How to properly use freezed in dart/flutter

I want to convert a Json with Freezed package in Dart/Flutter.

my json:

{"rates":{"btc":{"name":"Bitcoin","unit":"BTC","value":1.0,"type":"crypto"},"eth":{"name":"Ether","unit":"ETH","value":18.737,"type":"crypto"},"ltc":{"name":"Litecoin","unit":"LTC","value":747.585,"type":"crypto"},"bch":{"name":"Bitcoin Cash","unit":"BCH","value":194.048,"type":"crypto"},"bnb":{"name":"Binance Coin","unit":"BNB","value":150.81,"type":"crypto"},"eos":{"name":"EOS","unit":"EOS","value":67935.089,"type":"crypto"},"xrp":{"name":"XRP","unit":"XRP","value":95155.791,"type":"crypto"},"xlm":{"name":"Lumens","unit":"XLM","value":453990.358,"type":"crypto"},"link":{"name":"Chainlink","unit":"LINK","value":2609.98,"type":"crypto"},"dot":{"name":"Polkadot","unit":"DOT","value":6656.935,"type":"crypto"},"yfi":{"name":"Yearn.finance","unit":"YFI","value":6.75,"type":"crypto"},"usd":{"name":"US Dollar","unit":"$","value":52306.065,"type":"fiat"},"aed":{"name":"United Arab Emirates Dirham","unit":"DH","value":192111.287,"type":"fiat"},"ars":{"name":"Argentine Peso","unit":"$","value":43620885.106,"type":"fiat"},"aud":{"name":"Australian Dollar","unit":"A$","value":80483.238,"type":"fiat"},"bdt":{"name":"Bangladeshi Taka","unit":"৳","value":5730141.222,"type":"fiat"},"bhd":{"name":"Bahraini Dinar","unit":"BD","value":19714.26,"type":"fiat"},"bmd":{"name":"Bermudian Dollar","unit":"$","value":52306.065,"type":"fiat"},"brl":{"name":"Brazil Real","unit":"R$","value":259961.146,"type":"fiat"},"cad":{"name":"Canadian Dollar","unit":"CA$","value":70827.12,"type":"fiat"},"chf":{"name":"Swiss Franc","unit":"Fr.","value":46186.413,"type":"fiat"},"clp":{"name":"Chilean Peso","unit":"CLP$","value":50204408.056,"type":"fiat"},"cny":{"name":"Chinese Yuan","unit":"¥","value":372408.727,"type":"fiat"},"czk":{"name":"Czech Koruna","unit":"Kč","value":1235495.426,"type":"fiat"},"dkk":{"name":"Danish Krone","unit":"kr.","value":363157.719,"type":"fiat"},"eur":{"name":"Euro","unit":"€","value":48715.515,"type":"fiat"},"gbp":{"name":"British Pound Sterling","unit":"£","value":41686.312,"type":"fiat"},"gel":{"name":"Georgian Lari","unit":"₾","value":138872.604,"type":"fiat"},"hkd":{"name":"Hong Kong Dollar","unit":"HK$","value":409041.28,"type":"fiat"},"huf":{"name":"Hungarian Forint","unit":"Ft","value":18951574.97,"type":"fiat"},"idr":{"name":"Indonesian Rupiah","unit":"Rp","value":816333269.559,"type":"fiat"},"ils":{"name":"Israeli New Shekel","unit":"₪","value":189749.93,"type":"fiat"},"inr":{"name":"Indian Rupee","unit":"₹","value":4343668.469,"type":"fiat"},"jpy":{"name":"Japanese Yen","unit":"¥","value":7848695.164,"type":"fiat"},"krw":{"name":"South Korean Won","unit":"₩","value":69685263.504,"type":"fiat"},"kwd":{"name":"Kuwaiti Dinar","unit":"KD","value":16116.178,"type":"fiat"},"lkr":{"name":"Sri Lankan Rupee","unit":"Rs","value":16348295.799,"type":"fiat"},"mmk":{"name":"Burmese Kyat","unit":"K","value":109628916.276,"type":"fiat"},"mxn":{"name":"Mexican Peso","unit":"MX$","value":893162.687,"type":"fiat"},"myr":{"name":"Malaysian Ringgit","unit":"RM","value":250049.147,"type":"fiat"},"ngn":{"name":"Nigerian Naira","unit":"₦","value":79274550.235,"type":"fiat"},"nok":{"name":"Norwegian Krone","unit":"kr","value":553323.953,"type":"fiat"},"nzd":{"name":"New Zealand Dollar","unit":"NZ$","value":85818.875,"type":"fiat"},"php":{"name":"Philippine Peso","unit":"₱","value":2925609.024,"type":"fiat"},"pkr":{"name":"Pakistani Rupee","unit":"₨","value":14578223.75,"type":"fiat"},"pln":{"name":"Polish Zloty","unit":"zł","value":211703.518,"type":"fiat"},"rub":{"name":"Russian Ruble","unit":"₽","value":4811965.565,"type":"fiat"},"sar":{"name":"Saudi Riyal","unit":"SR","value":196165.112,"type":"fiat"},"sek":{"name":"Swedish Krona","unit":"kr","value":548320.826,"type":"fiat"},"sgd":{"name":"Singapore Dollar","unit":"S$","value":70481.9,"type":"fiat"},"thb":{"name":"Thai Baht","unit":"฿","value":1891095.47,"type":"fiat"},"try":{"name":"Turkish Lira","unit":"₺","value":1608285.988,"type":"fiat"},"twd":{"name":"New Taiwan Dollar","unit":"NT$","value":1640867.488,"type":"fiat"},"uah":{"name":"Ukrainian hryvnia","unit":"₴","value":1992749.746,"type":"fiat"},"vef":{"name":"Venezuelan bolívar fuerte","unit":"Bs.F","value":5237.406,"type":"fiat"},"vnd":{"name":"Vietnamese đồng","unit":"₫","value":1279502893.807,"type":"fiat"},"zar":{"name":"South African Rand","unit":"R","value":994222.714,"type":"fiat"},"xdr":{"name":"IMF Special Drawing Rights","unit":"XDR","value":39393.476,"type":"fiat"},"xag":{"name":"Silver - Troy Ounce","unit":"XAG","value":2311.686,"type":"commodity"},"xau":{"name":"Gold - Troy Ounce","unit":"XAU","value":26.191,"type":"commodity"},"bits":{"name":"Bits","unit":"μBTC","value":1000000.0,"type":"crypto"},"sats":{"name":"Satoshi","unit":"sats","value":100000000.0,"type":"crypto"}}}

