Private protocols and extensions rather than subclasses

notifier.swift

// I'm exploring replacing subclassing with protocols and extensions. This is super-simple with a subclass,
// but I always like to explore protocol options when I can.

typealias ObserverRemover = () -> Void

protocol Notifier: class {
    typealias T
    var observers: [NSUUID: (T -> Void)] { get set } // This is an implementation detail that I don't want to be public.
    var value: T { get set }
}

extension Notifier {
    func addObserver(didSet: (T -> Void)) -> (ObserverRemover) {
        let identifier = NSUUID()
        observers[identifier] = didSet  // But if it's not public (and writable!), I can't use it here.
        return { [weak self] in
            self?.removeObserver(identifier)
        }
    }
    // ...
}

// I've explored breaking out the private part this way to no success (marked public parts public for clarity)

public typealias ObserverRemover = () -> Void

// Declare the full public interface, including things that will be provided by extension
public protocol Notifier: class {
    typealias T
    var value: T { get set }
    func addObserver(didSet: (T -> Void)) -> (ObserverRemover)
    func notifyAllObservers()
}

private protocol _Notifier: Notifier { // Create a private protocol to provide local types with default implementations
    var observers: [NSUUID: (T -> Void)] { get set }
}

extension _Notifier {
    func addObserver(didSet: (T -> Void)) -> (ObserverRemover) { // This fails because addObserver is required to be both private and public
        let identifier = NSUUID()
        observers[identifier] = didSet
        return { [weak self] in
            self?.removeObserver(identifier)
        }
    }
    // ...
}