如何从外部访问有状态小部件的属性/方法？

import 'package:flutter/material.dart';

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

class _MyWidgetState extends State<MyWidget> {
  String _text = 'Hello World';

  void changeText(String newText) {
    setState(() {
      _text = newText;
    });
  }

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

class ParentWidget extends StatelessWidget {
  final MyWidget myWidget = MyWidget();

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        myWidget,
        RaisedButton(
          child: Text('Change Text'),
          onPressed: () {
            myWidget.changeText('New Text');
          },
        ),
      ],
    );
  }
}