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

In swift 3, Xcode 8.2:- checking battery state level

//Add observer
NotificationCenter.default.addObserver(self, selector: #selector(batteryStateDidChange), name: NSNotification.Name.UIDeviceBatteryStateDidChange, object: nil)


 //Fired when battery level changes

 func batteryStateDidChange(notification: NSNotification){
        //perform manipulation here
    }

Answer 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

Answer 3

We should remove notification also.

Ex.

deinit 
{
  NotificationCenter.default.removeObserver(self, name:NSNotification.Name(rawValue: "notify"), object: nil)

}

Answer 4

NSNotificationCenter add observer syntax in Swift 4.0 for iOS 11

  NotificationCenter.default.addObserver(self, selector: #selector(keyboardShow), name: NSNotification.Name.UIKeyboardWillShow, object: nil)

This is for keyboardWillShow notification name type. Other type can be selected from the available option

the Selector is of type @objc func which handle how the keyboard will show ( this is your user function )

Answer 5

Swift 5 Notification Observer

override func viewDidLoad() {
    super.viewDidLoad() 
    NotificationCenter.default.addObserver(self, selector: #selector(batteryLevelChanged), name: UIDevice.batteryLevelDidChangeNotification, object: nil)
}

@objc func batteryLevelChanged(notification : NSNotification){
    //do here code
}

override func viewWillDisappear(_ animated: Bool) {
    NotificationCenter.default.removeObserver(self, name: UIDevice.batteryLevelDidChangeNotification, object: nil)

}

Answer 6

1. Declare a notification name

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

2. You can add observer in two ways:

   Using Selector

   NotificationCenter.default.addObserver(self, selector: #selector(myFunction), name: .purchaseDidFinish, object: nil)
   
   @objc func myFunction(notification: Notification) {
       print(notification.object ?? "") //myObject
       print(notification.userInfo ?? "") //[AnyHashable("key"): "Value"]
   }
   

   or using block

   NotificationCenter.default.addObserver(forName: .purchaseDidFinish, object: nil, queue: nil) { [weak self] (notification) in
       guard let strongSelf = self else {
           return
       }
   
       strongSelf.myFunction(notification: notification)
   }
   
   func myFunction(notification: Notification) {
       print(notification.object ?? "") //myObject
       print(notification.userInfo ?? "") //[AnyHashable("key"): "Value"]
   }
   

3. Post your notification

   NotificationCenter.default.post(name: .purchaseDidFinish, object: "myObject", userInfo: ["key": "Value"])
   

from iOS 9 and OS X 10.11. It is no longer necessary for an NSNotificationCenter observer to un-register itself when being deallocated. more info

For a block based implementation you need to do a weak-strong dance if you want to use self inside the block. more info

Block based observers need to be removed more info

let center = NSNotificationCenter.defaultCenter()
center.removeObserver(self.localeChangeObserver)