Trying to implement publisher/subscriber in swift

Question

I'm actually trying to implement a pub/sub pattern into my Swift projects, I have few Agents (Singletons) that perform tasks, when their status change they should notify all the listener previously subscribed.

I get the error shaping my protocol, probably it should be conforming to the Equatable protocol, but I cannot see in which way (declaring protocol AvailableChatListener: Equatable resolves nothing).

I know that I could use the Apple's NSNotificationCenter (as nicely points here), but it's not a solution completely suitable for my requirements.

Here the code:

AvailableChatListener.swift

protocol AvailableChatListener {
    func onAvailableChatChange()
}

AvailableChatAgent.swift

class AvailableChatAgent {
  // MARK: - Singleton

  class var sharedInstance: AvailableChatAgent {
    struct Singleton {
      static let instance = AvailableChatAgent()
    }
    return Singleton.instance
  }

  // MARK: - Listeners

  private var availableChatListeners = [AvailableChatListener]()

  // MARK: - Public Properties

  // current Chats
  var currentChats = [Chat]()   {
    didSet  {
      statusChanged()
    }
  }

  func startObserving()   {
    // ...
    // perform tasks and change currentChats
    // ...
  }

  func stopObserving()    {
    // ...
    // stop tasks
    // ...
  }

  func statusChanged()    {

    // notify listeners
    for receiver in availableChatListeners {
      receiver.onAvailableChatChange()
    }
  }

  // ERROR #1
  func subscribeListener(listener: AvailableChatListener)    {
    availableChatListeners.append(listener)
  }

  // ERROR #1
  func unsubscribeListener(listener: AvailableChatListener)  {

    // ERROR #2
    if let index = availableChatListeners.indexOf(listener) {
      availableChatListeners.removeAtIndex(availableChatListeners.indexOf(listener)!)

      if availableChatListeners.count == 0 {
        self.stopObserving()
      }
    }
  }
}

And here where I use this logic (for example in a TableViewController)

ChatListTableViewController.swift

class ChatListTableViewController: UITableViewController, AvailableChatListener {

  // MARK: - Properties

  var availableChatAgent = AvailableChatAgent.sharedInstance

  // MARK: - ViewDidLoad

  override func viewDidLoad() {
    super.viewDidLoad()

    availableChatAgent.subscribeListener(self)
  }

  // MARK: - Implement AvailableChatListener

  func onAvailableChatChange()    {
    // perform task with the new data, for example update UI
  }
}

Thanks in advance.

UPDATE 1

Errors that I get:

1. protocol "AvailableChatListener' can only be used as a generic constraint because it has Self or associated type requirements
2. Cannot convert value of type 'AvailableChatListener' to expect argument type '@noescpae(AvailableChatListener) throws -> Bool'

Answer 1

Please try this method of removing your listener and see if it works. I have converted the object into an NSArray and then look for the index of the listener in that array. I am presuming that two listeners are equal if they are the same object and not for any other condition.

func unsubscribeListener(listener: AvailableChatListener)  {
    let arr = availableChatListeners as NSArray
    let index = arr.indexOfObject(listener)
    if index != NSNotFound {
        availableChatListeners.removeAtIndex(index)
        if availableChatListeners.count == 0 {
            self.stopObserving()
        }
    }
}

You should also declare your protocol this way (add @objc)

@objc protocol AvailableChatListener  {
    func onAvailableChatChange()
}

Answer 2

I had the hit same problem and did the following to resolve it. The use of Objective-C Sets requires the use of NSObject based subscribers to get over the Hashing problems mentioned.

import UIKit

protocol LogoListener {
    func logo(urlString url: String, hasImage image: UIImage)
}

class Server {
    var logoListeners = NSMutableSet()
    func addListener(listener: LogoListener) {
        logoListeners.add(listener)
        print("Added listener \(listener)")
    }
}

class Registration: NSObject, LogoListener {
    func logo(urlString url: String, hasImage image: UIImage) {
        savedImage = image
    }
    var savedImage: UIImage = UIImage()
    override init() {
        super.init()
        let server = Server()
        server.addListener(listener: self)
    }
}

let reg = Registration()