my model with freezed package:

import 'package:freezed_annotation/freezed_annotation.dart';

part 'exchange_rates_response.freezed.dart';
part 'exchange_rates_response.g.dart';

/// {@macro ExchangeRates}
@Freezed(toJson: false, fromJson: false)
class ExchangeRatesResponse with _$ExchangeRatesResponse {
  /// {@macro ExchangeRates}
  factory ExchangeRatesResponse({List<RatesResponse>? rates}) =
      _ExchangeRatesResponse;

  /// {@macro ExchangeRates}
  factory ExchangeRatesResponse.fromJson(Map<String, dynamic> json) {
    var ratesMap = json['rates'] as Map<String, dynamic>;
    var ratesValues = ratesMap.values;
    List<RatesResponse> rates =
        List.from(ratesValues.map((e) => RatesResponse.fromJson(e)).toList());

    return ExchangeRatesResponse(rates: rates);
  }
}

/// {@macro ExchangeRates}
@Freezed(toJson: false)
class RatesResponse with _$RatesResponse {
  /// {@macro ExchangeRates}
  const factory RatesResponse({
    String? name,
    String? unit,
    double? value,
    String? type,
  }) = _RatesResponse;

  /// {@macro ExchangeRates}
  factory RatesResponse.fromJson(Map<String, dynamic> json) =>
      _$RatesResponseFromJson(json);
}

my model ExchangeRatesResponse works fine. but i want to know is there a way to use freezed package in ExchangeRatesResponse.fromJson function instead of manually convert it

Solution

Create an implementation of JsonConverter and annotate either the corresponding field or the containing class

class ExchangeRateConverter implements JsonConverter<List<RatesResponse>?, Map<String, dynamic>> {
  const ExchangeRateConverter();

  @override
  List<RatesResponse>? fromJson(Map<String, dynamic> json) {
    var ratesValues = json.values;
    List<RatesResponse> rates =
        List.from(ratesValues.map((e) => RatesResponse.fromJson(e)).toList());
    return rates;
  }

  @override
  Map<String, dynamic> toJson(List<RatesResponse>? object) => {};
}

factory ExchangeRatesResponse({
    @ExchangeRateConverter() List<RatesResponse>? rates,
  }) = _ExchangeRatesResponse;