如何根据从外部传递的更改值动态更改自定义小部件中成员的值

import 'package:flutter/material.dart';

class CustomWidget extends StatefulWidget {
  @override
  _CustomWidgetState createState() => _CustomWidgetState();
}

class _CustomWidgetState extends State<CustomWidget> {
  String _value = '';

  void updateValue(String newValue) {
    setState(() {
      _value = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Text(_value);
  }
}

class ParentWidget extends StatefulWidget {
  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  String _externalValue = '';

  void updateExternalValue(String newValue) {
    setState(() {
      _externalValue = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        CustomWidget(),
        RaisedButton(
          child: Text('Update Value'),
          onPressed: () {
            updateExternalValue('New Value');
          },
        ),
      ],
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: ParentWidget(),
  ));
}