The argument type 'String? Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'

Question

I am trying to setup password and email validation and I am getting the error above. Any help would be greatly appreciated. The error above is in the main.dart code and has been bolded in the code.

validator.dart code

enum FormType { login, register }

class EmailValidator {
  static String? validate(String value) {
    return value.isEmpty ? "Email can't be empty" : null;
  }
}

class PasswordValidator {
  static String? validate(String value) {
    return value.isEmpty ? "Password can't be empty" : null;
  }
}

main.dart code

List<Widget>buildInputs() {
        return [
          TextFormField(
            validator: **EmailValidator.validate**,
            decoration: InputDecoration(labelText: 'Email'),
            onSaved: (value) => _email = value,
          ),
          TextFormField(
            validator: **PasswordValidator.validate**,
            decoration: InputDecoration(labelText: 'Password'),
            obscureText: true,
            onSaved: (value) => _password = value,
          ),
        ];
      }

score 7 · Accepted Answer · answered Apr 12 '22 at 11:58

7

If you check validator it returns a nullable String.

{String? Function(String?)? validator}

You can convert your validators like

class EmailValidator {
  static String? validate(String? value) {
    return value==null ||  value.isEmpty ? "Email can't be empty" : null;
  }
}

class PasswordValidator {
  static String? validate(String? value) {
    return value==null ||value.isEmpty ? "Password can't be empty" : null;
  }
}

answered Apr 12 '22 at 11:58

Md. Yeasin Sheikh

54,221
7
29
56

Glad to help you out, you can check more about [validation-logic](https://docs.flutter.dev/cookbook/forms/validation#2-add-a-textformfield-with-validation-logic) – Md. Yeasin Sheikh Apr 12 '22 at 12:02

score 1 · Answer 2 · answered Apr 12 '22 at 11:49

1

Your validate functions accept strictly non-null parameters. Change your function signature to static String? validate(String? value) (note the question mark after the second String) and it'll match the required signature.

answered Apr 12 '22 at 11:49

Piotr Szych

36
1
3

I have already tried that and it gives an error on IsEmpty saying: The property 'isEmpty' can't be unconditionally accessed because the receiver can be 'null'. Try making the access conditional (using '?.') or adding a null check to the target ('!') – Lloyd Wingrove Apr 12 '22 at 11:57

score 0 · Answer 3 · answered May 19 '23 at 14:06

The validator returns a null when all conditions are met, so you should handle that like this

static String? validateMobile(String? value) {
  //Indian Mobile number are of 10 digit only
    if (value!.length != 10) {
      return 'Mobile Number must be of 10 digit';
    } else {
      return null;
    }
  }

The argument type 'String? Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'

3 Answers3