Flutter TextInputFormatter for credit card number and expiry date

credit_card_input_formatters.dart

// Modified from https://youtu.be/4v4l6E8Sbj8?t=1025  (at around 17:05)
import 'package:flutter/services.dart';

/// A [TextInputFormatter] for the credit card number. Adds space after every four characters.
class CreditCardInputFormatter extends TextInputFormatter {
  final separator = ' ';

  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    return textManipulation(
      oldValue,
      newValue,
      textInputFormatter: FilteringTextInputFormatter.digitsOnly,
      formatPattern: (filteredString) {
        var buffer = StringBuffer();
        for (var i = 0; i < filteredString.length; i++) {
          buffer.write(filteredString[i]);
          if ((i + 1) % 4 == 0) {
            buffer.write(separator);
          }
        }
        return buffer.toString();
      },
    );
  }
}

/// A [TextInputFormatter] for the expiry date of a credit card. Adds " / " after the first two characters.
class ExpiryDateInputFormatter extends TextInputFormatter {
  final separator = ' / ';

  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    return textManipulation(
      oldValue,
      newValue,
      textInputFormatter: FilteringTextInputFormatter.digitsOnly,
      formatPattern: (filteredString) {
        return filteredString.length >= 2
            ? filteredString.substring(0, 2) + separator + filteredString.substring(2, filteredString.length)
            : filteredString;
      },
    );
  }
}

TextEditingValue textManipulation(
  TextEditingValue oldValue,
  TextEditingValue newValue, {
  required String Function(String filteredString) formatPattern,
  required TextInputFormatter textInputFormatter,
}) {
  // Remove all invalid characters
  newValue = textInputFormatter.formatEditUpdate(oldValue, newValue);
  var cursorPosition = newValue.selection.end;
  print(cursorPosition);
  final textWithSeparators = formatPattern(newValue.text);

  if (textWithSeparators == newValue.text) {
    return newValue;
  }
  bool isUserInput(String s) => newValue.text.contains(s);

  // Count number of inserted characters in the new string (up until the current cursor position)
  int insertedCharactersCount = 0;
  int initialCharactersCount = 0;

  for (var i = 0; i < textWithSeparators.length && initialCharactersCount < cursorPosition; i++) {
    final character = textWithSeparators[i];
    if (isUserInput(character)) {
      initialCharactersCount++;
    } else {
      insertedCharactersCount++;
    }
  }
  // Adjust cursor position according to number of inserted characters
  cursorPosition += insertedCharactersCount;

  return newValue.copyWith(
    text: textWithSeparators,
    selection: TextSelection.collapsed(offset: cursorPosition),
    composing: TextRange.empty,
  );
}