Null safety problem

[mudlee]

Hello! I wrote a custom validator which I add to a MultiValidator:

class NotNullOrEmptyValidator<T> extends FieldValidator<T> {
  NotNullOrEmptyValidator({required String errorText}) : super(errorText);

  @override
  bool isValid(T value) {
    return value != null;
  }
}

I use it like:

validators.add(NotNullOrEmptyValidator<File>(errorText: 'Required'));

So basically I just checked if the given file is null or not.

When validation occurs, I get this error back:

type 'Null' is not a subtype of type 'File' of 'value'

When the exception was thrown, this was the stack: 
#0      FieldValidator.call (package:form_field_validator/form_field_validator.dart)
#1      MultiValidator.isValid (package:form_field_validator/form_field_validator.dart:170:21)
#2      MultiValidator.call (package:form_field_validator/form_field_validator.dart:180:12)
#3      FormFieldState._validate (package:flutter/src/widgets/form.dart:468:37)
#4      FormFieldState.validate.<anonymous closure> (package:flutter/src/widgets/form.dart:461:7)

I also tried to change my logic and not use File, but just a simple String (path of the file), but still I get the same error, but with this message: type 'Null' is not a subtype of type 'String' of 'value'

[mudlee]

@Milad-Akarie ? :)

[Stefan Galler]

@mudlee Looks like that MultiValidator assumes that your Validator is of type String?.
So if you use it like NotNullOrEmptyValidator<String?>(errorText: 'Required') it would work.

class MultiValidator extends FieldValidator<String?> {
  final List<FieldValidator> validators;
  static String _errorText = '';

  MultiValidator(this.validators) : super(_errorText);

  @override
  bool isValid(value) {
    for (FieldValidator validator in validators) {
      if (validator.call(value) != null) {
        _errorText = validator.errorText;
        return false;
      }
    }
    return true;
  }

  @override
  String? call(dynamic value) {
    return isValid(value) ? null : _errorText;
  }
}