EventBusKit

A lightweight, type-safe event bus for Swift code.

EventBus is implemented as a thread-safe final class backed by NSLock. It supports weak owner lifecycle, token-based cancellation, one-shot subscriptions, ordered snapshot delivery, AsyncStream consumption, and a MainActor-friendly overload for UI owners.

• iOS 13+

Add the package dependency in Swift Package Manager and depend on the EventBus product.

import EventBus

struct UserDidLoginEvent: EventBusEvent {
    let userID: String
}

let bus = EventBus.default

final class AnalyticsOwner: @unchecked Sendable {}
let owner = AnalyticsOwner()

let token = bus.subscribe(owner: owner, to: UserDidLoginEvent.self) { _, event in
    print("Logged in:", event.userID)
}

bus.publish(UserDidLoginEvent(userID: "u-1"))
token.cancel()

Any event must conform to EventBusEvent (Sendable).

struct AppReadyEvent: EventBusEvent {
    let timestamp: Date
}

• Every subscribe(...) call creates a new independent subscription.
• One owner may have multiple subscriptions for the same event type.
• SubscriptionToken.cancel() cancels one specific subscription.
• unsubscribe(owner:from:) removes all subscriptions for one owner and event type.
• unsubscribeAll(for:) removes all subscriptions for one owner across all event types.

Consume events with for await:

let stream = bus.stream(AppReadyEvent.self)

Task {
    for await event in stream {
        print(event.timestamp)
    }
}

• publish(_:)
• subscribe(...) with (Owner, Event) handler
• subscribe(...) with Event-only handler
• subscribeOnce(...)
• subscribeOnMain(...)
• unsubscribe(owner:from:)
• unsubscribeAll(for:)
• stream(_:bufferingPolicy:)

• EventBus is thread-safe.
• Internal state is protected by NSLock.
• Dead subscriptions are cleaned lazily on subscribe and publish.
• publish(_:) uses snapshot delivery.
• Subscribers added during publish do not receive the current event.
• Subscribers removed during publish may already be in the snapshot, but CancellationState prevents cancelled handlers from running.
• Delivery order is registration order.

See:

• Sources/EventBus/Example/Demo.swift
• Sources/EventBus/Example/SwiftUIPlayground.swift

Unit tests are located in:

• Tests/EventBusTests/EventBusTests.swift

They cover delivery order, token-based cancellation, owner-based unsubscription, one-shot semantics, MainActor delivery, AsyncStream, and concurrent publishing.