要保护Flutter移动应用程序的注册API端点免受垃圾邮件或机器人攻击，可以通过使用Google reCAPTCHA进行验证来实现。

以下是一个使用Flutter插件flutter_recaptcha_v2的示例代码：

1. 使用依赖：

dependencies: flutter_recaptcha_v2: ^0.3.0

2. 导入flutter_recaptcha_v2插件：

import 'package:flutter_recaptcha_v2/flutter_recaptcha_v2.dart';

3. 创建并添加ReCaptcha API密钥，在初始化时，传递我们之前导入的secrets.dart文件中的API密钥。示例代码如下：

import 'package:secrets/secrets.dart';

class RegisterPage extends StatefulWidget { @override _RegisterPageState createState() => _RegisterPageState(); }

class _RegisterPageState extends State { final _formKey = GlobalKey(); final _scaffoldKey = GlobalKey(); String _email, _password;

// Step 1: Create a RecaptchaV2Controller instance RecaptchaV2Controller recaptchaV2Controller = RecaptchaV2Controller();

@override void dispose() { recaptchaV2Controller.dispose(); // dispose controller super.dispose(); }

@override Widget build(BuildContext context) { return Scaffold( key: _scaffoldKey, body: SafeArea( child: SingleChildScrollView( child: Form( key: _formKey, child: Column( children: [ // Step 2: Add RecaptchaV2 widget RecaptchaV2( apiKey: apiSecretKey, controller: recaptchaV2Controller, onVerifiedError: (err) { _scaffoldKey.currentState.showSnackBar( SnackBar(content: Text(err)), ); }, ), TextFormField( onSaved: (val) => _email = val, decoration: InputDecoration(labelText: 'Email'), validator: (val) => !val