关于flutter前端的表单设计问题

这是我用flask框架的代码，就是等到前端的post请求然后处理并返回数据

from flask import Flask, request
from flask_restful import Resource, Api
from flask_cors import CORS
import hashlib
import pymysql

app = Flask(__name__)
api = Api(app)
CORS(app)

conn = pymysql.connect(host='localhost', user='root', password='123456789', db='python_mysqlclient_test', charset='utf8mb4',
                       cursorclass=pymysql.cursors.DictCursor)


class Register(Resource):
    def post(self):
        data = request.json
        username = data.get('username')
        password = data.get('password')
        hash_password = hashlib.sha256(password.encode()).hexdigest()

        cursor = conn.cursor()
        cursor.execute('SELECT * FROM app01_userinfo WHERE username=%s', (username,))
        result = cursor.fetchone()

        if result:
            return {'message': 'User already exists.'}, 400

        cursor.execute('INSERT INTO app01_userinfo (username, password) VALUES (%s, %s)', (username, hash_password))
        conn.commit()

        return {'message': 'Registration successful.'}, 201


class Login(Resource):
    def post(self):
        data = request.json
        username = data.get('username')
        password = data.get('password')
        hash_password = hashlib.sha256(password.encode()).hexdigest()

        cursor = conn.cursor()
        cursor.execute('SELECT * FROM users WHERE username=%s AND password=%s', (username, hash_password))
        result = cursor.fetchone()

        if not result:
            return {'message': 'Invalid credentials.'}, 401

        return {'message': 'Login successful.'}, 200


api.add_resource(Register, '/register')
api.add_resource(Login, '/login')

if __name__ == '__main__':
    app.run(debug=True)

这是flutter前端的代码，我不知道前端代码是不是少了什么，好像运行不起来，有没有完整的代码补充，比如说Myapp状态类啥的，里面的body是用到了这个post请求的类，总之就是build这个地方不知道怎么写，因为没写过flutter表单，前端不完整，希望有能完整运行的代码补全

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

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

  @override
  State<RegisterScreen> createState() => _RegisterScreenState();
}

class _RegisterScreenState extends State<RegisterScreen> {
  final _formKey = GlobalKey<FormState>();
  final _usernameController = TextEditingController();
  final _passwordController = TextEditingController();


  Future<void> _register() async {
    final response = await http.post(
      Uri.parse('http://localhost:5000/register'),
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8',
      },
      body: jsonEncode(<String, String>{
        'username': _usernameController.text,
        'password': _passwordController.text,
      }),
    );

    if (response.statusCode == 201) {
      Navigator.pushReplacementNamed(context, '/home');
    } else {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(content: Text(jsonDecode(response.body)['message'])),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return const Placeholder();
  }
}