <translated json files>

This will generate Dart libraries, one per locale, which contain the translated versions. Your Dart libraries can import the primary file, named <prefix>messages_all.dart, and then call the initialization for a specific locale. Once that's done, any Intl.message calls made in the context of that locale will automatically print the translated version instead of the original.

  import "my_prefix_messages_all.dart";
  ...
  initializeMessages("dk").then(printSomeMessages);

Once the future returned from the initialization call returns, the message data is available.

Number Formatting and Parsing

To format a number, create a NumberFormat instance.

  var f = new NumberFormat("###.0#", "en_US");
  print(f.format(12.345));
    ==> 12.34

The locale parameter is optional. If omitted, then it will use the current locale. The format string is as described in NumberFormat

It's also possible to access the number symbol data for the current locale, which provides information as to the various separator characters, patterns, and other information used for formatting, as

  f.symbols

Current known limitations are that the currency format will only print the name of the currency, and does not support currency symbols, and that the scientific format does not really agree with scientific notation. Number parsing is not yet implemented.

Note that before doing any number formatting for a particular locale you must load the appropriate data by calling

  import 'package:intl/number_symbols_data_local.dart';
  ...
  initializeNumberFormatting(localeName, null).then(formatNumbers);

Once the future returned from the initialization call returns, the formatting data is available. Note that right now this includes all the data for a locales. We expect to make this use deferred loading to reduce code size.

Date Formatting and Parsing

To format a DateTime, create a DateFormat instance. These can be created using a set of commonly used skeletons taken from ICU/CLDR or using an explicit pattern. For details on the supported skeletons and patterns see DateFormat.

  new DateFormat.yMMMMEEEEd().format(aDateTime);
    ==> 'Wednesday, January 10, 2012'
  new DateFormat("EEEEE", "en_US").format(aDateTime);
    ==> 'Wednesday'
  new DateFormat("EEEEE", "ln").format(aDateTime);
    ==> 'mokɔlɔ mwa mísáto'

You can also parse dates using the same skeletons or patterns.

    new DateFormat.yMd("en_US").parse("1/10/2012");
    new DateFormat("Hms", "en_US").parse('14:23:01');

Skeletons can be combined, the main use being to print a full date and time, e.g.

    new DateFormat.yMEd().add_jms().format(new DateTime.now());
      ==> 'Thu, 5/23/2013 10:21:47 AM'

Known limitations: Time zones are not yet supported. Dart DateTime objects don't have a time zone, so are either local or UTC. Formatting and parsing Durations is not yet implemented.

Note that before doing any DateTime formatting for a particular locale, you must load the appropriate data by calling.

    import 'package:intl/date_symbol_data_local.dart';
    ...
    initializeDateFormatting("de_DE", null).then(formatDates);

Once the future returned from the initialization call returns, the formatting data is available.

There are other mechanisms for loading the date formatting data implemented, but we expect to deprecate those in favor of having the data in a library as in the above, and using deferred loading to only load the portions that are needed. For the time being, this will include all of the data, which will increase code size.

Bidirectional Text

The class BidiFormatter provides utilities for working with Bidirectional text. We can wrap the string with unicode directional indicator characters or with an HTML span to indicate direction. The direction can be specified with the RTL and LTR constructors, or detected from the text.

    new BidiFormatter.RTL().wrapWithUnicode('xyz');
    new BidiFormatter.RTL().wrapWithSpan('xyz');

Libraries

date_symbol_data

Date/time formatting symbols for all locales.

date_symbol_data_json

This file should be imported, along with date_format.dart in order to read locale data from files in the file system.

date_symbol_data_json

This file should be imported, along with date_format.dart in order to read locale data from files in the file system.

date_symbols
date_time_patterns

Date/time formatting symbols for a large subset of locales.

extract_messages

This is for use in extracting messages from a Dart program using the Intl.message() mechanism and writing them to a file for translation. This provides only the stub of a mechanism, because it doesn't define how the file should be written. It provides an IntlMessage class that holds the extracted data and parseString and parseFile methods which can extract messages that conform to the expected pattern:

generate_localized

This provides utilities for generating localized versions of messages. It does not stand alone, but expects to be given TranslatedMessage objects and generate code for a particular locale based on them.

intl

This library provides internationalization and localization. This includes message formatting and replacement, date and number formatting and parsing, and utilities for working with Bidirectional text.

intl_browser

This provides facilities for Internationalization that are only available when running in the web browser. You should import only one of this or intl_standalone.dart. Right now the only thing provided here is the ability to find the default locale from the browser.

intl_standalone

This provides facilities for Internationalization that are only available when running standalone. You should import only one of this or intl_browser.dart. Right now the only thing provided here is finding the operating system locale.

message_lookup_by_library

Message/plural format library with locale support. This can have different implementations based on the mechanism for finding the localized versions of messages. This version expects them to be in a library named e.g. 'messages_en_US'. The prefix is set in the "initializeMessages" call, which must be made for a locale before any lookups can be done.

number_symbol_data

Date/time formatting symbols for all locales.

number_symbols