Observer pattern with swift protocol extension

Observable.swift

import Foundation

public protocol Observer: Equatable {
    associatedtype EventType
    func listen(_ event: EventType)
}

public protocol Observable {
    associatedtype ObserverType: Observer
    associatedtype EventType
    var observers: [ObserverType] { get set }
    mutating func addObserver(_ observer: ObserverType)
    mutating func removeObserver(_ observer: ObserverType)
    func notify(_ event: EventType)
}

extension Observable where EventType == ObserverType.EventType {
    mutating public func addObserver(_ observer: ObserverType) {
        var os = observers
        os.append(observer)
        observers = os
    }
    mutating public func removeObserver(_ observer: ObserverType) {
        guard let i = observers.firstIndex(of: observer) else { return }
        observers.remove(at: i)
    }
    public func notify(_ event: EventType) {
        for o in observers {
            o.listen(event)
        }
    }
}

public enum News {
    case morning(String, Publisher)
    case noon   (String, Publisher)
    case evening(String, Publisher)
    case extra  (String, Publisher)
}

public struct Subscriber: Observer, Equatable {
    public typealias EventType = News
    public var id:   String
    public var name: String
    
    public func listen(_ event: EventType) {
        switch event {
        case .morning(let content, var publisher): read(content: content, publisher: &publisher)
        case .noon   (let content, var publisher): read(content: content, publisher: &publisher)
        case .evening(let content, var publisher): read(content: content, publisher: &publisher)
        case .extra  (let content, var publisher): read(content: content, publisher: &publisher)
        }
    }
    
    public func read(content: String, publisher: inout Publisher) {
        print("\(name) reads morning paper: \(content)\n")
        if content.contains(name) {
            publisher.removeObserver(self)
            print("\(name) unsubscribed\n")
        }
    }
}

public func ==(lhs: Subscriber, rhs: Subscriber) -> Bool {
    return lhs.id == rhs.id
}

public class Publisher: Observable {
    public var name: String
    public typealias ObserverType = Subscriber
    public typealias EventType    = News
    private var _observers: [ObserverType] = []
    public  var  observers: [ObserverType] {
        get { return _observers }
        set { _observers = newValue }
    }
    public init(name: String) {
        self.name = name
    }
}


var sentence_spring = Publisher(name: "Bunshun")

var kiyo     = Subscriber(id: "kiyo"   , name: "Kiyo")
var kimutaku = Subscriber(id: "kimura" , name: "Kimutaku")
var nakai    = Subscriber(id: "nakai"  , name: "Nakai")
var yoshiki  = Subscriber(id: "yoshiki", name: "yoshiki")


sentence_spring.addObserver(kimutaku)
sentence_spring.addObserver(nakai)
sentence_spring.addObserver(kiyo)
sentence_spring.addObserver(yoshiki)


sentence_spring.notify(.morning("Becky and GESU NO KIWAMI are in inappropriate relationship!!!", sentence_spring))

sentence_spring.removeObserver(nakai)
sentence_spring.removeObserver(kimutaku)

sentence_spring.notify(.evening("SMAP will be broken up!?"  , sentence_spring))
sentence_spring.notify(.extra  ("Kiyo is arrested!!"        , sentence_spring))
sentence_spring.notify(.extra  ("Sean K is pure japanease!!", sentence_spring))