currentState.validate() 导致 Flutter 应用崩溃

Question

我正在创建一个 Flutter 应用程序，当我导航到这个特定屏幕时出现问题。模拟器崩溃并显示一条消息：

NoSuchMethodError：在 null 上调用了方法“validate” 接收方：空 尝试调用：validate()

我不明白为什么要调用 validate() 方法，即使它包含在 _submit 函数中，该函数仅在按下屏幕中的按钮时才会调用。当我从 onPressed 中删除函数调用时：应用程序正常工作。

import 'package:flutter/material.dart';
import 'package:service_request/services/auth_services.dart';
import 'package:flutter/services.dart';

class SignupForm extends StatefulWidget {
  static const String id = 'signup_screen';

  @override
  _SignupFormState createState() => _SignupFormState();
}

class _SignupFormState extends State<SignupForm> {
  final _signUpFormKey = GlobalKey<FormState>();
  String _name, _email, _password;

  _submit() {
    if (_signUpFormKey.currentState.validate()) {
      _signUpFormKey.currentState.save();
      // Logging in the user w/ Firebase
      AuthService.signUpUser(context, _name, _email, _password);
    }
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text('Sign Up'),
        ),
        body: SingleChildScrollView(
          child: Form(
            key: _signUpFormKey,
            child: Padding(
              padding: EdgeInsets.all(20.0),
              child: Column(
                children: <Widget>[
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Name',
                      ),
                      validator: (input) => input.trim().isEmpty
                          ? 'Please enter a valid name'
                          : null,
                      onSaved: (input) => _name = input,
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      keyboardType: TextInputType.emailAddress,
                      validator: (input) => !input.contains('@')
                          ? 'Please enter a valid email'
                          : null,
                      onSaved: (input) => _email = input,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Email Address',
                      ),
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      validator: (input) => input.length < 6
                          ? 'Must be at least 6 characters'
                          : null,
                      onSaved: (input) => _password = input,
                      obscureText: true,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Password',
                      ),
                    ),
                  ),
                  Column(
                    children: <Widget>[
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.blue,
                            borderRadius: BorderRadius.circular(10.0),
                          ),
                          width: 350.0,
                          child: FlatButton(
                            onPressed: null,
                            child: Text(
                              'Sign Up',
                              style: TextStyle(
                                color: Colors.white,
                                fontSize: 18.0,
                              ),
                            ),
                          ),
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.red,
                            borderRadius: BorderRadius.circular(10.0),
                          ),
                          width: 350.0,
                          child: FlatButton(
                            onPressed: _submit(),
                            child: Text(
                              'Back to Log In',
                              style: TextStyle(
                                color: Colors.white,
                                fontSize: 18.0,
                              ),
                            ),
                          ),
                        ),
                      ),
                    ],
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Answer 1

_submit() 表示执行此函数。你必须删除()
您可以在下面复制粘贴运行完整代码

代码sn-p

FlatButton(
                            onPressed: _submit,
                            child: Text(
                              'Back to Log In',
                              style: TextStyle(
                                color: Colors.white,
                                fontSize: 18.0,
                              ),
                            ),
                          )

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

import 'package:flutter/services.dart';

class SignupForm extends StatefulWidget {
  static const String id = 'signup_screen';

  @override
  _SignupFormState createState() => _SignupFormState();
}

class _SignupFormState extends State<SignupForm> {
  final _signUpFormKey = GlobalKey<FormState>();
  String _name, _email, _password;

  _submit() {
    if (_signUpFormKey.currentState.validate()) {
      _signUpFormKey.currentState.save();
      // Logging in the user w/ Firebase
      //AuthService.signUpUser(context, _name, _email, _password);
    }
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text('Sign Up'),
        ),
        body: SingleChildScrollView(
          child: Form(
            key: _signUpFormKey,
            child: Padding(
              padding: EdgeInsets.all(20.0),
              child: Column(
                children: <Widget>[
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Name',
                      ),
                      validator: (input) => input.trim().isEmpty
                          ? 'Please enter a valid name'
                          : null,
                      onSaved: (input) => _name = input,
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      keyboardType: TextInputType.emailAddress,
                      validator: (input) => !input.contains('@')
                          ? 'Please enter a valid email'
                          : null,
                      onSaved: (input) => _email = input,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Email Address',
                      ),
                    ),
                  ),
                  Padding(
                    padding: EdgeInsets.symmetric(vertical: 10.0),
                    child: TextFormField(
                      validator: (input) => input.length < 6
                          ? 'Must be at least 6 characters'
                          : null,
                      onSaved: (input) => _password = input,
                      obscureText: true,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(
                          borderSide: BorderSide(
                            color: Colors.black54,
                          ),
                        ),
                        labelText: 'Password',
                      ),
                    ),
                  ),
                  Column(
                    children: <Widget>[
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.blue,
                            borderRadius: BorderRadius.circular(10.0),
                          ),
                          width: 350.0,
                          child: FlatButton(
                            onPressed: null,
                            child: Text(
                              'Sign Up',
                              style: TextStyle(
                                color: Colors.white,
                                fontSize: 18.0,
                              ),
                            ),
                          ),
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.red,
                            borderRadius: BorderRadius.circular(10.0),
                          ),
                          width: 350.0,
                          child: FlatButton(
                            onPressed: _submit,
                            child: Text(
                              'Back to Log In',
                              style: TextStyle(
                                color: Colors.white,
                                fontSize: 18.0,
                              ),
                            ),
                          ),
                        ),
                      ),
                    ],
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(        
        primarySwatch: Colors.blue,
      ),
      home: SignupForm(),
    );
  }
}

Answer 2

您正在调用方法而不是设置回调。编译器通常会选择它，但您的方法没有输入。 Dart 中的无类型方法返回一个动态对象，它可能是一个函数，这就是为什么你可以从你的小部件中调用它的原因。应该通过将其更改为 onPressed: _submit 或 onPressed: () => _submit() 来修复它，因为它们是函数而不是方法调用。