Error: Expected ',' before this. validator: validate!(String value),

Question

你好，我在 flutter 中遇到了这个错误：

lib/shared/components/components.dart:41:35: Error: Expected ',' before this.
      validator: validate!(String value),
                                  ^^^^^
lib/modules/login/login_screen.dart:36:33: Error: The argument type 'String? Function(String)' can't be assigned to the parameter type 'dynamic Function()?'.
                      validate: (String value){
                                ^
lib/shared/components/components.dart:41:35: Error: Getter not found: 'value'.
      validator: validate!(String value),
                                  ^^^^^
lib/shared/components/components.dart:41:27: Error: Too many positional arguments: 0 allowed, but 2 found.
Try removing the extra positional arguments.
      validator: validate!(String value),
                          ^


FAILURE: Build failed with an exception.

* Where:
Script 'C:\src\flutter\packages\flutter_tools\gradle\flutter.gradle' line: 1005

* What went wrong:
Execution failed for task ':app:compileFlutterBuildDebug'.
> Process 'command 'C:\src\flutter\bin\flutter.bat'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

components.dart代码为：

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

Widget defaultButton(
    {double width = double.infinity,
      Color background = Colors.blue,
      double radius = 10.0,
      Function()? function,
      required String text,
      bool isUpperCase = true}) =>
    Container(
      height: 40.0,
      width: width,
      child: MaterialButton(
          onLongPress: () {},
          onPressed: function!() ,
          child: Text(
            isUpperCase ? text.toUpperCase() : text.toLowerCase(),
            style: TextStyle(color: Colors.white),
          )),
      decoration: BoxDecoration(
        borderRadius: BorderRadiusDirectional.circular(radius),
        color: background,
      ),
    );

Widget defaultFormFeild({
  required TextEditingController controller,
  required TextInputType type,
  void Function(String)? onSubmit,
  void Function(String)? onChange,
  required Function()? validate,
  required var label,
  required IconData prefix,
}) =>
    TextFormField(
      controller: controller,
      keyboardType: type,
      onFieldSubmitted: onSubmit, //do null checking
      onChanged: onChange, //do null checking
      validator: validate!(),
      decoration: InputDecoration(
          labelText: label,
          prefixIcon: Icon(prefix),
          border: OutlineInputBorder()
      ),
    );

login_screen.dart代码：

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:udemy_flutter/shared/components/components.dart';
class LoginScreen extends StatelessWidget {
  var emailController = TextEditingController();
  var passController = TextEditingController();
  var formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.all(20.0),
        child: Center(
          child: SingleChildScrollView(
            child: Form(
              key: formKey,
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: [
                  const Text(
                      'Login',
                    style: TextStyle(
                      fontSize: 40.0,
                      fontWeight: FontWeight.bold
                    ),
                  ),
                  const SizedBox(
                    height: 40.0,
                  ),
                  defaultFormFeild(
                      controller: emailController,
                      label: 'Email',
                      prefix: Icons.email,
                      type: TextInputType.emailAddress,
                      validate: (String value){
                        if(value.isEmpty != null){
                          return 'Email Cannot Be Empty';
                        }
                        return null;
                      }
                  ),
                  const SizedBox(
                    height: 15.0,
                  ),
                  TextFormField(
                    controller: passController,
                    obscureText: true,
                    keyboardType: TextInputType.visiblePassword,
                    decoration: const InputDecoration(
                      border: OutlineInputBorder(),
                      labelText: 'Password',
                      prefixIcon: Icon(
                        Icons.lock
                      ),
                      suffixIcon: Icon(
                        Icons.remove_red_eye,
                      )
                    ),
                    onChanged: (value) {
                      print(value);
                    },
                    onFieldSubmitted: (value) {
                      print(value);
                    },
                    validator: (value) {
                      if(value!.isEmpty){
                        return 'Password cannot be empty';
                      }
                      return null;
                    },
                  ),
                  const SizedBox(
                    height: 10.0,
                  ),
                  defaultButton(
                    function: (){
                      print(emailController.text);
                      print(passController.text);
                    },
                    text: 'Login',
                  ),
                  const SizedBox(
                    height: 10.0,
                  ),
                  defaultButton(
                    text: 'ReGister',
                    function: () {
                      print('You have just clicked on register');
                    },
                    background: Colors.red,
                    isUpperCase: false
                  ),
                  Row(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      const Text('Don\'t you have an account?'),
                      TextButton(onPressed: () {}, child: const Text(
                        'Register Now'
                      ))
                    ],
                  )
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

谁能告诉我哪里错了我是 Flutter 平台的新手，我正在尝试构建屏幕用于各种用途，因为我在 Null 安全方面遇到问题并且我设法解决了它，但我不知道这段代码的确切问题是什么

Answer 1

TextFormField 接受 FormFieldValidator？作为 validator。 FormFieldValidator 的定义是：

typedef FormFieldValidator<T> = String? Function(T? value);

这与您对 Function()? validator 的定义不符。您需要更改类型以匹配 FormFieldValidator。

  void Function(String)? onChange,
  required Function()? validate,
  required var label,

会变成

  void Function(String)? onChange,
  required String Function(String)? validate
  required var label,

接下来还有第二个问题。您在分配它之前正在执行此功能。这是不必要的，会导致类型不匹配。在 defaultFormFeild 中，将代码更改为在将验证器传递给表单字段之前不执行验证器。

      onChanged: onChange, //do null checking
      validator: validate,
      decoration: InputDecoration(

Error: Expected ',' before this. validator: validate!(String value),

Error: Expected ',' before this. validator: validate!(String value),

flutter

flutter-layout

dart-null-safety

null-safety