src/EventEmitterTrait.php
public function on(string $event, callable $listener) : self
    {
        $this->listeners[$event][] = $listener;

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

        return $this;
    }
@param string   $event
@param callable $listener

@return $this