NotificationCenter — ουσία, αρχή λειτουργίας και αρχιτεκτονική ειδοποιήσεων

// Ορισμός προσαρμοσμένων ειδοποιήσεων
extension Notification.Name {
    static let dataDidUpdate =
        Notification.Name("com.app.dataDidUpdate")
    static let userLoggedOut =
        Notification.Name("com.app.userLoggedOut")
}

// Αποστολή ειδοποίησης με δεδομένα
let userInfo: [String: Any] = [
    "userId": 123,
    "timestamp": Date()
]
NotificationCenter.default.post(
    name: .dataDidUpdate,
    object: nil,
    userInfo: userInfo
)

// Εγγραφή σε ειδοποίηση (selector-based)
NotificationCenter.default.addObserver(
    self,
    selector: #selector(handleDataUpdate),
    name: .dataDidUpdate,
    object: nil
)

@objc func handleDataUpdate(_ notification: Notification) {
    guard let userId = notification.userInfo?["userId"] as? Int else { return }
    updateUI(for: userId)
}

// Εγγραφή σε ειδοποίηση (block-based, iOS 9+)
var observer: NSObjectProtocol?
observer = NotificationCenter.default.addObserver(
    forName: .dataDidUpdate,
    object: nil,
    queue: .main
) { [weak self] notification in
    guard let self else { return }
    self.handleNotification(notification)
}

// Καθυστερημένη αποστολή μέσω NotificationQueue
let notification = Notification(
    name: .dataDidUpdate,
    object: self,
    userInfo: ["progress": 0.5]
)

// Coalescing: πολλές ειδοποιήσεις συγχωνεύονται σε μία
NotificationQueue.default.enqueue(
    notification,
    postingStyle: .whenIdle,
    coalesceMask: .onName,
    forModes: [.common]
)

// Ασύγχρονη αποστολή μέσω DispatchQueue
DispatchQueue.main.async {
    NotificationCenter.default.post(name: .dataDidUpdate, object: nil)
}

protocol NotificationToken {
    func dispose()
}

extension NotificationCenter {
    func observe(
        name: NSNotification.Name,
        object: Any? = nil,
        queue: OperationQueue? = .main,
        using block: @escaping (Notification) -> Void
    ) -> NotificationToken {
        let observer = addObserver(forName: name, object: object,
                                   queue: queue, using: block)
        return NotificationTokenWrapper(observer: observer, center: self)
    }
}

// Χρήση με αυτόματη διαγραφή
class ViewModel {
    private var tokens: [NotificationToken] = []

    func startObserving() {
        let token = NotificationCenter.default.observe(
            name: .dataDidUpdate,
            queue: .main
        ) { [weak self] notification in
            self?.handleUpdate(notification)
        }
        tokens.append(token)
    }

    deinit {
        tokens.forEach { $0.dispose() }
    }
}

class SafeObserver {
    private var observers: [NSObjectProtocol] = []

    func addSubscriptions() {
        let token1 = NotificationCenter.default.addObserver(
            forName: .dataDidUpdate, object: nil,
            queue: .main) { [weak self] _ in
            self?.refreshData()
        }
        let token2 = NotificationCenter.default.addObserver(
            forName: .userLoggedOut, object: nil,
            queue: .main) { [weak self] _ in
            self?.logout()
        }
        observers.append(contentsOf: [token1, token2])
    }

    deinit {
        observers.forEach { NotificationCenter.default.removeObserver($0) }
    }

    private func refreshData() { }
    private func logout() { }
}

import Combine

class ReactiveViewModel {
    private var cancellables = Set<AnyCancellable>()

    func setupCombineSubscription() {
        NotificationCenter.default
            .publisher(for: .dataDidUpdate)
            .receive(on: DispatchQueue.main)
            .compactMap { $0.userInfo?["progress"] as? Float }
            .debounce(for: .seconds(0.3), scheduler: RunLoop.main)
            .sink { [weak self] progress in
                self?.progressLabel.text = "\(Int(progress * 100))%"
            }
            .store(in: &cancellables)
    }
}