NSNotificationCenter addObserver in Swift

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.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(batteryLevelChanged:) name:UIDeviceBatteryLevelDidChangeNotification object:nil];

Solution 1:

Swift 4.0 & Xcode 9.0+:

Send(Post) Notification:

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil)

OR

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil, userInfo: ["Renish":"Dadhaniya"])

Receive(Get) Notification:

NotificationCenter.default.addObserver(self, selector: #selector(self.methodOfReceivedNotification(notification:)), name: Notification.Name("NotificationIdentifier"), object: nil)

Function-Method handler for received Notification:

@objc func methodOfReceivedNotification(notification: Notification) {}

Swift 3.0 & Xcode 8.0+:

Send(Post) Notification:

NotificationCenter.default.post(name: Notification.Name("NotificationIdentifier"), object: nil)

Receive(Get) Notification:

NotificationCenter.default.addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(notification:)), name: Notification.Name("NotificationIdentifier"), object: nil)

Method handler for received Notification:

func methodOfReceivedNotification(notification: Notification) {
  // Take Action on Notification
}

Remove Notification:

deinit {
  NotificationCenter.default.removeObserver(self, name: Notification.Name("NotificationIdentifier"), object: nil)
}

Swift 2.3 & Xcode 7:

Send(Post) Notification

NSNotificationCenter.defaultCenter().postNotificationName("NotificationIdentifier", object: nil)

Receive(Get) Notification

NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(YourClassName.methodOfReceivedNotification(_:)), name:"NotificationIdentifier", object: nil)

Method handler for received Notification

func methodOfReceivedNotification(notification: NSNotification){
  // Take Action on Notification
}

For historic Xcode versions...

Send(Post) Notification

NSNotificationCenter.defaultCenter().postNotificationName("NotificationIdentifier", object: nil)

Receive(Get) Notification

NSNotificationCenter.defaultCenter().addObserver(self, selector: "methodOfReceivedNotification:", name:"NotificationIdentifier", object: nil)

Remove Notification

NSNotificationCenter.defaultCenter().removeObserver(self, name: "NotificationIdentifier", object: nil)
NSNotificationCenter.defaultCenter().removeObserver(self) // Remove from all notifications being observed

Method handler for received Notification

func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

Annotate either the class or the target method with @objc

@objc private func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

// Or

dynamic private func methodOfReceivedNotification(notification: NSNotification) {
  // Take Action on Notification
}

Solution 2:

It's the same as the Objective-C API, but uses Swift's syntax.

Swift 4.2 & Swift 5:

NotificationCenter.default.addObserver(
    self,
    selector: #selector(self.batteryLevelChanged),
    name: UIDevice.batteryLevelDidChangeNotification,
    object: nil)

If your observer does not inherit from an Objective-C object, you must prefix your method with @objc in order to use it as a selector.

@objc private func batteryLevelChanged(notification: NSNotification){     
    //do stuff using the userInfo property of the notification object
}

See NSNotificationCenter Class Reference, Interacting with Objective-C APIs