The Intl class provides a common entry point for internationalization related tasks. An Intl instance can be created for a particular locale and used to create a date format via anIntl.date(). Static methods on this class are also used in message formatting.

Message example:

'''I see ${Intl.plural(num_people,
          {'0': 'no one at all',
           '1': 'one other person',
           'other': '$num_people other people'})} in $place.''''

Usage examples:

 today(date) => Intl.message(
     "Today's date is $date",
     desc: 'Indicate the current date',
     examples: {'date' : 'June 8, 2012'});
 print(today(new Date.now());

 msg({num_people, place}) => Intl.message(
      '''I see ${Intl.plural(num_people,
        {'0': 'no one at all',
         '1': 'one other person',
         'other': '$num_people other people'})} in $place.'''',
     desc: 'Description of how many people are seen as program start.',
     examples: {'num_people': 3, 'place': 'London'});

Calling msg({'num_people': 2, 'place': 'Athens'}); would produce "I see 2 other people in Athens." as output in the default locale.

To use a locale other than the default, use the withLocale function.

  var todayString = new DateFormat("pt_BR").format(new Date.now());
  print(withLocale("pt_BR", () => today(todayString));

See tests/message_format_test.dart for more examples.

class Intl {
  /**
   * String indicating the locale code with which the message is to be
   * formatted (such as en-CA).
   */
  String _locale;

  /** The default locale. This defaults to being set from systemLocale, but
   * can also be set explicitly, and will then apply to any new instances where
   * the locale isn't specified.
   */
  static String _defaultLocale;

  /**
   * The system's locale, as obtained from the window.navigator.language
   * or other operating system mechanism. Note that due to system limitations
   * this is not automatically set, and must be set by importing one of
   * intl_browser.dart or intl_standalone.dart and calling findSystemLocale().
   */
  static String systemLocale = 'en_US';

  /**
   * Return a new date format using the specified [pattern].
   * If [desiredLocale] is not specified, then we default to [locale].
   */
  DateFormat date([String pattern, String desiredLocale]) {
    var actualLocale = (desiredLocale == null) ? locale : desiredLocale;
    return new DateFormat(pattern, actualLocale);
  }

  /**
   * Constructor optionally [aLocale] for specifics of the language
   * locale to be used, otherwise, we will attempt to infer it (acceptable if
   * Dart is running on the client, we can infer from the browser/client
   * preferences).
   */
  Intl([String aLocale]) {
    if (aLocale != null) {
      _locale = aLocale;
    } else {
      _locale = getCurrentLocale();
    }
  }

  /**
   * Returns a message that can be internationalized. It takes a
   * [message_str] that will be translated, which may be interpolated
   * based on one or more variables, a [desc] providing a description of usage
   * for the [message_str], and a map of [examples] for each data element to be
   * substituted into the message. For example, if message="Hello, $name", then
   * examples = {'name': 'Sparky'}. If not using the user's default locale, or
   * if the locale is not easily detectable, explicitly pass [locale].
   * The values of [desc] and [examples] are not used at run-time but are only
   * made available to the translators, so they MUST be simple Strings available
   * at compile time: no String interpolation or concatenation.
   * The expected usage of this is inside a function that takes as parameters
   * the variables used in the interpolated string, and additionally also a
   * locale (optional).
   * Ultimately, the information about the enclosing function and its arguments
   * will be extracted automatically but for the time being it must be passed
   * explicitly in the [name] and [args] arguments.
   */
  static String message(String message_str, {final String desc: '',
      final Map examples: const {}, String locale, String name,
      List<String> args}) {
    return messageLookup.lookupMessage(
        message_str, desc, examples, locale, name, args);
  }

  /**
   * Return the locale for this instance. If none was set, the locale will
   * be the default.
   */
  String get locale => _locale;

  /**
   * Return true if the locale exists, or if it is null. The null case
   * is interpreted to mean that we use the default locale.
   */
  static bool _localeExists(localeName) {
    return DateFormat.localeExists(localeName);
  }

