如何在TextFormField()上仅当在flutter中聚焦时才显示计数器文本？

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  FocusNode _focusNode = FocusNode();

  @override
  void dispose() {
    _focusNode.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return FocusScope(
      child: Column(
        children: [
          TextFormField(
            focusNode: _focusNode,
            decoration: InputDecoration(
              labelText: 'Input',
            ),
          ),
          Builder(
            builder: (BuildContext context) {
              if (_focusNode.hasFocus) {
                return Text('Counter: ${_getInputLength()}');
              } else {
                return Container();
              }
            },
          ),
        ],
      ),
    );
  }

  int _getInputLength() {
    // 获取输入文本的长度
    return 0;
  }
}