EventBus

EventBus is a safe-by-default alternative to Cocoa's NSNotificationCenter. It provides a type-safe API that can safely be used from multiple threads. It automagically removes subscribers when they are deallocated.

EventBus is to one-to-many notifications what a Delegate is to one-to-one notifications.

Usage

Simple Notifications:

Let's say you have a lottery that's supposed to notify all its participating players every time a new winning number is drawn:

import EventBus

protocol LotteryDraw {
    func didDraw(number: Int, in: Lottery)
}

class Lottery {
    private let eventBus = EventBus()

    func add(player: LottoPlayer) {
        self.eventBus.add(subscriber: player, for: LotteryDraw.self)
    }

    func draw() {
        let winningNumber = arc4random()
        self.eventBus.notify(LotteryDraw.self) { subscriber in
            subscriber.didDraw(number: Int(winningNumber), in: self)
        }
    }
}

class LottoPlayer : LotteryDraw {
    func didDraw(number: Int, in: Lottery) {
        if number == 123456 { print("Hooray!") }
    }
}

Complex Notifications:

Nice, but what if you would like to group a set of semantically related notifications (such as different stages of a process) into a common protocol? No problem! Your protocol can be of arbitrary complexity.

Consider this simple key-value-observing scenario:

import EventBus

protocol ValueChangeObserver {
    func willChangeValue(of: Publisher, from: Int, to: Int)
    func didChangeValue(of: Publisher, from: Int, to: Int)
}

class Publisher {
    private let eventBus = EventBus()

    var value: Int {
        willSet {
            self.eventBus.notify(ValueChangeObserver.self) { subscriber in
                subscriber.willChange(value: self.value, to: newValue)
            }
        }
        didSet {
            self.eventBus.notify(ValueChangeObserver.self) { subscriber in
                subscriber.didChange(value: oldValue, to: self.value)
            }
        }
    }

    func add(subscriber: ValueChangeObserver) {
        self.eventBus.add(subscriber: subscriber, for: ValueChangeObserver.self)
    }
}

class Subscriber : ValueChangeObserver {
    func willChangeValue(of: Publisher, from: Int, to: Int) {
        print("\(of) will change value from \(from) to \(to).")
    }

    func didChangeValue(of: Publisher, from: Int, to: Int) {
        print("\(of) did change value from \(from) to \(to).")
    }
}

Chaining

Sometimes it is desirable to have one event bus forward a carbon copy of all of its events to a second event bus. A possible scenario would be a proxy event bus that acts as a centralized facade to an arbitrary number of internal event buses.

let proxyEventBus = EventBus()

let eventBus1 = EventBus()
let eventBus2 = EventBus()
proxyEventBus.attach(chain: eventBus1, for SomeEvent.self)
proxyEventBus.attach(chain: eventBus2, for SomeEvent.self)

let subscriber1 = …
let subscriber2 = …
eventBus1.add(subscriber: subscriber1, for: SomeEvent.self)
eventBus2.add(subscriber: subscriber2, for: SomeEvent.self)

proxyEventBus.notify(SomeEvent.self) { subscriber in
    // …
}

// subscriber1 & subscriber2 are getting notified

For each event notified on either eventBus1 or eventBus2 a carbon copy will be forwarded to proxyEventBus .

Encapsulation

The API of EventBus is split up into these protocols:

EventRegistrable
EventSubscribable
EventChainable
EventNotifiable

This enables for ergonomic safe encapsulation …

public class Publisher {
    public var eventSubscribable: EventSubscribable {
        return self.eventBus
    }

    private let eventBus: EventBus = …

    // …
}

… preventing anybody else from issuing notifications on Publisher's event bus.

Debugging

As there is no way to see from the type of EventBus which kind of events are to be expected to be received from it, or allowed to notified on it, EventBus provides optional measures for catching misuse early and as an aid in general debugging.

Debug subscription of unregistered event types:

let eventBus = EventBus(options: [.warnUnknown])
eventBus.register(forEvent: FooEvent.self)

// Needed to silence warning:
// eventBus.register(forEvent: BarEvent.self)

let subscriber = …
eventBus.add(subscriber: subscriber, for: BarEvent.self)

// Console:
// Expected event of registered type (e.g. FooEvent), found: BarEvent.
// Info: Use a "Swift Error Breakpoint" on type "EventBus.UnknownEventError" to catch.

Debug notification of unregistered event types:

let eventBus = EventBus(options: [.warnUnknown])
eventBus.register(forEvent: FooEvent.self)

// Needed to silence warning:
// eventBus.register(forEvent: BarEvent.self)

eventBus.notify(FooEvent.self) { subscriber in
	// …
}

// Console:
// Expected event of registered type (e.g. FooEvent), found: BarEvent.
// Info: Use a "Swift Error Breakpoint" on type "EventBus.UnknownEventError" to catch.

Debug unhandled events:

let eventBus = EventBus(options: [.warnUnhandled])

// Needed to silence warning:
// eventBus.add(subscriber: …, for: FooEvent.self)

eventBus.notify(FooEvent.self) { subscriber in
	// …
}

// Console:
// Expected event of registered type (e.g. FooEvent), found: BarEvent.
// Info: Use a "Swift Error Breakpoint" on type "EventBus.UnhandledEventError" to catch.

Installation

The recommended way to add EventBus to your project is via Carthage:

github 'regexident/EventBus'

Or to add EventBus to your project is via CocoaPods:

pod 'Swift-EventBus'

License

EventBus is available under a MPL-2 license. See the LICENSE file for more info.

regexident / EventBus