dropdown flutter

String dropdownValue = 'One';

@override
Widget build(BuildContext context) {
  return DropdownButton<String>(
    value: dropdownValue,
    icon: const Icon(Icons.arrow_downward),
    iconSize: 24,
    elevation: 16,
    style: const TextStyle(
      color: Colors.deepPurple
    ),
    underline: Container(
      height: 2,
      color: Colors.deepPurpleAccent,
    ),
    onChanged: (String? newValue) {
      setState(() {
        dropdownValue = newValue!;
      });
    },
    items: <String>['One', 'Two', 'Free', 'Four']
      .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      })
      .toList(),
  );
}