formatEditUpdate method

Implementation

@override
TextEditingValue formatEditUpdate(
    TextEditingValue oldValue, TextEditingValue newValue) {
  /// provides placeholder text when user start editing
  if (oldValue.text.isEmpty) {
    oldValue = oldValue.copyWith(
      text: _placeholder,
    );
    newValue = newValue.copyWith(
      text: _fillInputToPlaceholder(newValue.text),
    );
    return newValue;
  }

  /// nothing changes, nothing to do
  if (newValue == _lastNewValue) {
    return oldValue;
  }
  _lastNewValue = newValue;

  int offset = newValue.selection.baseOffset;

  /// restrict user's input within the length of date form
  if (offset > 10) {
    return oldValue;
  }

  if (oldValue.text == newValue.text && oldValue.text.length > 0) {
    return newValue;
  }

  final String oldText = oldValue.text;
  final String newText = newValue.text;
  String? resultText;

  /// handle user editing, there're two cases:
  /// 1. user add new digit: replace '-' at cursor's position by user's input.
  /// 2. user delete digit: replace digit at cursor's position by '-'
  int index = _indexOfDifference(newText, oldText);
  if (oldText.length < newText.length) {
    /// add new digit
    String newChar = newText[index];
    if (index == 2 || index == 5) {
      index++;
      offset++;
    }
    resultText = oldText.replaceRange(index, index + 1, newChar);
    if (offset == 2 || offset == 5) {
      offset++;
    }
  } else if (oldText.length > newText.length) {
    /// delete digit
    if (oldText[index] != '/') {
      resultText = oldText.replaceRange(index, index + 1, '-');
      if (offset == 3 || offset == 6) {
        offset--;
      }
    } else {
      resultText = oldText;
    }
  }

  /// verify the number and position of splash character
  final splashes = resultText!.replaceAll(RegExp(r'[^/]'), '');
  int count = splashes.length;
  if (resultText.length > 10 ||
      count != 2 ||
      resultText[2].toString() != '/' ||
      resultText[5].toString() != '/') {
    return oldValue;
  }

  return oldValue.copyWith(
    text: resultText,
    selection: TextSelection.collapsed(offset: offset),
    composing: defaultTargetPlatform == TargetPlatform.iOS
        ? TextRange(start: 0, end: 0)
        : TextRange.empty,
  );
}