DateTimeTextFormatter

import 'dart:math';

import 'package:flutter/services.dart';

class DateTimeTextFormatter extends TextInputFormatter {
  static const _maxChars = 17;

  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    String separatorDate = '/';
    String spacing = ' ';
    String separatorTime = ':';
    var text = _format(
      newValue.text,
      oldValue.text,
      separatorDate,
      spacing,
      separatorTime,
    );

    return newValue.copyWith(
      text: text,
      selection: updateCursorPosition(
        oldValue,
        text,
      ),
    );
  }

  String _format(
    String value,
    String oldValue,
    String separatorDate,
    String spacing,
    String separatorTime,
  ) {
    var isErasing = value.length < oldValue.length;
    var isComplete = value.length > _maxChars + 2;

    if (!isErasing && isComplete) {
      return oldValue;
    }

    value = value.replaceAll(separatorDate, '');
    value = value.replaceAll(spacing, '');
    value = value.replaceAll(separatorTime, '');
    final result = <String>[];

    for (int i = 0; i < min(value.length, _maxChars); i++) {
      result.add(value[i]);

      if ((i == 1 || i == 3) && i != value.length - 1) {
        result.add(separatorDate);
      }

      if ((i == 7) && i != value.length - 1) {
        result.add(' ');
      }
      if ((i == 9 || i == 11) && i != value.length - 1) {
        result.add(separatorTime);
      }
    }
// (i == 1 || i == 3 || i == 7 || i == 9 || i == 11)
    return result.join();
  }

  TextSelection updateCursorPosition(
    TextEditingValue oldValue,
    String text,
  ) {
    var endOffset = max(
      oldValue.text.length - oldValue.selection.end,
      0,
    );

    var selectionEnd = text.length - endOffset;

    return TextSelection.fromPosition(TextPosition(offset: selectionEnd));
  }
}