Undeclared arguments passed to ViewHelper - typo3

Question

I upgraded a website from TYPO3 v7 to v9 and now I get the following error:
Undeclared arguments passed to ViewHelper \ViewHelpers\MyViewHelper: value, list. Valid arguments are: [...]
My current ViewHelper looks as follows:
<?php
namespace VP\News\ViewHelpers;
/**
* #package TYPO3
* #subpackage Fluid
*/
class InListViewHelper extends \TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper {
/**
* #param mixed $value value
* #param mixed $list list
* #return boolean
*/
public function render($value, $list) {
if (!is_array($list)) {
$list = str_replace(' ', '', $list);
$list = explode(',', $list);
}
return in_array($value, $list);
}
}

Answer 1

Some things have changed between v7 and v9 ViewHelpers in TYPO3 Fluid.
➊ You should extend from the abstract class TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper.
➋ You have to register the arguments you pass to the ViewHelper.
➌ Your ViewHelper looks more like a Condition-ViewHelper than an Abstract-ViewHelper.
The first point is self-explanatory. Simply change the name of the base class (the fully qualified class name). For the second point, you can use an additional method initializeArguments(). For example:
public function initializeArguments(): void
{
parent::initializeArguments();
$this->registerArgument('value', 'type', 'description');
...
}
You can find an example here.
However, your ViewHelper seems to check a condition ("is this element in a list?", "then...", "else..."). Therefore, it might be better to implement a Condition-ViewHelper.
This type of ViewHelper extends the class TYPO3Fluid\Fluid\Core\ViewHelper\AbstractConditionViewHelper and evaluates a condition using a method verdict() instead of render() or renderStatic().
You can find an example of a simple Condition-ViewHelper here.