如何在Flutter中使用输入格式化程序将用户的输入文本格式化为MM/DD/YYYY

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

TextEditingController _controller = TextEditingController();

TextField(
  controller: _controller,
  inputFormatters: [
    FilteringTextInputFormatter.digitsOnly,
    _DateInputFormatter(),
  ],
)

class _DateInputFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    final int newTextLength = newValue.text.length;
    int selectionIndex = newValue.selection.end;
    int usedSubstringIndex = 0;
    final StringBuffer newText = StringBuffer();

    if (newTextLength >= 1) {
      newText.write(newValue.text.substring(0, usedSubstringIndex = 1) + '/');
      if (newValue.selection.end >= 1) selectionIndex++;
    }
    if (newTextLength >= 3) {
      newText.write(newValue.text.substring(1, usedSubstringIndex = 3) + '/');
      if (newValue.selection.end >= 3) selectionIndex++;
    }
    if (newTextLength >= 5) {
      newText.write(newValue.text.substring(3, usedSubstringIndex = 5));
      if (newValue.selection.end >= 5) selectionIndex++;
    }

    // If the old value had selection, the new value should have the same selection.
    // Because we've inserted/changed some characters, we need to update the selection position.
    selectionIndex = math.min(selectionIndex, newText.length);
    selectionIndex = math.max(selectionIndex, 0);

    return TextEditingValue(
      text: newText.toString(),
      selection: TextSelection.collapsed(offset: selectionIndex),
    );
  }
}