What is ActivityPubSharp?

[warriordog]

Great question!

Are you familiar with the Fediverse and ActivityPub? If not, then check out those links first. The rest of this post will assume you have a baseline knowledge of the Fediverse ("fedi" from here on) or at least an understanding of federated social media in general.

ActivityPubSharp is an ambitious project to create a modular, safe, and easy library for interacting with other fedi software over ActivityPub. I describe this as "ambitious" because ActivityPub (AP) and its underlying standards - ActivityStreams (AS) and JSON-LD - are all designed for use in dynamically-typed languages that support union types. C# has neither of those features, which makes AP remarkably difficult to model and implement in C#.

Despite the challenge, this project aims to develop a standards-compliant implementation that exposes an ergonomic and conventional API for use in .NET project. A key goal is to simultaneously support high-level and low-level uses cases with equally first-class APIs, and to include optional bindings for common frameworks like ASP.NET and Entity Framework. The end result will be a collection of small, focused packages that can implicitly compose to construct the ideal API for any project.

todo architecture diagram here

New and existing projects can integrate ActivityPubSharp through Dependency Injection, with pre-configured safe defaults and plenty of available customization. Wherever possible, internal components have been developed with public interfaces that can be replaced by library users to customize even deeply buried logic. Most default implementations accept configuration objects compatible with the Options Pattern.

A simple use case might look like this, utilizing C# top-level statements and the mid-level API:

/**
 * Sample app that prints 5 most recent activities for a user
 */

// Import exactly what we need
using ActivityPub.Types;
using ActivityPub.Types.Collection;
using ActivityPub.Types.Extended.Activity;
using ActivityPub.Client;

// Create a container for Dependency Injection
// This can also be provided by the Generic Host, ASP.NET, or whatever your application would like to use.
using var container = new ServiceCollection();
container.AddActivityPubClient(); // Client module will add itself and all dependencies

// Create a client with default options
using var client = container.GetRequiredService<IActivityPubClient>();

// Make a request!
var responseObject = await client.Get<ASObject>("https://enby.life/users/9fpnzspgtp");

// Automatic narrowing based on "type" property
if (responseObject is not PersonActor person)
{
    // Low-level properties are still available for edge cases.
    var asTypes = String.Join(responseObject.Types);
    throw new ApplicationException($"Oops! Got unexpected object of type(s): {asTypes}");
}

// Ergonomic wrappers around complex models
var name = person.Name.ForLanguage("en", "us");
console.WriteLine($"Recent activity for {name}:");

// Easy handling of "Object | Link" constructions and paged collections.
// Links are recursively unwrapped up to a configurable limit.
var outboxResult = await client.ResolveCollection(person.Outbox);

// Collections support iteration (or direct List<T> access through the Items property)
var recentActivities = outboxResult.Take(5);
foreach (var activity in recentActivities)
{
    // Type narrowing is done even for collection elements
    var description = activity switch 
    {
        CreateActivity create => $"* Posted {create.Object.Url} at {create.Published}",
        DeleteActivity delete => $"* Deleted {delete.Object.Url} at {create.Published}", 
        UpdteActivity update => $"* Edited {edit.Object.Url} at {create.Published}", 
        LikeActivity like => $"* Liked {like.Object.Url} at {create.Published}",
        AnnounceActivity announce => $"* Boosted {announce.Object.Url} at {create.Published}",
        ASObject obj => $"* Did something at {object.Published}",
        _ => $"* Did a mysterious unknown thing!"
    };
    console.WriteLine(description);
}

We're not quite to that point yet, but progress is steady! Soon, ActivityPubSharp will be a viable base for microblogging platforms, federated blogs, bots, custom clients, and any other kind of fediverse application!