Simple i18n strings mapper

i18n-translation.dart

class Translation {
  // "zh-a-b-c" -> m"-c"
  static final _localeSuffixRE = RegExp(r"-[^-]+$");

  final bool localeCaseSensitive;
  var _currentLocale = "en";
  var _fallbackLocale = "en";
  Map<String, Map<String, String>> dict = Map();

  Translation({
    this.localeCaseSensitive = false,
    Map<String, Map<String, String>>? dict,
  }) {
    this.dict = dict ?? Map();
  }

  Translation clone() => Translation(dict: dict, localeCaseSensitive: localeCaseSensitive)
    ..setLocale(_currentLocale)
    ..setFallbackLocale(_fallbackLocale);

  void setLocale(String hyphenLocale) => _currentLocale = hyphenLocale;

  void setFallbackLocale(String hyphenLocale) => _fallbackLocale = hyphenLocale;

  String getLocale() => _currentLocale;

  String getFallbackLocale() => _fallbackLocale;

  /// Install a localized dictionary
  install(String hyphenLocale, Map<String, String> trs) {
    if (!localeCaseSensitive) hyphenLocale = hyphenLocale.toLowerCase();
    final existsTranslations = dict[hyphenLocale] ?? {};
    dict[hyphenLocale] = Map.fromEntries([
      ...existsTranslations.entries,
      ...trs.entries,
    ]);
  }

  /// Translate nullable
  String? tr(String key) => find(_currentLocale, key) ?? find(_fallbackLocale, key);

  /// Translate or fallback or key
  String tror(String key, [String? fallback]) => tr(key) ?? fallback ?? key;

  /// | Translations            | Key          | Result       | Reason        |
  /// | ----------------------- | ------------ | ------------ | ------------- |
  /// | `{ zh-CN }`             | `zh-CN`      | `zh-CN`      | Exact         |
  /// | `{ zh }`                | `zh-CN`      | `zh`         | StartsWith    |
  /// | `{ zh-CN, zh-HK }`      | `zh`         | `zh-CN`      | FirstKey      |
  /// | `{ zh-Hant, zh-HK }`    | `zh-SG`      | `zh-Hant`    | FirstKey      |
  /// | `{ zh-Hant-HK, zh-CN }` | `zh-Hant`    | `zh-Hant-HK` | StartsWith    |
  /// | `{ zh-Hant, zh-CN }`    | `zh-Hant-HK` | `zh-Hant`    | 2x StartsWith |
  String? find(String hyphenLocale, String key) {
    if (!localeCaseSensitive) hyphenLocale = hyphenLocale.toLowerCase();
    if (dict[hyphenLocale]?[key] != null) {
      return dict[hyphenLocale]![key];
    } else if (_localeSuffixRE.hasMatch(hyphenLocale)) {
      return find(hyphenLocale.replaceAll(_localeSuffixRE, ""), key);
    } else {
      final foundKeys = dict.keys.where((key) => key.startsWith(hyphenLocale));
      return foundKeys.isNotEmpty ? (dict[foundKeys.first]?[key]) : null;
    }
  }
}

translation_test.dart

void main() {
  
  test("install merge", () {
    final trs = Translation()
      ..install("en-US", {"hello": "hello"})
      ..install("en-US", {"color": "colour"})
      ..install("en-US", {"color": "color"});
    expect(
      "${trs.tr("hello")},${trs.tr("color")}",
      "hello,color",
    );
  });
  
  test("partial fallback", () {
    final trs = Translation()
      ..install("en-UK", {"color": "colour"})
      ..install("en", {"hello": "hello"})
      ..setLocale("en-UK");
    expect(trs.tr("hello"), "hello");
  });
  
  test("fallback locale", () {
    final trs = Translation()
      ..install("zh-CN", {"yanse": "颜色"})
      ..install("en-UK", {"yanse": "colour"})
      ..setLocale("rn-RU")
      ..setFallbackLocale("en");
    expect(trs.tr("yanse"), "colour");
  });
}