[Flutter][Widget] DropDownButton

참고 문서
https://dkswnkk.tistory.com/234

코드

List<String> dropdownList = ['1', '2', '3'];
String selectedDropdown = '1';

  ..
  ..

DropdownButton(
    value: selectedDropdown,
    items: dropdownList.map((String item) {
      return DropdownMenuItem<String>(
        child: Text('$item'),
        value: item,
      );
    }).toList(),
    onChanged: (dynamic value) {
      setState(() {
        selectedDropdown = value;
      });
    },
  ),