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 a particular WordPress post type to an ActivityPub object of (a) specified type(s). For example a transformer that can transform:
-`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.
Our primary goal is to 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 we achieve this using a modularized approach as proposed below. The following list indicates examples what might be those other benefits:
- Other types of content get federated in a more feature rich way:
- 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).
Every public WordPress post type may be transformed (have different transformers available) to a different ActivityPub object types. Transformations that are not
- 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.
- Highly unlikely: Do we really want to manage which transformers apply to what directly or do users prefer a setting like "let a certain plugin take care of this post type"? Then further configuration would have to be managed by the other plugins.
Currently, only one hard-coded transformer exists in [`includes/transformer/class post.php`](https://github.com/Automattic/wordpress-activitypub/blob/master/includes/transformer/class post.php).
With the design above the transformer does a lot and leaves a lot of responsibility to the developer implementing it.
Possible alternatives and solutions:
- The ActivityPub plugin could provide reusable Traits for common tasks.
- 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.
- Don't use an interface with implementations, (miss)use class extensions.
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.
[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.
