How to use setState in FutureBuilder?

Question

I have a function that accepts data via API:

//fetch data from API
  Future<List<CurrencyModel>?> _fetchCurrency() async {
    currencyList = [];
    final response = await http.get(
      Uri.parse(
          'https:...'),
    );
    if (response.statusCode == 200) {
      List<dynamic> values = [];
      values = json.decode(response.body);
      if (values.isNotEmpty) {
        for (int i = 0; i < values.length; i++) {
          if (values[i] != null) {
            Map<String, dynamic> map = values[i];
            currencyList.add(
              CurrencyModel.fromJson(map),
            );
          }
        }
        setState(() {
          currencyList;
        });
      }
      return currencyList;
    } else {
      throw Exception('Failed to load currencies');
    }
  }

I moved the logic of working with the API into a separate file, created a regular class with the Future function. How now to be with setState which was in Future? Because setState cannot be added to a regular class. How to add it to FutureBuilder? My code:

@override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Currencies'),
        centerTitle: true,
      ),
      body: FutureBuilder(
        future: client.fetchCurrency(),
        builder: (BuildContext context,
            AsyncSnapshot<List<CurrencyModel>?> snapshot) {
          if (snapshot.hasData) {
            return ListView.builder(
              itemCount: currencyList.length,
              itemBuilder: (context, index) => CurrencyCard(
                currencyList[index],
              ),
            );
          } else if (snapshot.hasError) {
            return Text(
              '${snapshot.error}',
            );
          }
          return const Center(
            child: CircularProgressIndicator(),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: startTimer,
        child: const Icon(Icons.update_sharp),
      ),
    );
  }

Answer 1

It is easy:
1 Declare a function to process your list

List<CurrencyModel>? _processOnEnd(List<CurrencyModel>? value){

   //write what you need inside this function

   return value;
}

2 Call it inside then method of your Future

FutureBuilder(
    future: client.fetchCurrency().then(_processOnEnd),