  /**
   * Given [newLocale] return a locale that we have data for that is similar
   * to it, if possible.
   * If [newLocale] is found directly, return it. If it can't be found, look up
   * based on just the language (e.g. 'en_CA' -> 'en'). Also accepts '-'
   * as a separator and changes it into '_' for lookup, and changes the
   * country to uppercase.
   * Note that null is interpreted as meaning the default locale, so if
   * [newLocale] is null it will be returned.
   */
  static String verifiedLocale(String newLocale, Function localeExists,
                               {Function onFailure: _throwLocaleError}) {
    // TODO(alanknight): Previously we kept a single verified locale on the Intl
    // object, but with different verification for different uses, that's more
    // difficult. As a result, we call this more often. Consider keeping
    // verified locales for each purpose if it turns out to be a performance
    // issue.
    if (newLocale == null) return systemLocale;
    if (localeExists(newLocale)) {
      return newLocale;
    }
    for (var each in
        [canonicalizedLocale(newLocale), _shortLocale(newLocale)]) {
      if (localeExists(each)) {
        return each;
      }
    }
    return onFailure(newLocale);
  }

  /**
   * The default action if a locale isn't found in verifiedLocale. Throw
   * an exception indicating the locale isn't correct.
   */
  static String _throwLocaleError(String localeName) {
    throw new ArgumentError("Invalid locale '$localeName'");
  }

  /** Return the short version of a locale name, e.g. 'en_US' => 'en' */
  static String _shortLocale(String aLocale) {
    if (aLocale.length < 2) return aLocale;
    return aLocale.substring(0, 2).toLowerCase();
  }

  /**
   * Return a locale name turned into xx_YY where it might possibly be
   * in the wrong case or with a hyphen instead of an underscore.
   */
  static String canonicalizedLocale(String aLocale) {
    // Locales of length < 5 are presumably two-letter forms, or else malformed.
    // Locales of length > 6 are likely to be malformed. In either case we
    // return them unmodified and if correct they will be found.
    // We treat C as a special case, and assume it wants en_ISO for formatting.
    // TODO(alanknight): en_ISO is probably not quite right for the C/Posix
    // locale for formatting. Consider adding C to the formats database.
    if (aLocale == "C") return "en_ISO";
    if ((aLocale.length < 5) || (aLocale.length > 6)) return aLocale;
    if (aLocale[2] != '-' && (aLocale[2] != '_')) return aLocale;
    var lastRegionLetter = aLocale.length == 5 ? "" : aLocale[5].toUpperCase();
    return '${aLocale[0]}${aLocale[1]}_${aLocale[3].toUpperCase()}'
           '${aLocale[4].toUpperCase()}$lastRegionLetter';
  }

  /**
   * Support method for message formatting. Select the correct plural form from
   * [cases] given [howMany].
   */
  static String plural(var howMany, Map cases, [num offset=0]) {
    // TODO(efortuna): Deal with "few" and "many" cases, offset, and others!
    return select(howMany.toString(), cases);
  }

  /**
   * Format the given function with a specific [locale], given a
   * [message_function] that takes no parameters. The [message_function] can be
   * a simple message function that just returns the result of `Intl.message()`
   * it can be a wrapper around a message function that takes arguments, or it
   * can be something more complex that manipulates multiple message
   * functions.
   *
   * In either case, the purpose of this is to delay calling [message_function]
   * until the proper locale has been set. This returns the result of calling
   * [msg_function], which could be of an arbitrary type.
   */
  static withLocale(String locale, Function message_function) {
    // We have to do this silliness because Locale is not known at compile time,
    // but must be a static variable in order to be visible to the Intl.message
    // invocation.
    if (_defaultLocale == null) _defaultLocale = systemLocale;
    var oldLocale = _defaultLocale;
    _defaultLocale = locale;
    var result = message_function();
    _defaultLocale = oldLocale;
    return result;
  }

  /**
   * Support method for message formatting. Select the correct exact (gender,
   * usually) form from [cases] given the user [choice].
   */
  static String select(String choice, Map cases) {
    if (cases.containsKey(choice)) {
      return cases[choice];
    } else if (cases.containsKey('other')){
      return cases['other'];
    } else {
      return '';
    }
  }

  /**
   * Accessor for the current locale. This should always == the default locale,
   * unless for some reason this gets called inside a message that resets the
   * locale.
   */
  static String getCurrentLocale() {
    if (_defaultLocale == null) _defaultLocale = systemLocale;
    return _defaultLocale;
  }
}