For https://medium.com/@pavarnos/lazy-event-dispatching-with-symfony-1fc8813c44e1

LazyEventDispatcher.php

<?php
use ISV\Service\Cache\CacheInterface;
use ISV\Service\Cache\FileCache;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\EventDispatcher\EventDispatcher;

/**
 * Overrides the symfony event dispatcher so that we don't call addListener() 400 times for every request when 90%
 * of requests never dispatch an event.
 *
 * When dispatch() is called, we addListener() first and then dispatch().
 *
 * Stole some ideas from ContainerAwareEventDispatcher
 */
class LazyEventDispatcher extends EventDispatcher
{
    const CACHE_KEY = 'LazyEventDispatcher';

    // array index in $listeners
    const SERVICE = 0;
    const METHOD = 1;
    const PRIORITY = 2;

    /** @var CacheInterface */
    private $cache;

    /** @var array eventName => array of ['service', 'method', 'priority'] one per listener */
    private $listeners = [];

    /** @var ContainerInterface */
    private $container;

    /**
     * LazyEventDispatcher constructor.
     * @param ContainerInterface $container
     * @param CacheInterface     $cache for unit testing: so we can inject a different cache if needed
     */
    public function __construct(ContainerInterface $container, CacheInterface $cache = null)
    {
        $this->container = $container;
        $this->cache = $cache;
    }

    /**
     * @return array
     */
    public function getLazyListeners()
    {
        if (empty($this->listeners)) {
            $this->listeners = $this->getCache()->get(self::CACHE_KEY);
            if (empty($this->listeners)) {
                $this->listeners = [];
            }
        }
        return $this->listeners;
    }

    /**
     * save them to cache so we can load them later.
     * called at compile time.
     */
    public function saveLazyListeners()
    {
        ksort($this->listeners);
        $this->getCache()->set(self::CACHE_KEY, $this->listeners);
    }

    /**
     * Adds an event listener that listens on the specified events. Called by a container compiler class at build time.
     *
     * @param string $eventName   The event to listen for
     * @param string $serviceName usually a class name
     * @param string $methodName  eg onPersonDelete
     * @param int    $priority    The higher this value, the earlier an event
     *                            listener will be triggered in the chain (defaults to 0)
     */
    public function addLazyListener($eventName, $serviceName, $methodName, $priority = 0)
    {
        $this->listeners[$eventName][] = [
            self::SERVICE  => $serviceName,
            self::METHOD   => $methodName,
            self::PRIORITY => $priority,
        ];
    }

    /**
     * {@inheritdoc}
     */
    public function dispatch($eventName, Event $event = null)
    {
        if ($this->hasListeners($eventName)) {
            return parent::dispatch($eventName, $event);
        }

        $allListeners = $this->getLazyListeners();
        if (!isset($allListeners[$eventName])) {
            return parent::dispatch($eventName, $event);
        }

        // add them then dispatch
        foreach ($allListeners[$eventName] as $handler) {
            $service = $this->container->get($handler[self::SERVICE]);
            $this->addListener($eventName, [$service, $handler[self::METHOD]], $handler[self::PRIORITY]);
        }
        return parent::dispatch($eventName, $event);
    }

    /**
     * no need to clear the cache explicitly: it will be automatically rebuilt when the container is rebuilt
     * @return CacheInterface
     */
    private function getCache()
    {
        if (empty($this->cache)) {
            $this->cache = new FileCache(CACHE_PATH);
        }
        return $this->cache;
    }
}