Flutter -为文本字段构建自定义计数器小部件-无法获取currentLength

import 'package:flutter/material.dart';

class CustomCounterTextField extends StatefulWidget {
  @override
  _CustomCounterTextFieldState createState() => _CustomCounterTextFieldState();
}

class _CustomCounterTextFieldState extends State<CustomCounterTextField> {
  final TextEditingController _controller = TextEditingController();
  int _currentLength = 0;

  @override
  void initState() {
    super.initState();
    _controller.addListener(_updateLength);
  }

  @override
  void dispose() {
    _controller.removeListener(_updateLength);
    _controller.dispose();
    super.dispose();
  }

  void _updateLength() {
    setState(() {
      _currentLength = _controller.text.length;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        TextField(
          controller: _controller,
        ),
        SizedBox(height: 10),
        Text('当前长度: $_currentLength'),
      ],
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: Scaffold(
      appBar: AppBar(title: Text('自定义计数器')),
      body: Center(child: CustomCounterTextField()),
    ),
  ));
}