Create a plugin for Memory Limit Policy

This article is to describe how to create a plugin for the Memory Limit Policy module.

The context

For this exercise, we will assume that our site has a content type “Article” which has a “Tags” field. This “Tags” field is an entity reference to one or multiple taxonomy terms from the “Tags” vocabulary. When the article is being displayed, if the “tags” field is filled, a process is triggered to determine the best articles to suggest to the visitor based on the selected tags. Because of the complexity of this process, if the field has more than 2 values, the default PHP memory_limit of 128M will not be enough. In that context, it would not be possible to use the “Path” constraint plugin from memory limit policy. 

The plugin

Assuming we have a module project_article to handle the custom code around the article feature. We first need to add memory_limit_policy as a dependency in project_article.info.yml.

Then, we create a src/Plugin/MemoryLimitConstraint/taggedArticles.php file in project_article module. The class taggedArticles must extend MemoryLimitConstraintBase and implement the ContainerFactoryPluginInterface interface. It must implement the create, getSummary and evaluate methods. Constraint plugins are based on annotations. To register the plugin, we must give an id, a title and a description. As a first step, we will simply make a constraint which always applies. This is the code we should have at this step:

<?php

namespace Drupal\project_article\Plugin\MemoryLimitConstraint;

use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\memory_limit_policy\Annotation\MemoryLimitConstraint;
use Drupal\memory_limit_policy\MemoryLimitConstraintBase;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Custom Memory limit constraint for articles.
 *
 * @MemoryLimitConstraint(
 *   id = "tagged_articles",
 *   title = @Translation("Tagged articles"),
 *   description = @Translation("Apply for articles which the 'tags' field is filled.")
 * )
 */
class TaggedArticles extends MemoryLimitConstraintBase implements ContainerFactoryPluginInterface {

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getSummary() {
    return $this->t('Apply to articles which the "tags" field is filled.');
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate() {
    return TRUE;
  }
}

We now need to update the evaluate method to return TRUE only if the current page is an article node and if the tags field has more than 2 values. We can use the current_route_match service from Drupal core to determine the route and get the parameters we need to decide if the constraint applies or not. After updating the create method to invoke the current_route_match service, defining a __construct method to access the service and updating the evaluate method, our TaggedArticles.php plugin looks like this:

<?php

namespace Drupal\project_article\Plugin\MemoryLimitConstraint;

use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\memory_limit_policy\Annotation\MemoryLimitConstraint;
use Drupal\memory_limit_policy\MemoryLimitConstraintBase;
use Drupal\node\Entity\Node;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Custom Memory limit constraint for articles.
 *
 * @MemoryLimitConstraint(
 *   id = "tagged_articles",
 *   title = @Translation("Tagged articles"),
 *   description = @Translation("Apply for articles which the 'tags' field is filled.")
 * )
 */
class TaggedArticles extends MemoryLimitConstraintBase implements ContainerFactoryPluginInterface {

  /**
   * Route match service.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;

  /**
   * Constructs constraint plugin.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param RouteMatchInterface $route_match
   *   The route match service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, RouteMatchInterface $route_match) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);

    $this->routeMatch = $route_match;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('current_route_match')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getSummary() {
    return $this->t('Apply to articles which the "tags" field is filled.');
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate() {
    $route_name = $this->routeMatch->getRouteName();

    if ($route_name === 'entity.node.canonical') {
      $node = $this->routeMatch->getParameter('node');

      if ($node instanceof Node && $node->bundle() === 'article') {
        $terms = $node->get('field_tags')->getValue();

        // Apply the constraint if "field_tags" has more than 2 values.
        if (count($terms) > 2) {
          return TRUE;
        }
      }
    }
    return FALSE;
  }
}

At this point, we can configure a policy as described in this post and confirm it applies properly by using the debug headers.

However, this exercise would not be exhaustive without covering the configuration form. As of now, our plugin does not require any specific setting. When we select it, the configuration form only shows the “Negate the constraint” option which is provided by default. Let’s push the exercise further and avoid hard coding the limit of 2 “tags” to be referenced for the constraint to apply.

We need to implement the buildConfigurationForm and submitConfigurationForm method and define the schema of our configuration.

We create a config/schema/memory_limit_policy_tagged_articles.schema.yml file in the project_article module. Our configuration will be very simple as it only defines the number of tags to be filled for the constraint to apply.

memory_limit_policy.constraint.plugin.tagged_articles:
  type: memory_limit_policy.constraint.plugin
  mapping:
    tags_threshold:
      type: integer
      label: 'Number of tags threshold'

The configuration form only has one input field to enter the number of tags. We can update the getSummary method so it displays the entered value. We finally need to update the evaluate method to use the configured value as the threshold. Here is the final code of our plugin:

<?php

namespace Drupal\project_article\Plugin\MemoryLimitConstraint;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\memory_limit_policy\Annotation\MemoryLimitConstraint;
use Drupal\memory_limit_policy\MemoryLimitConstraintBase;
use Drupal\node\Entity\Node;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Custom Memory limit constraint for articles.
 *
 * @MemoryLimitConstraint(
 *   id = "tagged_articles",
 *   title = @Translation("Tagged articles"),
 *   description = @Translation("Apply for articles which the 'tags' field is filled.")
 * )
 */
class TaggedArticles extends MemoryLimitConstraintBase implements ContainerFactoryPluginInterface {

  /**
   * Route match service.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;

  /**
   * Constructs constraint plugin.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param RouteMatchInterface $route_match
   *   The route match service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, RouteMatchInterface $route_match) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);

    $this->routeMatch = $route_match;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('current_route_match')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getSummary() {
    return $this->t(
      'Apply to articles which the "tags" field has more than @tags_threshold values.',
      [
        '@tags_threshold' => $this->configuration['tags_threshold'],
      ]
    );
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildConfigurationForm($form, $form_state);

    $form['tags_threshold'] = [
      '#type' => 'number',
      '#title' => $this->t('Tags threshold'),
      '#description' => $this->t('The minimum number of tags to filled in the article for the constraint to apply.'),
      '#default_value' => $this->getConfiguration()['tags_threshold'] ?? 2,
      '#min' => 0,
      '#step' => 1,
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::submitConfigurationForm($form, $form_state);

    $this->configuration['tags_threshold'] = $form_state->getValue('tags_threshold');
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate() {
    $route_name = $this->routeMatch->getRouteName();

    if ($route_name === 'entity.node.canonical') {
      $node = $this->routeMatch->getParameter('node');

      if ($node instanceof Node && $node->bundle() === 'article') {
        $terms = $node->get('field_tags')->getValue();

        // Apply the constraint if "field_tags" has more than the configured values.
        if (count($terms) > $this->configuration['tags_threshold']) {
          return TRUE;
        }
      }
    }
    return FALSE;
  }
}

If the plugin you wrote is generic enough to be used by other projects, feel free to open an issue on the memory limit policy module’s issue queue.