class _MyPage2State extends State<MyPage2> {
  String dropdownValue = "One";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: DropdownButton<String>(
          value: dropdownValue,
          onChanged: (String newValue) {
            if (newValue == dropdownValue) {
              print("do nothing");
            } else {
              setState(() {
                dropdownValue = newValue;
              });
            }
          },
          items: <String>['One', 'Two', 'Free', 'Four']
              .map<DropdownMenuItem<String>>((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(
                value,
                style: TextStyle(
                    color: value == dropdownValue ? Colors.grey : Colors.black),
              ),
            );
          }).toList(),
        ),
      ),
    );
  }
}