Flutter 下拉 API 来电

Question

error:

Exception: type 'List<dynamic>' is not a subtype of type 'List<DropdownMenuItem<int>>?'

Dropdown:

FutureBuilder(
future: customerServices.fetchApiFromUrl(),

builder:
    (BuildContext context, AsyncSnapshot snapshot) {
  if (!snapshot.hasData) {
    return const Center(
        child: CircularProgressIndicator());
  } else {
    try {
      final snapshotData = snapshot.data;
      return DropdownButton(
        borderRadius: BorderRadius.circular(15),
        value: dropdownValue,
        icon: const Icon(
          Icons.arrow_downward,
          color: Colors.white,
        ),
        elevation: 16,
        style: const TextStyle(color: Colors.black),
        underline: Container(
          height: 2,
          color: Colors.white,
        ),
        onChanged: (int? newValue) {},
        items: snapshotData.map((Customer customer) {
          return DropdownMenuItem(child: Text(customer.first_name!), value: customer.id);
        }).toList()
      );
    } catch (e) {
      throw Exception(e);
    }
  }
},

),

API Call

Future fetchApiFromUrl() async {
    final SharedPreferences sharedPreferences =
        await SharedPreferences.getInstance();
   
    List<Customer> data = CustomerListResponse.fromJson(jsonDecode(response.body)).results!;
 
    if (response.statusCode == 200) {
      if (kDebugMode) {
        log("res ${(data)}");
      }
      return data;
    }
  }

Answer 1

It seems related to the type it infers from your snapshot.map().toList() .

Try giving explicit type to it as snapshot.map<DropdownMenuItem<int>>().toList() , see the below code for this:

return DropdownButton(
        ...
        onChanged: (int? newValue) {},
        items: snapshotData.map<DropdownMenuItem<int>>((Customer customer) {
          return DropdownMenuItem(child: Text(customer.first_name!), value: customer.id);
        }).toList()
      );

You can also try changing the onChanged() to dynamic by not typecasting the parameter as:

onChanged: (newValue) {},

As the type of onChanged() is void Function(dynamic), see the below image:

Sure, it does work by typecasting it but you can try this as well as the version I use, I can't.