如何在状态改变时保留新的TextFormField值？

class MyFormWidget extends StatefulWidget {
  @override
  _MyFormWidgetState createState() => _MyFormWidgetState();
}

class _MyFormWidgetState extends State<MyFormWidget> {
  String _textValue = '';

  @override
  Widget build(BuildContext context) {
    return TextFormField(
      onChanged: (value) {
        setState(() {
          _textValue = value;
        });
      },
      decoration: InputDecoration(
        labelText: 'Enter a value',
      ),
    );
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('My App'),
        ),
        body: MyFormWidget(),
      ),
    );
  }
}