Listener Aggregates

Listener aggregates exist to facilitate two operations:

  • Attaching many listeners at once.
  • Attaching stateful listeners.

A listener aggregate is a class implementing Zend\EventManager\ListenerAggregateInterface, which defines two methods:

attach(EventManagerInterface $events, $priority = 1);
detach(EventManagerInterface $events);

To attach an aggregate to an event manager, you pass the event manager to the aggregate's attach() method; in that method, you will then attach listeners to the events you are interested in.

Implementation

To implement ListenerAggregateInterface, you need to define the attach() and detach() methods. A typical implementation will look something like this:

use Zend\EventManager\EventInterface;
use Zend\EventManager\EventManagerInterface;
use Zend\EventManager\ListenerAggregateInterface;

class Aggregate implements ListenerAggregateInterface
{
    private $listeners = [];

    public function attach(EventManagerInterface $events, $priority = 1)
    {
        $this->listeners[] = $events->attach('something', [$this, 'onSomething'], $priority);
        $this->listeners[] = $events->attach('else', [$this, 'onElse'], $priority);
        $this->listeners[] = $events->attach('again', [$this, 'onAgain'], $priority);
    }

    public function detach(EventManagerInterface $events)
    {
        foreach ($this->listeners as $index => $listener) {
            $events->detach($listener);
            unset($this->listeners[$index]);
        }
    }

    public function onSomething(EventInterface $event)
    {
        // handle event
    }

    public function onElse(EventInterface $event)
    {
        // handle event
    }

    public function onAgain(EventInterface $event)
    {
        // handle event
    }
}

Because the logic for detaching is essentially the same in all implementations, we provide two facilities for implementing this:

  • Zend\EventManager\AbstractListenerAggregate is an abstract class that defines the $listeners property and the detach() method. You may extend it in order to create an implementation.
  • Zend\EventManager\ListenerAggregateTrait is a trait that defines the $listeners property and the detach() method. You may implement Zend\EventManager\ListenerAggregateInterface and use this trait to implement the detach() logic.

Usage

To use an aggregate listener, you need to attach it to the event manager. As noted in the intro to this section, you do so by passing the event manager to the aggregate's attach() method:

// Assume $events is an EventManager instance, and $aggregate is an instance of
// the Aggregate class defined earlier.
$aggregate->attach($events);

Recommendations

  • We recommend using listener aggregates when you have several listeners that are related and/or share common dependencies and/or business logic. This helps keep the logic in the same location, and helps reduce dependencies.

  • We recommend using the verbiage on<Event Name> to name your listener methods. This helps hint that they will be triggered on an event, and semantically ties them to the specific event name.

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