How to implement dropdownbutton into a function in flutter

Im working on a company project. But I can't simply get the idea of how to implement a basic dropdown button into a function but I can't seem to make the values change in the dropdown function what do you think im doing wrong here's my code:

Widget buildDropdownField({
  required String dropdownHeader,
  required String dropdownValue,
}) {
  return Column(
    children: <Widget>[
      Text(dropdownHeader),
      const SizedBox(
        height: 10,
      ),
      //dropdownField
      DropdownButton<String>(
        value: dropdownValue,
        icon: const Icon(Icons.arrow_downward),
        elevation: 16,
        style: const TextStyle(color: Colors.deepPurple),
        underline: Container(
          height: 2,
          color: Colors.deepPurpleAccent,
        ),
        onChanged: (String? newValue) {
          setState(() {
            dropdownValue = newValue!;
          });
        },
        items: <String>['-', 'Geçti', 'Kaldı', 'Belirsiz']
            .map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
      )
    ],
  );
}

Wrap with StatefulBuilder it will work.

Widget buildDropdownField(
      {required String dropdownHeader, required String dropdownValue}) {
    return Column(
      children: <Widget>[
        Text(dropdownHeader),
        const SizedBox(
          height: 10,
        ),
        StatefulBuilder(
          builder: (_, setDropState) {
            return DropdownButton<String>(
              value: dropdownValue,
              icon: const Icon(Icons.arrow_downward),
              elevation: 16,
              style: const TextStyle(color: Colors.deepPurple),
              underline: Container(
                height: 2,
                color: Colors.deepPurpleAccent,
              ),
              onChanged: (String? newValue) {
                setDropState(() {
                  dropdownValue = newValue!;
                });
              },
              items: <String>['-', 'Geçti', 'Kaldı', 'Belirsiz']
                  .map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value),
                );
              }).toList(),
            );
          },
        )
      ],
    );
  }