如何在flutter中验证单个表单域

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Form Validation'),
        ),
        body: Padding(
          padding: EdgeInsets.all(16.0),
          child: Form(
            key: _formKey,
            child: Column(
              children: <Widget>[
                TextFormField(
                  validator: (value) {
                    if (value.isEmpty) {
                      return 'Please enter some text';
                    }
                    return null;
                  },
                  decoration: InputDecoration(
                    labelText: 'Username',
                  ),
                ),
                RaisedButton(
                  onPressed: () {
                    if (_formKey.currentState.validate()) {
                      // 验证通过，执行相应操作
                      // 可以在这里提交表单或执行其他逻辑
                    }
                  },
                  child: Text('Submit'),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}