Feature: #108148 - Union types for ViewHelpers

Description

Fluid 5 brings support for union types in ViewHelper argument definitions. Previously, it was necessary to specify an argument as mixed if more than one type should be possible. Now it is possible to specify multiple types separated by a pipe character (|).

The following built-in PHP types can also be used:

iterable
countable
callable

Example:

use TYPO3\CMS\Core\Resource\File;
use TYPO3\CMS\Core\Resource\FileReference;
use TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper;

class MyViewHelper extends AbstractViewHelper
{
    public function initializeArguments(): void
    {
        $this->registerArgument(
            'file',
            File::class . '|' . FileReference::class,
            'a file object'
        );
        $this->registerArgument(
            'items',
            'iterable',
            'a list of items'
        );
    }
}

This feature also applies to the Argument ViewHelper <f:argument>:

<f:argument name="file" type="TYPO3\CMS\Core\Resource\File|TYPO3\CMS\Core\Resource\FileReference" />

Note that union types disable automatic type conversion by Fluid, so it might be necessary to specify more types to keep ViewHelpers flexible. Example:

$this->registerArgument(
    'ids',
    'array|string|int',
    'a list of ids, either comma-separated or as array'
);

Impact

Custom ViewHelper implementations have more options to specify an API with strict type requirements and can avoid manual type checks.