Write a specialized ActivityPub transformer for an Event-Custom-Post-Type

NOTE: This documentation is also likely to be userful for content 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 object.

Hooks

To make the WordPress ActivityPub plugin use a custom transformer 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  && 'my_event_post_type'  === $wp_object->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 a 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-LD 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;
}

The ActivityPub object classes contain dynamic getter and setter functions: set_<property-name> and get_<property-name>. Of course, the property with property-name must exist for these to work. The function transform_object_properties tries to set all properties known to the ActivityPub object where a function called get_<property-name> exists in the current transformer class.

How to add new properties

Adding new properties is not encouraged to do at the transformer level. It's recommended to create a proper target ActivityPub object first. The target ActivityPub object also controls the the JSON-LD context via the constant JSON_LD_CONTEXT. Example.

Properties

NOTE: Within PHP all properties are snake_case, they will be transformed to the according CamelCase by the ActivityPub plugin. So if to you set start_time by using the ActivityPub objects class nfunction set_start_time or implementing a getter function in the transformer class called get_start_time the property startTime will be set accordingly in the JSON representation of the resulting ActivityPub object.

You can find all available event related properties in the event class along documentation and with links to the specifications.

Mandatory Properties for an Event

In order to ensure your events are comatible with other ActivityPub Event implementations there are several required properties that must be set by your transformer.

type: if using the Activitypub\Activity\Extended_Object\Event class the type will default to Event without doing anything.
startTime: the events start time
name: the title of the event

Recommended properties for an Event in order to achieve good interoperability with other ActivityPub platforms

summary: Other ActivityPub platforms that don't natively support event should use the summary (and the name) to display it as an converted object type. For example Mastodon converts an Event object to a Note. It is recommended to write the summary as text-centered with minimal HTML markup and that it contains the most important event details like place, time, etc.
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