Notifications and userInfo with Swift 3.0

Swift 3.0 brought quite a few changes to the Swift language, including the removal of NS prefixes from Foundation framework types — the Great Renaming. NSThread became Thread, NSData became Data, and so on.

This means the usage of NSNotificationCenter — oh sorry, NotificationCenter — with userInfo needs to be updated. This is a significant difference between Swift 2 and Swift 3.

The way to get the default NotificationCenter is now let nc = NotificationCenter.default. Additionally, the model of using a selector when receiving notifications has changed to specifying a closure or function to execute.

For example, in Swift 2 you would write:

let nc = NSNotificationCenter.defaultCenter()
nc.addObserver(self,
               selector: #selector(ViewController.catchNotification),
               name: "MyNotification",
               object: nil)

While in Swift 3 it's written as:

let nc = NotificationCenter.default // Note: default is a property, not a method call
nc.addObserver(forName:Notification.Name(rawValue:"MyNotification"),
               object:nil, queue:nil,
               using:catchNotification)

The example above sets up the notification center to deliver MyNotification notifications to the catchNotification function, which has a method signature of (Notification) -> Void. Alternatively, you can use a closure:

let nc = NotificationCenter.default // Note: default is a property, not a method call
nc.addObserver(forName:Notification.Name(rawValue:"MyNotification"),
               object:nil, queue:nil) {
  notification in
  // Handle the notification
}

Posting Notifications

Now let's look at how to post notifications. The postNotificationName method from Swift 2.0 has been replaced by post in Swift 3.0.

let nc = NotificationCenter.default
nc.post(name:Notification.Name(rawValue:"MyNotification"),
        object: nil,
        userInfo: ["message":"Hello there!", "date":Date()])

userInfo takes [AnyHashable: Any]? as a parameter, which is called a dictionary literal in Swift. Note that the values in userInfo don't need to be of the same type (as indicated by the Any); here a String type and a Date type are sent together.

Handling Notifications

Using guard syntax to unwrap and validate expected data from userInfo is a great approach.

  func catchNotification(notification:Notification) -> Void {
    print("Catch notification")

    guard let userInfo = notification.userInfo,
          let message  = userInfo["message"] as? String,
          let date     = userInfo["date"]    as? Date else {
      print("No userInfo found in notification")
      return
    }

    let alert = UIAlertController(title: "Notification!",
                                  message:"\(message) received at \(date)",
                                  preferredStyle: UIAlertControllerStyle.alert)
    alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.default, handler: nil))
    self.present(alert, animated: true, completion: nil)

  }

To verify the guard behavior, try calling post using a String type or another object type instead of Date(). You'll see No userInfo found in notification in the console output.

Sample Code

You can try the code above in a simple iOS project. Create a Single View Application project and replace the contents of ViewController.swift with the following:

import UIKit

class ViewController: UIViewController {

  let myNotification = Notification.Name(rawValue:"MyNotification")

  override func viewDidLoad() {
    super.viewDidLoad()

    let nc = NotificationCenter.default
    nc.addObserver(forName:myNotification, object:nil, queue:nil, using:catchNotification)
  }

  override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    let nc = NotificationCenter.default
    nc.post(name:myNotification,
            object: nil,
            userInfo:["message":"Hello there!", "date":Date()])
  }

  func catchNotification(notification:Notification) -> Void {
    print("Catch notification")

    guard let userInfo = notification.userInfo,
          let message  = userInfo["message"] as? String,
          let date     = userInfo["date"]    as? Date else {
        print("No userInfo found in notification")
        return
    }

    let alert = UIAlertController(title: "Notification!",
                                  message:"\(message) received at \(date)",
                                  preferredStyle: UIAlertControllerStyle.alert)
    alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.default, handler: nil))
    self.present(alert, animated: true, completion: nil)
  }
}

Key points:

A Notification's "name" is no longer a String type but a Notification.Name type, so when declaring a notification, use let myNotification = Notification.Name(rawValue:"MyNotification"). This allows us to use myNotification anywhere a Notification.Name is needed, such as in NotificationCenter.addObserver and NotificationCenter.post.
It is recommended to use a separate catchNotification method rather than tangled-up code blocks.

That's it — clean and effective!

Comments Section

Improved way of declaring and using notifications:

First declare the notification name:

public extension Notification {
  public class MyApp {
     public static let MyNotification = Notification.Name("Notification.MyApp.MyNotification")
  }
}

Post a notification using the notification name:

NotificationCenter.default.post(name: Notification.MyApp.MyNotification, object: self)

Observe the notification:

NotificationCenter.default.addObserver(forName: Notification.MyApp.MyNotification, object: nil, queue: OperationQueue.main) {
      pNotification in

  // Your code here
}