symfony_event_dispatcher_lazy_listener.php

<?php
// This code is what the Symfony Dependency Injection container performs
// under the hood when registering a listener.
$eventDispatcher->addListener(
    KernelEvents::REQUEST,
    function ($event) {
        // only when here, MyListener and ExpensiveDependency trigger autoloading
        // and the time of calling their constructor is spent.
        (new MyListener(new ExpensiveDependency))->onKernelRequest($event);
    },
);

// dispatch "unwraps" the listener by calling the closure, only then MyListener
// and the ExpensiveDependency are created. 
$eventDispatcher->dispatch(
    new RequestEvent(/** ... */), KernelEvents::REQUEST)
);