starbase_events

Crates.io

An async event emitter for the starbase application framework. This crate works quite differently than other event systems, as subscribers can mutate the event and its data. Because of this, we cannot use message channels, and must take extra precaution to satisfy Send + Sync requirements.

Creating events

Events must derive Event, or implement the Event trait. Events can be any type of struct, but the major selling point is that events are mutable, allowing inner content to be modified by subscribers.

use starbase_events::Event;
use app::Project;

#[derive(Debug, Event)]
pub struct ProjectCreatedEvent(pub Project);

Creating emitters

An Emitter is in charge of managing subscribers, and dispatching an event to each subscriber, while taking into account the execution flow and once subscribers.

Every event will require its own emitter instance.

use starbase_events::Emitter;

let project_created = Emitter::<ProjectCreatedEvent>::new();
let cache_check: Emitter<CacheCheckEvent> = Emitter::new();

Using subscribers

Subscribers are async functions that are registered into an emitter, and are executed when the emitter emits an event. They are passed the event object as a Arc<RwLock<T>>, allowing for the event and its inner data to be accessed mutably or immutably.

use starbase_events::{EventResult, EventState};

async fn update_root(event: Arc<RwLock<ProjectCreatedEvent>>) -> EventResult<ProjectCreatedEvent> {
  let event = event.write().await;

  event.0.root = new_path;

  Ok(EventState::Continue)
}

emitter.on(subscriber).await; // Runs multiple times
emitter.once(subscriber).await; // Only runs once

Furthermore, we provide a #[subscriber] function attribute that streamlines the function implementation. For example, the above subscriber can be rewritten as:

#[subscriber]
async fn update_root(mut event: ProjectCreatedEvent) {
  event.0.root = new_path;
}

When using #[subscriber], the following benefits apply:

The return type is optional.
The return value is optional if EventState::Continue.
Using mut event or &mut Event will acquire a write lock, otherwise a read lock.
Omitting the event parameter will not acquire any lock.

Controlling the event flow

Subscribers can control the event execution flow by returning EventState, which supports the following variants:

Continue - Continues to the next subscriber (default).
Stop - Stops after this subscriber, discarding subsequent subscribers.
Return - Like Stop but also returns a value for interception.

#[subscriber]
async fn continue_flow(mut event: CacheCheckEvent) {
  Ok(EventState::Continue)
}

#[subscriber]
async fn stop_flow(mut event: CacheCheckEvent) {
  Ok(EventState::Stop)
}

#[subscriber]
async fn return_flow(mut event: CacheCheckEvent) {
  Ok(EventState::Return(path_to_cache)))
}

For Return flows, the type of value returned is inferred from the event. By default the value is a unit type (()), but can be customized with #[event] for derived events, or type Value when implemented manually.

use starbase_events::{Event, Emitter};
use std::path::PathBuf;

#[derive(Event)]
#[event(value = "PathBuf")]
pub struct CacheCheckEvent(pub PathBuf);

// OR

pub struct CacheCheckEvent(pub PathBuf);

impl Event for CacheCheckEvent {
  type Value = PathBuf;
}

Emitting and handling results

When an event is emitted, subscribers are executed sequentially in the same thread so that each subscriber can mutate the event if necessary. Because of this, events do not support references for inner values, and instead must own everything.

An event can be emitted with the emit() method, which requires an owned event (and owned inner data).

let (event, result) = emitter.emit(ProjectCreatedEvent(owned_project)).await?;

// Take back ownership of inner data
let project = event.0;