Demonstrates how Apple's NotificationCenter works

MyNotificationCenter.swift

import Foundation

protocol MyNotificationCenterInterface {
  func addObserver(_ observer: Any, selector: Selector, name: NSNotification.Name?, object: Any?)
  func post(_ notification: Notification)
}

struct NotificationObserverData {
  let observer: Any,
      selector: Selector,
      object: Any?
}

class MyNotificationCenter: MyNotificationCenterInterface {
  private var map: [NSNotification.Name: [NotificationObserverData]] = [:]

  static let shared =  MyNotificationCenter()

  func addObserver(_ observer: Any, selector: Selector, name: NSNotification.Name?, object: Any?) {
    guard let name = name else { return }
    let data = NotificationObserverData(observer: observer, selector: selector, object: object)
    if var values = map[name] {
      values.append(data)
      map[name] = values
    } else {
      map[name] = [data]
    }
  }

  func post(_ notification: Notification) {
    guard let dataItems = map[notification.name] else { return }
    for item in dataItems {
      guard equals(item.object, notification.object) else {
        print("Unauthorised object sending the notification. Blocked!")
        return
      }
      
      if let objectObserver = item.observer as? NSObject {
       objectObserver.performSelector(onMainThread: item.selector, with: item.object, waitUntilDone: true)
      } else {
        //Note: This gets executed a bit late than performSelector
        DispatchQueue.main.async {
          Thread.detachNewThreadSelector(item.selector, toTarget: item.observer, with: item.object)
        }
      }
    }
  }
}

let notifName = NSNotification.Name.init("hello")

class A: NSObject {
  let b = B()

  override init() {
    super.init()
    let selector = #selector(b.tellMe(sender:))
    MyNotificationCenter.shared.addObserver(b, selector: selector, name: notifName, object: self)
  }
  
  func notifyB() {
    MyNotificationCenter.shared.post(Notification(name: notifName, object: self))
  }
}

class B {
  @objc func tellMe(sender: Any) {
    print("Notification received! with sender: \(sender)")
  }
}

let a = A()
a.notifyB()


//This equals method credits to: https://stackoverflow.com/a/61050386/3632958
func equals(_ x : Any?, _ y : Any?) -> Bool {
    guard x is AnyHashable else { return false }
    guard y is AnyHashable else { return false }
    return (x as! AnyHashable) == (y as! AnyHashable)
}