localization example

localization.dart

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

const appSupportedLocales = <Locale>[
  Locale('ru'),
  Locale('en'),
];

class AppLocalisationDelegate extends LocalizationsDelegate<AppLocalizations> {
  @override
  bool isSupported(Locale locale) {
    return appSupportedLocales.contains(locale);
  }

  @override
  bool shouldReload(LocalizationsDelegate old) {
    return false;
  }

  @override
  Future<AppLocalizations> load(Locale locale) async {
    final appLocalisation = AppLocalizations(locale);
    await appLocalisation.load();
    return appLocalisation;
  }
}

class AppLocalizations {
  final Locale locale;
  Map keys;

  AppLocalizations(this.locale);

  Map<String, String> flatten(Map map, {String prefix}) {
    final output = Map<String, String>();

    map.forEach((key, value) {
      final keyWithPrefix = prefix != null ? '$prefix.$key' : key;
      if (value is Map) {
        output.addAll(flatten(value, prefix: keyWithPrefix));
      } else {
        output[keyWithPrefix] = value;
      }
    });

    return output;
  }

  Future load() async {
    final data =
        await rootBundle.loadString('assets/i18n/${locale.languageCode}.json');
    final dataDecoded = json.decode(data);
    keys = flatten(dataDecoded);
  }

  static AppLocalizations of(BuildContext context) {
    return Localizations.of<AppLocalizations>(context, AppLocalizations);
  }

  String translate(String key) {
    final text = keys[key];
    return text != null ? text : key;
  }
}

String translate(BuildContext context, key) =>
    Localizations.of<AppLocalizations>(context, AppLocalizations)
        .translate(key);