Feature: #88770 - PSR-14 based EventDispatcher

Description

A new EventDispatcher system is added to extend TYPO3's Core behaviour via PHP code. In the past, this was done via Extbase's SignalSlot and TYPO3's custom hook system. The new EventDispatcher system is a fully capable replacement for new code in TYPO3, as well as a possibility to migrate away from previous TYPO3 solutions.

PSR-14 [https://www.php-fig.org/psr/psr-14/] is a lean solution that builds upon wide-spread solutions for hooking into existing PHP code (Frameworks, CMS and the like).

PSR-14 consists of four components:

1. An EventDispatcher object that is used to trigger an Event. TYPO3 has a custom EventDispatcher implementation for now, however all EventDispatchers of all frameworks are implementing \Psr\EventDispatcher\EventDispatcherInterface thus it is possible to replace the event dispatcher with another. The EventDispatcher's main method dispatch() is called in TYPO3 Core or extensions, that receives a PHP object and is then handed to all available listeners.

2. A ListenerProvider object that contains all listeners which have been registered for all events. TYPO3 has a custom ListenerProvider that collects all listeners during compile time. This component is not exposed outside of TYPO3's Core Framework.

3. Various Event objects. An event object can be any PHP object and is called from TYPO3 Core or an extension ("Emitter") containing all information to be transported to the listeners. By default, all registered listeners get triggered by an Event, however, if an Event has the interface \Psr\EventDispatcher\StoppableEventInterface implemented, a listener can stop further execution of other event listeners. This is especially useful if the listeners are candidates to provide information to the emitter. This allows to finish event dispatching, once this information has been acquired.

If an event can be modified, appropriate methods should be available, although due to PHP's nature of handling objects and the PSR-14 Listener signature, it cannot be guaranteed to be immutable.

4. Listeners: Extensions and PHP packages can add listeners that are registered. They are usually associated to Event objects by the name of the event (FQCN) to be listened on. It is the task of the ListenerProvider to provide configuration mechanisms to represent this relationship.

The main benefits of the EventDispatcher approach over Hooks and Extbase's SignalSlot Dispatcher is an implementation which helps extension authors to better understand the possibilities by having a strongly typed system based on PHP. In addition, it serves as a bridge to also incorporate other Events provided by frameworks that support PSR-14.

Impact

TYPO3's EventDispatcher serves as the basis to replace all Signal/Slots and hooks in the future, however for the time being, hooks and registered Slots work the same way as before, unless migrated to an EventDispatcher-like code, whereas a PHP E_USER_DEPRECATED error can be triggered.

Some hooks / signal/slots might not be replaced 1:1 to EventDispatcher, but rather superseded with a more robust or future-proof API.

Registration:

If an extension author wants to provide a custom Event Listener, an according entry with the tag event.listener can be added to the Configuration/Services.yaml file of that extension.

Example:

services:
  MyCompany\MyPackage\EventListener\NullMailer:
    tags:
      - name: event.listener
        identifier: 'myListener'
        event: TYPO3\CMS\Core\Mail\Event\AfterMailerInitializationEvent
        before: 'redirects, anotherIdentifier'

The tag name event.listener identifies that a listener should be registered.

The custom PHP class \MyCompany\MyPackage\EventListener\NullMailer serves as the listener, whereas the identifier is a common name so orderings can be built upon the identifier, the optional before and after attributes allow for custom sorting against identifier.

The event attribute is the FQCN of the Event object.

If no attribute method is given, the class is treated as Invokable, thus __invoke method is called.

An example listener, which hooks into the Mailer API to modify Mailer settings to not send any emails, could look like this:

namespace MyCompany\MyPackage\EventListener;
use TYPO3\CMS\Core\Mail\Event\AfterMailerInitializationEvent;

class NullMailer
{
    public function __invoke(AfterMailerInitializationEvent $event): void
    {
        $event->getMailer()->injectMailSettings(['transport' => 'null']);
    }
}

An extension can define multiple listeners.

Once the emitter is triggering an Event, this listener is called automatically. Be sure to inspect the Event PHP class to fully understand the capabilities provided by an Event.

Best Practices:

1. When configuring Listeners, it is recommended to add one Listener class per Event type, and have it called via __invoke().

2. When creating a new Event PHP class, it is recommended to add a Event suffix to the PHP class, and to move it into an appropriate folder e.g. Classes/Database/Event to easily discover Events provided by a package. Be careful about the context that should be exposed.

3. Emitters (TYPO3 Core or Extension Authors) should always use Dependency Injection to receive the EventDispatcher object as a constructor argument, where possible, by adding a type declaration for \Psr\EventDispatcher\EventDispatcherInterface .

Any kind of Event provided by TYPO3 Core falls under TYPO3's Core API deprecation policy, except for its constructor arguments, which may vary. Events that should only be used within TYPO3 Core, are marked as @internal, just like other non-API parts of TYPO3, but @internal Events will be avoided whenever technically possible.