Future::then PHP 메소드

    public function then($onfulfill, $onreject = NULL)
    {
        if (!is_callable($onfulfill)) {
            $onfulfill = NULL;
        }
        if (!is_callable($onreject)) {
            $onreject = NULL;
        }
        $next = new Future();
        if ($this->state === self::FULFILLED) {
            $this->privateResolve($onfulfill, $next, $this->value);
        } elseif ($this->state === self::REJECTED) {
            $this->privateReject($onreject, $next, $this->reason);
        } else {
            array_push($this->subscribers, array('onfulfill' => $onfulfill, 'onreject' => $onreject, 'next' => $next));
        }
        return $next;
    }