flutter - TextFormField 验证器不起作用 - 在 null 上调用了方法 'validate'

Question

我正在尝试为新的 flutter 应用构建一个简单的登录页面。小部件树是非常基本的。它由一个包含 Scaffold 的 MaterialApp 组成。 Scaffold 的“家”是一个 StatefulWidget，其中包含带有两个 TextFormField 的 Form。

TextFormFields 有“验证器”代码来检查空值。

有一个按钮在按下时会调用方法“validateAndSave”。在这个方法中，我为 GlobalKey 的一个实例调用了“验证”方法。

单击按钮时，验证器似乎没有触发，我收到以下错误消息:

I/flutter (10459): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter (10459): The following NoSuchMethodError was thrown while handling a gesture:
I/flutter (10459): The method 'validate' was called on null.
I/flutter (10459): Receiver: null
I/flutter (10459): Tried calling: validate()
I/flutter (10459): 
I/flutter (10459): When the exception was thrown, this was the stack:
I/flutter (10459): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter (10459): #1      _LoginForm.validateAndSave (package:playground/main.dart:33:13)

我尝试在项目上运行 flutter clean 并重新运行应用程序。我仍然收到错误消息。

这是我正在运行的代码。

import 'package:flutter/material.dart';

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

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

class LoginForm extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _LoginForm();
}

class _LoginForm extends State<LoginForm>{
  // GlobalKey<FormState> formKey = new GlobalKey();
  final _formKey = new GlobalKey<FormState>();
  String _email;
  String _password;

  void validateAndSave(){
    final form = _formKey.currentState;
    if(form.validate())
    {
      print ('Form is valid');
    }
    else
    {
      print('form is invalid');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
        padding: EdgeInsets.only(left: 15.0, right: 15.0),
        child: Form(
            child: Column(
              // mainAxisAlignment: MainAxisAlignment.center,
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  SizedBox(height: 150.0,),
                  //input field for email
                  TextFormField(
                    decoration: InputDecoration(labelText: 'Email'),
                    validator: (value) => value.isEmpty ? 'Email cannot be blank':null,
                    //       onSaved: (value) => _email = value,
                  ),
                  //input field for password
                  TextFormField(
                    decoration: InputDecoration(labelText: 'Password'),
                    obscureText: true,
                    validator: (value) => value.isEmpty ? 'Password cannot be blank':null,
                    //     onSaved: (value) => _password = value,
                  ),
                  RaisedButton(
                    child: Text ('Login', style: TextStyle(fontSize: 20.0),),
                    onPressed: validateAndSave,
                  )
                ]
            )
        )
    );
  }
}

我觉得我错过了代码中的某些内容。在解决这个问题时，我将不胜感激。

Answer 1

您没有将 key 添加到表单中，此代码应该可以工作:

class _LoginForm extends State<LoginForm> {
  // GlobalKey<FormState> formKey = GlobalKey();
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  String _email;
  String _password;

  void validateAndSave() {
    final FormState form = _formKey.currentState;
    if (form.validate()) {
      print('Form is valid');
    } else {
      print('Form is invalid');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.only(left: 15.0, right: 15.0),
      child: Form(
        key: _formKey,
        child: Column(
          // mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            SizedBox(
              height: 150.0,
            ),
            // input field for email
            TextFormField(
              decoration: InputDecoration(labelText: 'Email'),
              validator: (value) =>
                  value.isEmpty ? 'Email cannot be blank' : null,
              // onSaved: (value) => _email = value,
            ),
            // input field for password
            TextFormField(
              decoration: InputDecoration(labelText: 'Password'),
              obscureText: true,
              validator: (value) =>
                  value.isEmpty ? 'Password cannot be blank' : null,
              // onSaved: (value) => _password = value,
            ),
            RaisedButton(
              child: Text(
                'Login',
                style: TextStyle(fontSize: 20.0),
              ),
              onPressed: validateAndSave,
            ),
          ],
        ),
      ),
    );
  }
}