NSNotificationCenter addObserver in Swift

Question

How do you add an observer in Swift to the default notification center? I\'m trying to port this line of code that sends a notification when the battery level changes.

Answer 1

Swift 3.0 in Xcode 8

Swift 3.0 has replaced many "stringly-typed" APIs with struct "wrapper types", as is the case with NotificationCenter. Notifications are now identified by a struct Notfication.Name rather than by String. See the Migrating to Swift 3 guide.

Previous usage:

// Define identifier
let notificationIdentifier: String = "NotificationIdentifier"

// Register to receive notification
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(_:)), name: notificationIdentifier, object: nil)

// Post a notification
NSNotificationCenter.defaultCenter().postNotificationName(notificationIdentifier, object: nil)

New Swift 3.0 usage:

// Define identifier
let notificationName = Notification.Name("NotificationIdentifier")

// Register to receive notification
NotificationCenter.default.addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification), name: notificationName, object: nil)

// Post notification
NotificationCenter.default.post(name: notificationName, object: nil)

All of the system notification types are now defined as static constants on Notification.Name; i.e. .UIDeviceBatteryLevelDidChange, .UIApplicationDidFinishLaunching, .UITextFieldTextDidChange, etc.

You can extend Notification.Name with your own custom notifications in order to stay consistent with the system notifications:

// Definition:
extension Notification.Name {
    static let yourCustomNotificationName = Notification.Name("yourCustomNotificationName")
}

// Usage:
NotificationCenter.default.post(name: .yourCustomNotificationName, object: nil)