This document describes proposals for the evolvment of the [WordPress ActivityPub plugin](https://github.com/Automattic/WordPress-ActivityPub/) in the field of the interaction with other WordPress plugins that may add additional ActivityPub features.
- **Transformers:** a piece of code that converts items of particular WordPress post types to an ActivityPub object. For example a transformer that can transform:
- every post type to a `Note`
-`post` or `page` to `Note`, `Article`, `Image`, `Audio` or `Article` depending on the [post-format](https://wordpress.org/documentation/article/post-formats/)
-`tribe_events` (Event post the of [The Event Calendar](https://wordpress.org/plugins/the-events-calendar/)) to `Event`
-`podcast` (Podcast post of [Podlove Podcast Publisher](https://github.com/podlove/podlove-publisher)) to `PodcastEpisode` as ([proposed and implemented by Castopod](https://code.castopod.org/adaures/castopod/-/blob/main/app/Libraries/PodcastEpisode.php))
- should be aware (or even control) the whole chain from creating some content within WordPress to being published via ActivityPub. For sure, it should be the only plugin that directly sends and receives ActivityPub.
- should not get any more complex by default, the out-of-the box functionality/features and simplicity should be similar to version 1.0.0.
- offers other plugins the possibility to register custom transformers.
- (maybe) offers other plugins the possibility to register new WordPress actor types, e.g. the `tribe_organizer` post type of [The Event Calendar](https://wordpress.org/plugins/the-events-calendar/).
- handles the controls collisions of actor names:
* collisions that are already present during activation of a WordPress actor type on the settings page
* collisions that happen afterwards (e.g. a new user registering with the same username as the blog-wide actor)
* (maybe) show notifications/warnings when using an actor name which has been used and followed by in the past, as there may be issues due to caching of the old public key.
## Goals of the WordPress Event Fedeartion project
The event federations project goal is make it possible that events created within an event plugin can federate properly as an ActivityPub object with Type `Event` along with all the meta-data belonging to the event, as found in common ActivityPub implementations, e.g. of Mobilizon, in order to ensure maximum compatibly between those services.
Nevertheless, we see a lot of other cases that can benefit, if the primary goals of the event federation project are achieved this using a modularized approach as proposed below. The following list indicates examples what might be those other benefits:
- ActivityPub offers features how to deal with multilingual content ([contentMap - see Example 115](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-contentmap)) which could be implemented differently by multilingual and translation plugins (example search for the current use of `->set_content_map` within the ActivityPub plugin).
**Note:** CastoPop, to give an example, has currently decided that podcast episodes will not be sent to followers as a `Create` activity directly containing the `PodcastEpisode` object, but will send a `Note` which is linking to the `PodcastEpisode`, because most Fediverse implementations currently ignore by default all object types they don't know. This is likely the same reason [flohmarkt](https://codeberg.org/grindhold/flohmarkt) is currently also using the `Note` an not `Product` even though that one would be available in the offical specification.
All publicly accessible WordPress post types can become active for ActivityPub federation when selecting a transformer available for the relevant post type. The transformer determines the target ActivityPub object type, which can vary depending on variables like the post type or post format. If there is no specific transformer available for a given post type, it cannot be assigned through the user interface.
- Should not differ too much from the current view: Maybe completely hide the current activity object type part of the settings and only show the buttons for `enable` and `disable` and move the transformer table to an advanced settings page.
- Maybe make the tables more advanced and cleary show the origin of the transformer (built-in, etc.) and maybe a Pop-Up with a description, or even a link to a configuration page of the transformer.
Currently, only one hard-coded transformer exists in `includes/transformer/class post.php` ([Source](https://github.com/Automattic/wordpress-activitypub/blob/master/includes/transformer/class-post.php)).
- Highly unlikely: The ActivityPub plugin provides an even more height level framework for adding transformers. For example for events the ActivityPub plugin could provide a built-in transformer to the object type `Event` which only needs a mapping (might make things harder, instead of making them easier). See Appendix.
More comprehensive actor management would benefit our project aims and potentially meet the needs of others in the future. Nevertheless, **its importance is considerably lower than that of Transformer management**. In the future, the following factors may become more important when larger websites should be using the ActivityPub plugin.
Currently the whole logic controlling this is written within the `Activity_Dispatcher` [class](https://github.com/Automattic/wordpress-activitypub/blob/master/includes/class-activity-dispatcher.php#L21) which is currently triggered only within the `Scheduler` [class](https://github.com/Automattic/wordpress-activitypub/blob/master/includes/class-scheduler.php#L14-L15).
The specifications allow for a lot of flexibility in their use. As WordPress websites serve a variety of tasks and goals, providing more detailed capabilities and options in this area **may prove challenging to accomplish in a user-friendly manner**.
- For maximum Mobilizon compatibility we would love to have simply an actor of type `Application`, preferable called `@relay@wordpress.site`, that announces all events. Nonetheless, in theory, a Mobilizon instance should have the capability to follow any other actor as well.
- Additionally, for example, organizers in [The Event Calendar](https://wordpress.org/plugins/the-events-calendar/) could also offer their own actor of type `Organizer` or `Group` to publicize events, or create events if the `author`-actors are not enabled.
- Optional: As events are sometimes published a long time before they start, adding the posibility that events get announced again at a scheduled time before the start, e.g. by sending an `Announce` activity.
Other WordPress actor types might have valid use cases, like actors for categories, or specific post types in general: "I only want to see the blog updates, but I do not want to spam my timeline with each product they post on their site, even if they choose to federate them."
Each ActivityPub actor must have a [unique ID](https://www.w3.org/TR/activitypub/#obj-id). In our case this is a HTTPS URI.
But in reality [webfinger](https://webfinger.net/) is used: `@actor-name@instance.tld`. Actors like Persons and Groups and Applications have Public Keys attached to them, as well as their ID is probably cached by most software. So assigning some actor-name to something new also can cause unintended behavior.
If multiple plugins want to individually federate their content (like events, products and blog posts or even the built-in WordPress categories), they must be able to have something like an actor API, the ActivityPub plugin can register. The ActivityPub plugin being the main coordinator is necessary because actors are globally unique.
- For existing (collision was created before the installation/activation of the plugin)
- For new collisions (collision was/will be created after the installation/activation of the plugin)
It seems like it is the best if collisions are avoided in the first place by encouraging the use of prefixes, like `category_<category>`.
Anyway it seems the question has to be raised wheter activitypub should store a history of all past and currently mapped actors. Currently the ActivityPub plugin is still written with WordPress users being the default ActivityPub actors in mind. This also reflects in things like the class which is responsible for the actor lookup is still called `Users` and yet handles the `Blog` and the `Application` user too, see the [Source](https://github.com/Automattic/wordpress-activitypub/blob/master/includes/collection/class-users.php).
[youtube-dl](https://github.com/ytdl-org/youtube-dl) is solving a similar problem with "extractors" that return data to the processing-chain of youtube-dl. They provide a base class that includes useful functions for common issues e.g. automatic testing, geo-bypassing, login/cookie/header management. The most [basic extractor](https://github.com/ytdl-org/youtube-dl#adding-support-for-a-new-site) is about 40 lines of code.
They can also chain extractors recursivly, like a extractor for mastodon videos, that gets the video from the post and returns the source of the video, that could be youtube, vimeo or anything else youtube-dl supports.
