intaleq/intaleq_driver/lib/controller/functions/digit_obsecur_formate.dart

import 'package:flutter/services.dart';

class DigitObscuringFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    final maskedText = maskDigits(newValue.text);
    return newValue.copyWith(
        text: maskedText,
        selection: updateCursorPosition(maskedText, newValue.selection));
  }

  String maskDigits(String text) {
    final totalDigits = text.length;
    final visibleDigits = 4;
    final hiddenDigits = totalDigits - visibleDigits * 2;

    final firstVisibleDigits = text.substring(0, visibleDigits);
    final lastVisibleDigits = text.substring(totalDigits - visibleDigits);

    final maskedDigits = List.filled(hiddenDigits, '*').join();

    return '$firstVisibleDigits$maskedDigits$lastVisibleDigits';
  }

  TextSelection updateCursorPosition(
      String maskedText, TextSelection currentSelection) {
    final cursorPosition = currentSelection.baseOffset;
    final cursorOffset =
        currentSelection.extentOffset - currentSelection.baseOffset;
    final totalDigits = maskedText.length;
    const visibleDigits = 4;
    final hiddenDigits = totalDigits - visibleDigits * 2;

    final updatedPosition = cursorPosition <= visibleDigits
        ? cursorPosition
        : hiddenDigits + visibleDigits + (cursorPosition - visibleDigits);

    return TextSelection.collapsed(
        offset: updatedPosition, affinity: currentSelection.affinity);
  }
}