NotificationCenter

import Foundation

// Define custom notification names
extension Notification.Name {
    static let userDidLogin   = Notification.Name("userDidLogin")
    static let dataDidRefresh = Notification.Name("dataDidRefresh")
}

// Receive notifications using a closure
let observer1 = NotificationCenter.default.addObserver(
    forName: .userDidLogin,
    object: nil,
    queue: .main
) { notification in
    let userName = notification.userInfo?["name"] as? String ?? "Unknown"
    print("Login notification received: \(userName)")
}

let observer2 = NotificationCenter.default.addObserver(
    forName: .dataDidRefresh,
    object: nil,
    queue: .main
) { _ in
    print("Data refresh notification received")
}

// Post a notification (you can attach data via userInfo)
NotificationCenter.default.post(
    name: .userDidLogin,
    object: nil,
    userInfo: ["name": "Alice", "timestamp": Date()]
)

NotificationCenter.default.post(name: .dataDidRefresh, object: nil)

// Post multiple notifications
NotificationCenter.default.post(
    name: .userDidLogin,
    object: nil,
    userInfo: ["name": "Bob"]
)

// Remove observers (always remove to prevent memory leaks)
NotificationCenter.default.removeObserver(observer1)
NotificationCenter.default.removeObserver(observer2)

// No notifications are received after removal
NotificationCenter.default.post(
    name: .userDidLogin,
    object: nil,
    userInfo: ["name": "Carol"]
)
print("No notifications are received after removal")

// Example: receiving a system notification
let appObserver = NotificationCenter.default.addObserver(
    forName: NSNotification.Name("NSApplicationDidBecomeActiveNotification"),
    object: nil,
    queue: .main
) { _ in
    print("The app became active")
}

NotificationCenter is well suited for loosely coupled communication between screens and modules. The sender and receiver do not need to know about each other.

When using addObserver(forName:object:queue:using:), always store the returned observer token in a variable. Failing to remove an observer can cause memory leaks or unintended notification delivery. Always call removeObserver in deinit or when the view disappears.

For timer-based processing with Timer, see Timer.