A statically typed NotificationCenter in Swift

Friday. Jan 20, 2017 ~ 2 mins

Event hubs, event bus, notification center… all names for an architectural solution to decouple components of your program. You do some work, and then you announce to the world you did, but you don’t care if anybody is listening.

There’s an Apple-provided implementation available for this when doing mobile development for iOS, it’s called NotificationCenter. An example of where you’d use this could be to post a notification every time the current user changes.

To use it, you come up with a magic name for an event, let’s say userChanged, and every time we post this notification we vow to include the new user in userInfo dictionary, let’s say userInfo["user"] holds the new User instance.

My main problem with this, is what’s to say that:

I won’t misspell the event name? (usrChanged)
I won’t misspell the user key in userInfo? (usr, newUser)
I won’t forget to include the new User at all?
I won’t send the wrong type as the new user? (perhaps nil?)

I have good news though. Swift’s generics allow us to make a very nice solution that alleviates all these problems!

The above definition of Hub, Events and Event<T> allows us to:

Ensure nobody ever misstypes an event by extending Events to allow usage of of the dot short-hand on observe and post:
```
extension Events {
  static let userUpdated = Event<User?>("UserUpdated")
}
```

Guarantee that all observed events of type T actually contains a T:

hub.observe(.userUpdated) { (user: User?) in /* … */ }

Guarantee that all posted events of type T actually includes a T:

hub.post(.userUpdated, User(name: "Alice"))
hub.post(.userUpdated, nil)

And all of this is built on top of Apple’s own NotificationCenter and your normal Swift generics!

There’s a GitHub repository for the code in this post, you’re very welcome to send pull-requests: https://github.com/Burgestrand/swift-hub. That’s all for now, thanks for staying!

Kim Burgestrand

Heartfelt software development. Sometimes other things.