Write a specialized ActivityPub transformer for an Event Custom Post Type

NOTE: This documentation is also likely to be userfu for other types other than events.

The ActivityPub plugin offers a basic support for all post types out of the box, but it also allows the registration of external transformers. A transformer is a class that implements the abstract transformer class and is responsible for generating the ActivityPub JSON representation of an WordPress post or comment.

Hooks

To make the WordPress ActivityPub plugin use our custom transformer we simply add a filter to the activitypub_transformer hook which provides access to the transformer factory. The transformer factory determines which transformer is used to transform a WordPress object to ActivityPub.

function register_custom_transformer_for_my_post_type( $transformer, $wp_object, $object_class ) {
    // Return a custom transformer, if the WordPress object is a post and the post type matches.
    if ( 'WP_Post' === $object_class  && $wp_object->post_type === 'my_event_post_type' ) {
        require_once __DIR__ . '/includes/activitypub/transformer/class-my-custom-transformer.php';
        return new My_Event_Post_Type_Transformer( $wp_object );
    }
    return $transformer; // Return the default transformer.
}

add_filter(
    'activitypub_transformer',
    'register_custom_transformer_for_my_post_type',
    10,
    3
)

NOTE: issues could arrise if this filter gets applied multiple times, be sure to take the priority argument of the add_filter into account.

Writing an event transformer class

If you are writing a transformer for your custom post type it is recommended to start by extending the upstream generic post transformer. It contains useful default implementations for generating the attachments, rendering a proper content in HTML from either blocks or the classic editor, extracting tags and more.

/**
 * ActivityPub Transformer for my_event_post_type.
 */
class My_Event_Post_Type_Transformer extends Activitypub\Transformer\Post; {

The main function which controls the transformation is to_object. This one is called by the ActivityPub plugin to get the resulting ActivityPub representation as an PHP-array. The convertion to json takes place later and you don't need to cover that.

/**
  * Transform the WordPress Object into an ActivityPub Event Object.
  *
  * @return Activitypub\Activity\Extended_Object\Event
  */
public function to_object() {
  $activitypub_object = new Activitypub\Activity\Extended_Object\Event();
  $activitypub_object = $this->transform_object_properties( $activitypub_object );

  return $activitypub_object;
}

How to add new properties

How to customize the JSON-LD context

Mandatory properties

type

	/**
	 * Returns the ActivityStreams 2.0 Object-Type for an Event.
	 *
	 * @see https://www.w3.org/TR/activitystreams-vocabulary/#dfn-event
	 * @return string The Event Object-Type.
	 */
	protected function get_type() {
		return 'Event';
	}

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