emitter.php

<?php

interface EventDispatcher
{
    public function listen(string $event, callable $listener);
    public function emit(object $event): object;
}

class ExampleEventDispatcher implements EventDispatcher
{
    protected $listeners = [];

    public function listen(string $event, callable $listener)
    {
        $this->listeners[$event][] = $listener;
    }

    public function emit(object $event)
    {
        $listeners = $this->listeners[get_class($event)] ?? [];

        foreach ($listeners as $listener) {
            $listener($event);
        }

        return $event;
    }
}

interface CanStopPropagation {
    public function stopPropagation();
    public function hasStoppedPropagation(): bool;
}

class PropagationAwareEventDispatcher implements EventDispatcher
{
    protected $listeners = [];

    public function listen(string $event, callable $listener)
    {
        $this->listeners[$event][] = $listener;
    }

    public function emit(object $event)
    {
        $listeners = $this->listeners[get_class($event)] ?? [];
        $canStopPropagation = $event instanceof CanStopPropagation;

        foreach ($listeners as $listener) {
            if ($canStopPropagation && $event->hasStoppedPropagation()) {
                goto end;
            }
            $listener($event);
        }
        
        end:
        return $event;
    }
}

usage.php

<?php

$dispatcher = new ExampleEventDispatcher();

class SomethingHappened {}

class ListenerOfSomethingHappened {
    public function __invoke(SomethingHappened $event)
    {
        var_dump($event);
    }
}

$dispatcher->listen(SomethingHappened::class, new ListenerOfSomethingHappened());

$dispatcher->emit(new SomethingHappened());