Feature: #108524 - Configuration file to register global Fluid namespaces 

Overriding existing ViewHelpers 

TYPO3 reads and merges Configuration/Fluid/Namespaces.php files from all loaded extensions in the usual loading order, which can be manipulated by declaring dependencies in composer.json and possibly ext_emconf.php. If an extension registers a namespace that has already been registered by another extension, these namespaces will be merged by Fluid. This allows extensions to override ViewHelpers of another extension selectively.

Example (extension2 depends on extension1):

<?php

return [
    'myext' => ['MyVendor\\MyExtension1\\ViewHelpers'],
];

<?php

return [
    'myext' => ['MyVendor\\MyExtension2\\ViewHelpers'],
];

Resulting namespace definition:

[
    'myext' => [
        'MyVendor\\MyExtension1\\ViewHelpers',
        'MyVendor\\MyExtension2\\ViewHelpers',
    ],
];

Namespaces are processed in reverse order, which means that <myext:demo /> would first check for EXT:my_extension2/Classes/ViewHelpers/DemoViewHelper.php, and would fall back to EXT:my_extension1/Classes/ViewHelpers/DemoViewHelper.php.

PSR-14 event to modify namespaces 

The new \ModifyNamespacesEvent is introduced, which allows modification of the whole namespaces array before it is being passed to Fluid. This allows for example to:

• completely redefine an existing namespace (instead of extending it)
• add namespaces conditionally
• modify order of merged namespaces

Example:

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\EventListener;

use TYPO3\CMS\Core\Attribute\AsEventListener;
use TYPO3\CMS\Fluid\Event\ModifyNamespacesEvent;

#[AsEventListener]
final readonly class ModifyNamespacesListener
{
    public function __invoke(ModifyNamespacesEvent $event): void
    {
        $namespaces = $event->getNamespaces();
        // Replace existing "theme" namespace completely
        $namespaces['theme'] = ['MyVendor\\MyExtension\\ViewHelpers'];
        $event->setNamespaces($namespaces);
    }
}

Note that namespaces might still be imported locally from within a template file, which is unaffected by this event.

Backwards-compatible namespaces in extensions 

There are several ways to provide backwards-compatible global namespaces in extensions, depending on the concrete use case:

• preferred: Define namespace in TYPO3_CONF_VARS (with version check) and in new Namespaces.php. This means that in TYPO3 v14 installations the new Namespaces.php can already be used to extend the namespace.
• alternative: Define namespace both in TYPO3_CONF_VARS (without version check) and in new Namespaces.php. This means however that the namespace can only be extended with TYPO3_CONF_VARS, not with the new Namespaces.php.
• keep TYPO3_CONF_VARS until support for < v14 is dropped by the extension. This can only be extended with TYPO3_CONF_VARS as well.
• implement own merging logic in ModifyNamespacesEvent if necessary.

Example for preferred option:

<?php

return [
    'myext' => ['MyVendor\\MyExtension\\ViewHelpers'],
];

<?php

if ((new \TYPO3\CMS\Core\Information\Typo3Version())->getMajorVersion() < 14) {
    $GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces']['myext'][] = 'MyVendor\\MyExtension\\ViewHelpers';
}

To sum up:

• Namespaces.php can only extend namespaces defined in Namespaces.php.
• TYPO3_CONF_VARS can extend both TYPO3_CONF_VARS and Namespaces.php.
• ModifyNamespacesEvent can modify everything.