public function setCallback($callback)
    {
        // If the provided $value is not a closure, make it one now for easy resolution.
        if (!is_callable($callback)) {
            $callback = function () use ($callback) {
                return $callback;
            };
        }

        if ($this->protected) {
            return $this;
        }

        $this->callback = $callback;

        return $this;
    }