Alternative observer pattern in swift, with playground example

Observers.swift

//: Playground - noun: a place where people can play
import UIKit

private class Weak: Hashable {
    let hashValue: Int
    weak var value: AnyObject?
    
    init(value: AnyObject) {
        self.value = value
        self.hashValue = ObjectIdentifier(value).hashValue
    }
    
    static func ==(lhs: Weak, rhs: Weak) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }
}

class Provider<ObserverType> {
    private var weakObservers = Set<Weak>()
    private var observers: [ObserverType] {
        weakObservers = Set(weakObservers.filter{ $0.value != nil })
        return weakObservers.flatMap{ $0.value as? ObserverType }
    }
    
    func add(observer: ObserverType) {
        let weak = Weak(value: observer as AnyObject)
        guard weak.value != nil else { return assertionFailure("ObserverType must be class bound") }
        weakObservers.insert(weak)
    }
    
    func remove(observer: ObserverType) {
        let weak = Weak(value: observer as AnyObject)
        guard weak.value != nil else { return assertionFailure("ObserverType must be class bound") }
        weakObservers.remove(weak)
    }
    
    func post(block: (ObserverType) -> Void) {
        observers.forEach(block)
    }
}


// Example usage:
struct User {
    let name: String
}

protocol UserObserver: class {
    func didUpdate(user: User, in provider: UserProvider)
}

class UserProvider: Provider<UserObserver> {
    func updateUser() {
        //Update user as needed, then push update to observers:
        let user = User(name: "John Doe")
        post{ $0.didUpdate(user: user, in: self) }
    }
}

class PresenterA: UserObserver {
    func didUpdate(user: User, in provider: UserProvider) {
        print("A: \(user.name)")
    }
}

class PresenterB: UserObserver {
    func didUpdate(user: User, in provider: UserProvider) {
        print("B: \(user.name)")
    }
}

let provider = UserProvider()
let presenterA = PresenterA()
let presenterB = PresenterB()

provider.add(observer: presenterA)
provider.add(observer: presenterB)

print("updating user")
provider.updateUser()

provider.remove(observer: presenterA)

print("updating user")
provider.updateUser()

/*
 Output:
 updating user
 A: John Doe
 B: John Doe
 updating user
 B: John Doe
*/