app_localizations.dart

import 'dart:async';
import 'dart:convert';

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

class AppLocalizations {
  static const LocalizationsDelegate<AppLocalizations> delegate =
      _AppLocalizationsDelegate();

  final Locale locale;

  // Helper method to keep the code in the widgets concise
  // Localizations are accessed using an InheritedWidget "of" syntax
  Map<String, String> _localizedStrings;

  // Static member to have a simple access to the delegate from the MaterialApp
  AppLocalizations(this.locale);

  Future load() async {
    // Load the language JSON file from the "lang" folder
    String jsonString = await rootBundle.loadString("assets/lang/lang.json");
    Map<String, dynamic> jsonMap = json.decode(jsonString);

    _localizedStrings = jsonMap.map((String key, dynamic value) {
      return MapEntry(key, value["${locale.languageCode}"].toString());
    });
  }

  String translate(String key) => _localizedStrings[key];

  // This method will be called from every widget which needs a localized text
  static AppLocalizations of(BuildContext context) =>
      Localizations.of<AppLocalizations>(context, AppLocalizations);
}

// LocalizationsDelegate is a factory for a set of localized resources
// In this case, the localized strings will be gotten in an AppLocalizations object
class _AppLocalizationsDelegate
    extends LocalizationsDelegate<AppLocalizations> {
  // This delegate instance will never change (it doesn't even have fields!)
  // It can provide a constant constructor.
  static const List<String> _supportedLocales = ["en", "it", "pt","de"];

  const _AppLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) =>
      _supportedLocales.contains(locale.languageCode);

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

  @override
  bool shouldReload(_AppLocalizationsDelegate old) => false;
}