Error: The parameter 'onSubmit' can't have a value - flutter

Question

Hi I have created a default form field, in a separate dart file called components, and I have login_screen.dart also.
there are many errors I don't know how to fix it, and make the code work, I will put down the code of the component.dart code:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
Widget defaultButton(
{double width = double.infinity,
Color background = Colors.blue,
double radius = 10.0,
required 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,
Function onSubmit,
Function onChange,
required Function validate,
required var label,
required IconData prefix,
}) =>
TextFormField(
controller: controller,
keyboardType: type,
onFieldSubmitted: onSubmit(),
onChanged: onChange(),
validator: validate(),
decoration: InputDecoration(
labelText: label,
prefixIcon: Icon(prefix),
border: OutlineInputBorder()
),
);
and here is the code of the login_screen.dart:
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'
))
],
)
],
),
),
),
),
),
);
}
}
Idk if all the problems came from a null-safety feature in a flutter because I'm new in this technology.

Answer 1

Why
The reason this happens is because with null safety enabled, your functions onSubmit and onChange can't be null.
Solution
I would do it this way:
Widget defaultFormFeild({
required TextEditingController controller,
required TextInputType type,
Function? onSubmit, //Add question mark
Function? onChange, //Add question mark
required Function validate,
required var label,
required IconData prefix,
}) =>
TextFormField(
controller: controller,
keyboardType: type,
onFieldSubmitted: onSubmit != null? onSubmit() : null, //do null checking
onChanged: onChange != null? onChange() : null, //do null checking
validator: validate(),
decoration: InputDecoration(
labelText: label,
prefixIcon: Icon(prefix),
border: OutlineInputBorder()
),
);