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How to create and expose computed properties to the REST API in Drupal 8

In Drupal 8.5.0, the "processed" property of text fields is available in REST which means that REST apps can render the HTML output of a textarea without worrying about the filter formats.

In this post, I will show you how you can add your own processed fields to be output via the REST API.

by Jibran Ijaz /

The "processed" property mentioned above is what is known as a computed property on the textarea field.

The ability to make the computed properties available for the REST API like this can be very helpful. For example, when the user inputs the raw value and Drupal performs some complex logical operations on it before showing the output.

Drupal fieldable entities can also have computed properties and those properties can also be exposed via REST. I used the following solution to expose the data of an entity field which takes raw data from the users and perform some complex calculations on it.

First of all, we need to write hook_entity_bundle_field_info to add the property and because it is a computed field we don't need to implement hook_entity_field_storage_info.

<?php // my_module/my_module.module /** * @file * Module file for my_module. */ use Drupal\my_module\FieldStorageDefinition; use Drupal\my_module\Plugin\Field\MyComputedItemList /** * Implements hook_entity_bundle_field_info(). */ function my_module_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) { $fields = []; // Add a property only to nodes of the 'my_bundle' bundle. if ($entity_type->id() === 'node' && $bundle === 'my_bundle') { // It is not a basefield so we need a custom field storage definition see // $fields['my_computed_property'] = FieldStorageDefinition::create('string') ->setLabel(t('My computed property')) ->setDescription(t('This is my computed property.')) ->setComputed(TRUE) ->setClass(MyComputedItemList::class) ->setReadOnly(FALSE) ->setInternal(FALSE) ->setDisplayOptions('view', [ 'label' => 'hidden', 'region' => 'hidden', 'weight' => -5, ]) ->setDisplayOptions('form', [ 'label' => 'hidden', 'region' => 'hidden', 'weight' => -5, ]) ->setTargetEntityTypeId($entity_type->id()) ->setTargetBundle($bundle) ->setName('my_computed_property') ->setDisplayConfigurable('form', FALSE) ->setDisplayConfigurable('view', FALSE); } return $fields; }

Then we need the MyComputedItemList class to perform some magic. This class will allow us to set the computed field value.

<?php // my_module/src/Plugin/Field/MyComputedItemList.php namespace Drupal\my_module\Plugin\Field; use Drupal\Core\Field\FieldItemList; use Drupal\Core\TypedData\ComputedItemListTrait; /** * My computed item list class. */ class MyComputedItemList extends FieldItemList { use ComputedItemListTrait; /** * {@inheritdoc} */ protected function computeValue() { $entity = $this->getEntity(); if ($entity->getEntityTypeId() !== 'node' || $entity->bundle() !== 'my_bundle' || $entity->my_some_other_field->isEmpty()) { return; } $some_string = some_magic($entity->my_some_other_field); $this->list[0] = $this->createItem(0, $some_string); }

The field we add is not a base field so we can't use \Drupal\Core\Field\BaseFieldDefinition. There is an open core issue to address that but in tests there is a workaround using a copy of \Drupal\entity_test\FieldStorageDefinition:

<?php // my_module/src/FieldStorageDefinition.php namespace Drupal\my_module; use Drupal\Core\Field\BaseFieldDefinition; /** * A custom field storage definition class. * * For convenience we extend from BaseFieldDefinition although this should not * implement FieldDefinitionInterface. * * @todo Provide and make use of a proper FieldStorageDefinition class instead: * */ class FieldStorageDefinition extends BaseFieldDefinition { /** * {@inheritdoc} */ public function isBaseField() { return FALSE; } }

Last but not least we need to announce our property definition to the entity system so that it can keep track of it. As it is an existing bundle we can write an update hook. Otherwise, we'd need to implement hook_entity_bundle_create.

<?php // my_module/my_module.install /** * @file * Install file for my module. */ use Drupal\my_module\FieldStorageDefinition; use Drupal\my_module\Plugin\Field\MyComputedItemList; /** * Adds my computed property. */ function my_module_update_8001() { $fields['my_computed_property'] = FieldStorageDefinition::create('string') ->setLabel(t('My computed property')) ->setDescription(t('This is my computed property.')) ->setComputed(TRUE) ->setClass(MyComputedItemList::class) ->setReadOnly(FALSE) ->setInternal(FALSE) ->setDisplayOptions('view', [ 'label' => 'hidden', 'region' => 'hidden', 'weight' => -5, ]) ->setDisplayOptions('form', [ 'label' => 'hidden', 'region' => 'hidden', 'weight' => -5, ]) ->setTargetEntityTypeId('node') ->setTargetBundle('my_bundle') ->setName('my_computed_property') ->setDisplayConfigurable('form', FALSE) ->setDisplayConfigurable('view', FALSE); // Notify the storage about the new field. \Drupal::service('field_definition.listener')->onFieldDefinitionCreate($fields['my_computed_property']); }

The beauty of this solution is that I don't have to write a custom serializer to normalize the output. Drupal Typed Data API is doing all the heavy lifting.

Related Drupal core issues:

Posted by Jibran Ijaz
Senior Drupal Developer


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