setState的作用是更新屏幕/小部件的UI。你可以在一个函数中使用setState，但它必须在一个有状态的类中。有状态类允许您重新构建用户界面。在您的代码中，您已经在statefulwidget类之外声明了该函数。你能做的是：

class NewScreen extends StatefulWidget {
  const NewScreen({Key? key}) : super(key: key);

  @override
  State<NewScreen> createState() => _NewScreenState();
}

class _NewScreenState extends State<NewScreen> {

void showCustomDialog(
  BuildContext context,
) {
  var number = 10;

  showDialog(
    context: context,
    builder: (BuildContext context) {
    return Column(
      children: [
        Text(
          number,
          style: TextStyle(color: Colors.grey[700]),
        ),
        MyServiceField(
          hint: 'hello',
          onChange: (fieldData) {
            var fieldNum = int.parse(fieldData);
            setState(                     // Now you can use setState as you have declared your function in a sateful widget.
              number = fieldNum
            );
          },
        ),
        ],
    );
  });
}

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Setstaet in Function'),
        backgroundColor: Colors.green,
      ),
      body: Container(
          child: Center(
         child: Padding(
        padding: const EdgeInsets.symmetric(horizontal: 20.0),
        child: Container(
          child: Text(
            'User Interface Changed!!',
          ),
        )),
      )),
    );
  }
}

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当您在StatefulWidget内部而不是外部时，可以调用setState方法，除非您传递回调。此时，您可以将对话框中的小部件提取为statefulwidget：

class CustomWidget extends StatefulWidget {
  const CustomWidget({super.key});

  @override
  State<CustomWidget> createState() => _CustomWidgetState();
}

class _CustomWidgetState extends State<CustomWidget> {
  var number = 10;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Text(
            number.toString(),
            style: TextStyle(color: Colors.grey[700]),
          ),
          TextField(
            onChanged: (fieldData) {
              var fieldNum = int.parse(fieldData);
              setState(() => number = fieldNum);
            },
          ),
        ],
      ),
    );
  }
}

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并在对话框中使用它，如下所示：

void showCustomDialog(BuildContext context) {
  showDialog(
      context: context,
      builder: (BuildContext context) {
        return const CustomWidget();
      });
}

型