isRtlLanguage method

bool isRtlLanguage ([String languageString ])

Check if a BCP 47 / III languageString indicates an RTL language.

i.e. either:

  • a language code explicitly specifying one of the right-to-left scripts, e.g. "az-Arab", or
  • a language code specifying one of the languages normally written in a right-to-left script, e.g. "fa" (Farsi), except ones explicitly specifying Latin or Cyrillic script (which are the usual LTR alternatives).

The list of right-to-left scripts appears in the 100-199 range in http://www.unicode.org/iso15924/iso15924-num.html, of which Arabic and Hebrew are by far the most widely used. We also recognize Thaana, N'Ko, and Tifinagh, which also have significant modern usage. The rest (Syriac, Samaritan, Mandaic, etc.) seem to have extremely limited or no modern usage and are not recognized. The languages usually written in a right-to-left script are taken as those with Suppress-Script: Hebr|Arab|Thaa|Nkoo|Tfng in http://www.iana.org/assignments/language-subtag-registry, as well as Sindhi (sd) and Uyghur (ug). The presence of other subtags of the language code, e.g. regions like EG (Egypt), is ignored.

Implementation 

static bool isRtlLanguage([String languageString]) {
  var language = languageString ?? Intl.getCurrentLocale();
  if (_lastLocaleCheckedForRtl != language) {
    _lastLocaleCheckedForRtl = language;
    _lastRtlCheck = _rtlLocaleRegex.hasMatch(language);
  }
  return _lastRtlCheck;
}