Date Field input formatter and validator with example

date_input_formatter.dart

import 'package:flutter/services.dart';

class DateInputFormatter extends TextInputFormatter {
  final String separator;
  static const _maxLength = 8;
  DateInputFormatter({required this.separator});
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    // Remove any non-numeric characters from the new value
    String sanitizedValue = newValue.text.replaceAll(RegExp('[^0-9]'), '');

    // If the sanitized value is longer than the max length, truncate it
    if (sanitizedValue.length > _maxLength) {
      sanitizedValue = sanitizedValue.substring(0, _maxLength);
    }

    // Add dashes to the sanitized value at the appropriate positions
    String formattedValue = '';
    for (int i = 0; i < sanitizedValue.length; i++) {
      if (i == 2 || i == 4) {
        formattedValue += separator;
      }
      formattedValue += sanitizedValue[i];
    }

    // If the new value is the same as the formatted value, no need to update the text editing value
    if (newValue.text == formattedValue) {
      return newValue;
    }

    // Otherwise, create and return a new text editing value with the formatted value
    return TextEditingValue(
      text: formattedValue,
      selection: TextSelection.collapsed(offset: formattedValue.length),
    );
  }
}

example.dart

                        TextFormField(
                          controller: idVerificationVm.dobField,
                          inputFormatters: [DateInputFormatter(separator: '/')],
                          validator: (dob) => validateDate(
                            dob,
                            '/',
                            1900,
                            DateTime.now().year - 18,
                          ),
                          keyboardType: TextInputType.number,
                        ),

validator_mixin.dart

mixin ValidatorMixin {

  String? validateDate(
      String? date, String separator, int? earliestYear, int? latestYear) {
    if (date == null || date.trim().isEmpty) {
      return 'Date of Birth is required';
    }

    final components = date.split(separator);
    if (components.length != 3) {
      return 'Enter a valid date';
    }

    final month = int.tryParse(components[1]);
    final day = int.tryParse(components[0]);
    final year = int.tryParse(components[2]);

    if (month == null || month > 12 || month < 1) {
      return 'Date month cannot be ${components[1]}';
    }

    if (year == null) {
      return 'Provide a valid year for this date';
    }

    if (earliestYear != null && year < earliestYear) {
      return 'Year cannot be earlier than $earliestYear';
    }

    if (latestYear != null && year > latestYear) {
      return 'Year cannot be after $latestYear';
    }

    final days = DateUtils.getDaysInMonth(year, month);
    if (day == null || day > days || day < 1) {
      return 'Date day cannot be ${components[0]}';
    }
    return null;
  }

}