Response Context

This feature is available in Contao 4.12 and later.

This section describes the “Response Context” concept. A concept that is special to Contao and thus deserves its own section in the documentation.

Motivation

Content Management Systems do not share all the same properties as static site generators, or your good old Symfony based website. In a traditional Symfony application, you as the developer control 100% of the output of a particular HTML website. Speaking in Symfony terms, your controller controls the full Response instance, from its HTTP headers to its content, everything from the opening <html> to the closing </html>, all the scripts, the stylesheets, the page title and the description.

In Contao, however, most of the content is dynamically created. That’s why it’s a Content Management System after all. Let’s take e.g. a news reader front end module as an example. It displays the detail of a certain news entry and thus also would like to adjust the <title> tag in the <head> section of the HTML document. The controller, however, does not know if that news reader front end module was placed on that page and the front end module on the other hand does not even know if there is a <title> element it can update. The news reader element is a fragment only so it can be used in a completely different context other than a regular HTML web page. E.g. it could be rendered as an ESI fragment, a partial loaded via Ajax or rendered into an e-mail. In all cases there is no <title> element to update.

The “Response Context” is here to solve this problem.

Process overview

  1. Contao determines the responsible Page Controller based on the URL that was requested by the visitor.
  2. The Controller knows in which context it is used. It has access to the Request and produces the Response. It now defines the ResponseContext and its capabilities. It then renders the different fragments into its content.
  3. The different fragments can access the current ResponseContext (could also be that there’s none!) and can check for its capabilities. Depending on what they can do, they can decide what they want to do (e.g. update the page title).
  4. The Page Controller applies the changes of the ResponseContext in the way it thinks is correct, produces the Response and finishes the current ResponseContext.

Creating a Response Context

The ResponseContext class is a container for capabilities. Basically, a capability is just a “service” that can be accessed by its class name or in case it implements interfaces, by their respective interfaces. Setting and accessing the current ResponseContext is simplified by the ResponseContextAccessor:

<?php

// src/Controller/Page/ExamplePageController.php
namespace App\Controller\Page;

use Contao\CoreBundle\ServiceAnnotation\Page;
use Contao\PageModel;
use Contao\CoreBundle\Routing\ResponseContext\HtmlHeadBag\HtmlHeadBag;
use Contao\CoreBundle\Routing\ResponseContext\ResponseContext;
use Contao\CoreBundle\Routing\ResponseContext\ResponseContextAccessor;
use Symfony\Component\HttpFoundation\Response;

/**
 * @Page
 */
class ExamplePageController
{
    private ResponseContextAccessor $responseContextAccessor;
    
    public function __construct(ResponseContextAccessor $responseContextAccessor)
    {
        $this->responseContextAccessor = $responseContextAccessor;
    }
    
    public function __invoke(Request $request, PageModel $pageModel): Response
    {
        $responseContext = new ResponseContext();
        $responseContext->add(new HtmlHeadBag());
        $this->responseContextAccessor->setResponseContext($responseContext);
        
        // Render elements, front end modules, everything can access the current context
        // by checking e.g. $responseContext->has(HtmlHeadBag::class) and do something with it.
        
        // Setting e.g. the <title>
        $myHtmlContent = '<html><head><title>%s</title></head><body>Content</body></html>';
        $myHtmlContent = sprintf($myHtmlContent, $responseContext->get(HtmlHeadBag::class)->getTitle());
        
        $response = new Response($myHtmlContent);
        $this->responseContextAccessor->finalizeCurrentContext($response);
        return $response;
    }
}

The ResponseContext also contains an addLazy() method which you should always prefer over add() if you can. Just because your controller provides a capability in the context does not mean there’s even any module or element going to use it. We can save some resources by making them lazy.

The core capabilities/services

The Contao core-bundle ships with currently two core services which you can reuse.

Have a look at the CoreResponseFactory in case you want to reuse the default Contao ResponseContext.

The HtmlHeadBag

The class name of the HtmlHeadBag service already implies, that this will have more capabilities in the future. The goal should be to be able to manage everything within <head> dynamically. Right now, it’s a very basic service that allows you to override <title> by calling setTitle(). Same goes for setMetaDescription() and setMetaRobots().

The JsonLdManager

The JsonLdManager is a central place to manage JSON-LD data collected within all the elements. It is capable of managing multiple schemas, but the most likely use case will be schema.org data which is why this is used as an example here:

<?php

use Contao\CoreBundle\Routing\ResponseContext\JsonLd\JsonLdManager;
use Contao\CoreBundle\Routing\ResponseContext\ResponseContext;
use Spatie\SchemaOrg\ImageObject;

// This is how the JsonLdManager is created
$schemaManager = new JsonLdManager(new ResponseContext());

// This is how you would access it from the current context (in case it exists)
$schemaManager = $this->responseContextAccessor->getCurrentContext()->get(JsonLdManager::class);

// Get the graph for schema.org
$graph = $schemaManager->getGraphForSchema(JsonLdManager::SCHEMA_ORG);

// Add a new ImageObject
$graph->add((new ImageObject())->name('Name')->caption('Caption'));

/**
 * This will generate the following now:
 * 
 * <script type="application/ld+json">
 * [
 *     {
 *         "@context": "https:\/\/schema.org",
 *         "@graph": [
 *             {
 *                 "@type": "ImageObject",
 *                 "name": "Name",
 *                 "caption": "Caption"
 *             }
 *         ]
 *     }
 * ]
 * </script>
 */
$schemaManager->collectFinalScriptFromGraphs()

Future

The Response Context will likely be the place where additional possibilities will be introduced such as

  • Adding/managing <script> tags
  • Adding/managing <link> tags
  • Adding/managing <meta> tags