ios broadcasting event using NotificationCenter

1. Create a custom event call AButtonIsTapped

extension Notification.Name {
    static let AButtonIsTapped = Notification.Name("AButtonIsTapped")
}

2. Create some observer objects.

private var buttonTapObserver: NSObjectProtocol?
private var buttonTapObserver2: NSObjectProtocol?
private var buttonTapObserver3: NSObjectProtocol?    

3. Initialize the observers and start observe.

// listens to the AButtonIsTapped event and it can come from any object
self.buttonTapObserver = NotificationCenter.default.addObserver(
    forName: .AButtonIsTapped,
    object: nil,
    queue: OperationQueue.main,
    using: { notification in
        print("observer111111: A button is tapped")
}
)

// listens to the AButtonIsTapped event and it has to come from the tapButton object
self.buttonTapObserver2 = NotificationCenter.default.addObserver(
    forName: .AButtonIsTapped,
    object: self.tapButton,
    queue: OperationQueue.main,
    using: { notification in
        print("observer222222: A button is tapped")
}
)

// listens to the AButtonIsTapped event and it has to come from self, this view controller
self.buttonTapObserver3 = NotificationCenter.default.addObserver(
    forName: .AButtonIsTapped,
    object: self,
    queue: OperationQueue.main,
    using: { notification in
        print("observer333333: A button is tapped")
}
)

4. Send event to the NotificationCenter.

// post the AButtonIsTapped event with the tapButton object, only buttonTapObserver and buttonTapObserver1 will observe this, buttonTapObserver3 will not because it only observes object coming from this view controller
NotificationCenter.default.post(name: .AButtonIsTapped, object: sender)

5. Removing observers. If your app targets iOS 9.0 and later or macOS 10.11 and later, you don’t need to unregister an observer in its dealloc method. // https://developer.apple.com/documentation/foundation/notificationcenter/1413994-removeobserver

if let observer1 = buttonTapObserver1 {
    print("Removing buttonTapObserver1")
    NotificationCenter.default.removeObserver(observer1)
}
if let observer2 = buttonTapObserver2 {
    print("Removing buttonTapObserver2")
    NotificationCenter.default.removeObserver(observer2)
}
if let observer3 = buttonTapObserver3 {
    print("Removing buttonTapObserver3")
    NotificationCenter.default.removeObserver(observer3)
}

6. All of the above in one ViewController. The event can be posted from another controller/object/view and be observed in this or anywhere in the app that observes that particular event.

import UIKit

extension Notification.Name {
    static let AButtonIsTapped = Notification.Name("AButtonIsTapped")
}

class ViewController: UIViewController {
    
    private var buttonTapObserver: NSObjectProtocol?
    private var buttonTapObserver2: NSObjectProtocol?
    private var buttonTapObserver3: NSObjectProtocol?
    
    @IBOutlet weak var tapButton: UIButton!
    
        override func viewDidLoad() {
        super.viewDidLoad()
        
        // listens to the AButtonIsTapped event and it can come from any object
        self.buttonTapObserver = NotificationCenter.default.addObserver(
            forName: .AButtonIsTapped,
            object: nil,
            queue: OperationQueue.main,
            using: { notification in
                print("observer111111: A button is tapped")
        }
        )
        
        // listens to the AButtonIsTapped event and it has to come from the tapButton object
        self.buttonTapObserver2 = NotificationCenter.default.addObserver(
            forName: .AButtonIsTapped,
            object: self.tapButton,
            queue: OperationQueue.main,
            using: { notification in
                print("observer222222: A button is tapped")
        }
        )
        
        // listens to the AButtonIsTapped event and it has to come from self, this view controller
        self.buttonTapObserver3 = NotificationCenter.default.addObserver(
            forName: .AButtonIsTapped,
            object: self,
            queue: OperationQueue.main,
            using: { notification in
                print("observer333333: A button is tapped")
        }
        )
    }

    
    @IBAction func buttonTapped(_ sender: UIButton) {
        // post the AButtonIsTapped event with the tapButton object, only buttonTapObserver and buttonTapObserver1 will observe this, buttonTapObserver3 will not because it only observes object coming from this view controller
        NotificationCenter.default.post(name: .AButtonIsTapped, object: sender)
    }
    
}

Complete example in Github