I know my question is strange and I should change the response in the Magento way.

I have an observer subscribed to controller_front_send_response_before event.


<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    <event name="controller_front_send_response_before">
        <observer name="roland_controller_front_send_response_before"


namespace Roland\HelloWorld\Model\Observers;

use \Magento\Framework\Event\ObserverInterface;

class ResponseBefore implements ObserverInterface {

    public function execute(\Magento\Framework\Event\Observer $observer) {



    public function playWithResponse($response) {

        $head = '<script>alert("'.time().'");</script>';
        $response->setBody(preg_replace('/<\/head>/', $head . '</head>', $response->getBody(), 1));


It works fine when Full page cache not enabled. Every time I refresh the page, I get an alert with the current server time. When Full page cache is enabled, I would like to make my code cached into Full page cache. Currently my observer called on every page load as the Full page cache comes before the controller_front_send_response_before event.

So I checked how's Magento 2 Full page cache works in magento/module-page-cache/Observer/Process/LayoutRenderElement.php

    public function execute(\Magento\Framework\Event\Observer $observer)
    $event = $observer->getEvent();
    /** @var \Magento\Framework\View\Layout $layout */
    $layout = $event->getLayout();
    if ($this->isFullPageCacheEnabled() && $layout->isCacheable()) {
        $name = $event->getElementName();
        /** @var \Magento\Framework\View\Element\AbstractBlock $block */
        $block = $layout->getBlock($name);
        $transport = $event->getTransport();
        if ($block instanceof \Magento\Framework\View\Element\AbstractBlock) {
            $blockTtl = $block->getTtl();
            $output = $transport->getData('output');
            if (isset($blockTtl) && $this->isVarnishEnabled()) {
                $output = $this->_wrapEsi($block, $layout);
            } elseif ($block->isScopePrivate()) {
                $output = sprintf(
                    '<!-- BLOCK %1$s -->%2$s<!-- /BLOCK %1$s -->',
            $transport->setData('output', $output);

I saw that it caches every block, but I haven't got any further.

Here is the part of my $event->getElementName(): mailcheck_init emailCapture copyright before.body.end page.wrapper root

I thought if I change the content in the observer before the page cache that might worth, but I have no luck.

if($event->getElementName() == 'root'){
    $transport = $event->getTransport();
    $transport->setData('output', 'test');

Do you have any suggestion how should I achieve the desired result?

2 Answers 2


So, finally I have a real solution. So I was able to track down Magento2 full page cache and turned out that it does the caching as a plugin: Magento\PageCache\Model\App\FrontController\BuiltinPlugin in the aroundDispatch.


Does the work of storing the serialized $response object.

So the solution was to create a plugin for the after event:


<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    <type name="Magento\Framework\Controller\ResultInterface">
        <plugin name="change-result-before-cache" type="Roland\HelloWorld\Model\Plugins\ChangeResult"/>



namespace Roland\HelloWorld\Model\Plugins;

use Magento\Framework\App\Response\Http as ResponseHttp;

class ChangeResult {

    public function afterRenderResult(ResponseHttp $response) {
        $response->setBody(preg_replace('/<\/head>/', '<script>alert("' . time() . '");</script>' . '</head>', $response->getBody(), 1));


The result

When page cache enabled, you should always see the same timestamp for every refresh.

Related docs

How Magento2 plugins work - Alan Storm Magento 2 Full page caching - Alan Storm


@Roland Soós your answer is good but when I am apply facing some error below code resolve my error.

  use Magento\Framework\App\Response\Http as ResponseHttp;

Add above class

  protected $response;

  public function __construct(
       ResponseHttp $response
  ) {
       $this->response = $response;

  public function afterRenderResult(\Magento\Framework\Controller\ResultInterface $process) 
       $this->response->setBody(preg_replace('/<\/head>/', '<script>alert("' . time() . '");</script>' . '</head>', $this->response->getBody(), 1));
  • Hi, this is called but it gives Fatal error: Uncaught TypeError: Argument 2 passed to Magento\PageCache\Model\Controller\Result\BuiltinPlugin::afterRenderResult() must implement interface Magento\Framework\Controller\ResultInterface, null given error. Have you faced this? Please let me know I am facing the issue. Thanks
    – Pribhav
    Jun 28, 2019 at 6:35

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