protected function getCallable()
    {
        $callback = null;
        if (is_object($this->runnable) && $this->runnable instanceof Runnable) {
            $callback = array($this->runnable, 'run');
        } elseif (is_callable($this->runnable)) {
            $callback = $this->runnable;
        } else {
            $callback = array($this, 'run');
        }

        return $callback;
    }