public function getUnregisteredDeviceTokens(array $devices_token)
    {
        $tokens = [];

        if (!empty($this->feedback->tokenFailList)) {
            $tokens = $this->feedback->tokenFailList;
        }
        if (!empty($this->feedback->apnsFeedback)) {
            $tokens = array_merge($tokens, array_pluck($this->feedback->apnsFeedback, 'devtoken'));
        }

        return $tokens;
    }