show simple dependency injection EDIT: Ok really just dependency inheritance

DependencyInjection.swift

// do DI easy with protocol / extension?
class PostcardStore {
    let postcards = ["France", "Romania"]

    func calcPostage(from: String, to: String) -> Int {
        return 42 // good enough until tests tell us otherwise
    }
}

struct Context {
    var notificationCenter = {
        NSNotificationCenter.defaultCenter()
    }
    var locationManager = {
        CLLocationManager()
    }
}

protocol InjectableContext {
    var postcardStore: PostcardStore { get }
    var context: Context { get }
}

extension InjectableContext {
    var postcardStore: PostcardStore {
        return PostcardStore()
    }
    var context: Context {
        return Context()
    }
}

class Controller: InjectableContext {
    // needs store!
    func mailCards() {
        let cards = postcardStore.postcards
        let postage = postcardStore.calcPostage(cards.first!, to: cards.last!)
        print(postage)
        context.locationManager
    }
}

let mainController = Controller()
mainController.mailCards() // should use DI so we can test

I like the idea of having small, well-formed dependencies that vend specific sets of functionality. I whipped this example up in class quickly, to prove the language concept. the example needs much better structure and naming. To take it further, could have an "Injector" that tests for protocol conformance and if so sets the various properties.