EventDispatcher (PSR-14 Events)¶

The EventDispatcher system was added to extend TYPO3's Core behaviour in TYPO3 v10.0 via PHP code. In the past, this was done via Extbase's SignalSlot and TYPO3's custom hook system. The EventDispatcher system is a fully capable replacement for new code in TYPO3, as well as a possibility to migrate away from previous TYPO3 solutions.

Benni Mack: "Don't get hooked, listen to events! PSR-14 within TYPO3 v10" @ TYPO3 Developer Days 2019

Hint

For a basic example on listening to an event see chapter Listen to an event in the extension development how-to section.

Hint

Additional background on the implementation can be found at https://usetypo3.com/psr-14-events.html

Quick start¶

Dispatching an event¶

Create an event class.

An event class is basically a plain PHP object with getters for immutable properties and setters for mutable properties. It contains a constructor for all properties:

EXT:some_extension/Classes/Event/DoingThisAndThatEvent.php¶

final class DoingThisAndThatEvent {
   private string $mutableProperty;
   private int $immutableProperty;

   public function __construct(string $mutableProperty, int $immutableProperty) {
      // ...
   }

   // Getter for both properties, setters only for $mutableProperty;
}

Read more about implementing event classes.

Inject the EventDispatcher

If you are in a controller the EventDispatcher already got injected and in this case you can omit this step.

If the EventDispatcher is not yet available, you need have it injected:

EXT:some_extension/Classes/SomeClass.php¶

use Psr\EventDispatcher\EventDispatcherInterface;

final class SomeClass {
    private EventDispatcherInterface $eventDispatcher;

    public function injectEventDispatcher(EventDispatcherInterface $eventDispatcher): void
    {
        $this->eventDispatcher = $eventDispatcher;
    }
}

Dispatch the event

Create an event object with the data that should be passed to the listeners. Use the data of mutable properties however it suits your business logic:

EXT:some_extension/Classes/SomeClass.php¶

public function doSomething() {
    // ..
    /** @var DoingThisAndThatEvent $event */
    $event = $this->eventDispatcher->dispatch(
        new DoingThisAndThatEvent("foo", 2)
    );
    $someChangedValue = $event->getMutableProperty();
    // ...
}

Description of PSR-14 in the context of TYPO3¶

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 the following four components:

The EventDispatcher object¶

The EventDispatcher object is used to trigger an Event. TYPO3 has a custom EventDispatcher implementation. In PSR-14 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 will then be handed to all available listeners.

The ListenerProvider¶

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.

The events¶

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.

The listeners¶

Extensions and PHP packages can add listeners that are registered via YAML. They are usually associated to Event objects by the fully qualified name of the event to be listened on. It is the task of the ListenerProvider to provide configuration mechanisms to represent this relationship.

Advantages of the EventDispatcher over hooks¶

The main benefits of the EventDispatcher approach over hooks 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 on TYPO3 Core development in the future¶

TYPO3's EventDispatcher serves as the basis to replace all hooks in the future, however for the time being, hooks 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 might not be replaced 1:1 to EventDispatcher, but rather superseded with a more robust or future-proof API.

Implementing an event listener in your extension¶

Hint

For a basic example on listening to an event see chapter Listen to an event in the extension development how-to section.

Registering the event listener¶

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.

EXT:some_extension/Configuration/Services.yaml¶

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

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

The custom PHP class Vendor\MyExtension\EventListener\NullMailer serves as the listener whose handleEvent method is called once the event is dispatched. 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 the identifier of other listeners.

If no attribute method is given, the class is treated as invokable, thus its __invoke method will be called:

EXT:my_extension/Configuration/Services.yaml¶

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

Changed in version 11.3: The event tag can be omitted if the listener implementation has a corresponding event type in the method signature. In that case the event class is automatically derived from the method signature of the listener implementation.

The event listener class¶

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

EXT:some_extension/Classes/EventListener/NullMailer.php¶

namespace Vendor\SomeExtension\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¶

When configuring listeners, it is recommended to add one listener class per event type, and have it called via __invoke().
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 like Classes/Database/Event to easily discover events provided by a package. Be careful about the context that should be exposed.
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. Events marked as @internal should be avoided whenever technically possible.

Debugging event handling¶

A complete list of all registered event listeners can be viewed in the the module System > Configuration > Event Listeners (PSR-14). The system extension lowlevel has to be installed for this module to be available.

To debug all events that are actually dispatched during a frontend request you can use the admin panel:

Go to Admin Panel > Debug > Events and see all dispatched events. The system extension adminpanel has to be installed for this module to be available.