protected function checkEtag($etag)
{
    return isset($_SERVER['HTTP_IF_NONE_MATCH'])&&$_SERVER['HTTP_IF_NONE_MATCH']==$etag;
}

protected function checkLastModified($lastModified)
{
    return isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])&&@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE'])>=$lastModified;
}

protected function generateEtag($seed)
{
    return '"'.base64_encode(sha1(serialize($seed),true)).'"';
}

protected function getEtagValue()
{
    if($this->etagSeedExpression)
        return $this->generateEtag($this->evaluateExpression($this->etagSeedExpression));
    elseif($this->etagSeed)
        return $this->generateEtag($this->etagSeed);
    return false;
}

protected function getLastModifiedValue()
{
    if($this->lastModifiedExpression)
    {
        $value=$this->evaluateExpression($this->lastModifiedExpression);
        if(is_numeric($value)&&$value==(int)$value)
            return $value;
        elseif(($lastModified=strtotime($value))===false)
            throw new CException(Yii::t('yii','Invalid expression for CHttpCacheFilter.lastModifiedExpression: The evaluation result "{value}" could not be understood by strtotime()',
                array('{value}'=>$value)));
        return $lastModified;
    }

    if($this->lastModified)
    {
        if(is_numeric($this->lastModified)&&$this->lastModified==(int)$this->lastModified)
            return $this->lastModified;
        elseif(($lastModified=strtotime($this->lastModified))===false)
            throw new CException(Yii::t('yii','CHttpCacheFilter.lastModified contained a value that could not be understood by strtotime()'));
        return $lastModified;
    }
    return false;
}

public function preFilter($filterChain)
{
    // Only cache GET and HEAD requests
    if(!in_array(Yii::app()->getRequest()->getRequestType(), array('GET', 'HEAD')))
        return true;

    $lastModified=$this->getLastModifiedValue();
    $etag=$this->getEtagValue();

    if($etag===false&&$lastModified===false)
        return true;

    if($etag)
        header('ETag: '.$etag);

    if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])&&isset($_SERVER['HTTP_IF_NONE_MATCH']))
    {
        if($this->checkLastModified($lastModified)&&$this->checkEtag($etag))
        {
            $this->send304Header();
            $this->sendCacheControlHeader();
            return false;
        }
    }
    elseif(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
    {
        if($this->checkLastModified($lastModified))
        {
            $this->send304Header();
            $this->sendCacheControlHeader();
            return false;
        }
    }
    elseif(isset($_SERVER['HTTP_IF_NONE_MATCH']))
    {
        if($this->checkEtag($etag))
        {
            $this->send304Header();
            $this->sendCacheControlHeader();
            return false;
        }

    }

    if($lastModified)
        header('Last-Modified: '.gmdate('D, d M Y H:i:s', $lastModified).' GMT');

    $this->sendCacheControlHeader();
    return true;
}

protected function send304Header()
{
    $httpVersion=Yii::app()->request->getHttpVersion();
    header("HTTP/$httpVersion 304 Not Modified");
}

protected function sendCacheControlHeader()
{
    if(Yii::app()->session->isStarted)
    {
        session_cache_limiter('public');
        header('Pragma:',true);
    }
    header('Cache-Control: '.$this->cacheControl,true);
}