如何在表单中间使用Flutter CircularProgressIndicator()？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Form with CircularProgressIndicator'),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              // 表单部分
              TextFormField(
                decoration: InputDecoration(
                  labelText: '用户名',
                ),
              ),
              TextFormField(
                decoration: InputDecoration(
                  labelText: '密码',
                ),
              ),
              RaisedButton(
                child: Text('登录'),
                onPressed: () {
                  // 处理登录逻辑
                },
              ),
              SizedBox(height: 20), // 用于创建一些间距

              // CircularProgressIndicator部分
              CircularProgressIndicator(),
            ],
          ),
        ),
      ),
    );
  }
}