我创建了一个登录页面，我需要将这些内容添加到我的用户名中。如何在flutter中验证我的表单

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Form Validation',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyFormPage(),
    );
  }
}

class MyFormPage extends StatefulWidget {
  @override
  _MyFormPageState createState() => _MyFormPageState();
}

class _MyFormPageState extends State<MyFormPage> {
  final _formKey = GlobalKey<FormState>();
  String _username;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Form Validation'),
      ),
      body: Form(
        key: _formKey,
        child: Column(
          children: <Widget>[
            TextFormField(
              validator: (value) {
                if (value.isEmpty) {
                  return 'Please enter your username';
                }
                return null;
              },
              onSaved: (value) {
                _username = value;
              },
              decoration: InputDecoration(
                labelText: 'Username',
              ),
            ),
            RaisedButton(
              onPressed: () {
                if (_formKey.currentState.validate()) {
                  _formKey.currentState.save();
                  // 执行表单提交操作
                  // 可以在这里调用后端API进行验证等操作
                  showDialog(
                    context: context,
                    builder: (BuildContext context) {
                      return AlertDialog(
                        title: Text('Success'),
                        content: Text('Username: $_username'),
                        actions: <Widget>[
                          FlatButton(
                            child: Text('OK'),
                            onPressed: () {
                              Navigator.of(context).pop();
                            },
                          ),
                        ],
                      );
                    },
                  );
                }
              },
              child: Text('Submit'),
            ),
          ],
        ),
      ),
    );
  }
}