I've created a form using Flutter which has date picker.

User is supposed to pick his/her date of birth using it to make sure if the user is 16 and above. How do I validate date of birth to age 16?

Here are the parts of the code:

class _WelcomeScreenState extends State<WelcomeScreen> {
  TextEditingController dateinput = TextEditingController();
  final formKey = GlobalKey<FormState>();
  String name = "";

  @override
  void initState() {
    dateinput.text = ""; //set the initial value of text field
    super.initState();
  }

--

GestureDetector(
                child: TextField(
                  style: TextStyle(color: Colors.white),
                  controller:
                      dateinput, //editing controller of this TextField
                  decoration: InputDecoration(
                      labelStyle: TextStyle(color: Colors.white),
                      icon: Icon(Icons.calendar_today),
                      iconColor: Colors.white, //icon of text field
                      labelText: "Enter Date Of Birth" //label text of field
                      ),
                  readOnly:
                      true, //set it true, so that user will not able to edit text
                  onTap: () async {
                    DateTime? pickedDate = await showDatePicker(
                        context: context,
                        initialDate: DateTime.now(),
                        firstDate: DateTime(
                            1900), //DateTime.now() - not to allow to choose before today.
                        lastDate: DateTime(2040));

                    if (pickedDate != null) {
                      print(
                          pickedDate); //pickedDate output format => 2021-03-10 00:00:00.000
                      String formattedDate =
                          DateFormat('dd-MM-yyyy').format(pickedDate);
                      print(
                          formattedDate); //formatted date output using intl package =>  2021-03-16
                      //you can implement different kind of Date Format here according to your requirement

                  setState(() {
                    dateinput.text =
                        formattedDate; //set output date to TextField value.
                  });
                } else {
                  print("Date is not selected");
                }
              },
            ),
          ),

CodePudding user response：

A naive approach would be to just construct a DateTime object from the selected birth date and to then compute DateTime.now().difference(birthDate).inDays / 365. That doesn't account for leap days, and maybe it's close enough, but it's not how a human would compute age.

When attempting to solve a programming problem, one of the first things you usually should ask yourself is: How would you solve this without a computer?

To determine if someone is at least 16 years old, you would take the current date, subtract 16 from the year, use the same month and day¹, and see if their birthday is on or before that date, ignoring the time. So just do that:

extension IsAtLeastYearsOld on DateTime {
  bool isAtLeastYearsOld(int years) {
    var now = DateTime.now();
    var boundaryDate = DateTime(now.year - years, now.month, now.day);

    // Discard the time from [this].
    var thisDate = DateTime(year, month, day);

    // Did [thisDate] occur on or before [boundaryDate]?
    return thisDate.compareTo(boundaryDate) <= 0;
  }
}

void main() {
  // The results below were obtained with 2022-06-11 as the current date.
  print(DateTime(2006, 6, 10).isAtLeastYearsOld(16)); // Prints: true
  print(DateTime(2006, 6, 11).isAtLeastYearsOld(16)); // Prints: true
  print(DateTime(2006, 6, 12).isAtLeastYearsOld(16)); // Prints: false
}

¹ This should be fine even if the current date is a leap day since DateTime will convert February 29 into March 1 for non-leap years.

CodePudding user response：

With a function for calculate :

int calculateAge(DateTime birthDate) {
  DateTime currentDate = DateTime.now();
  int age = currentDate.year - birthDate.year;
  if (birthDate.month > currentDate.month) {
    age--;
  } else if (currentDate.month == birthDate.month) {
    if (birthDate.day > currentDate.day) {
      age--;
    }
  }
  return age;
}