How do you create custom notifications in Swift 3?

Question

In Objective-C, a custom notification is just a plain NSString, but it's not obvious in the WWDC version of Swift 3 just what it should be.

Colobus

cocoacasts.com/what-is-notification-name-and-how-to-use-it – Mango

Answer 1

You could also use a protocol for this

protocol NotificationName {
    var name: Notification.Name { get }
}

extension RawRepresentable where RawValue == String, Self: NotificationName {
    var name: Notification.Name {
        get {
            return Notification.Name(self.rawValue)
        }
    }
}

And then define your notification names as an enum anywhere you want. For example:

class MyClass {
    enum Notifications: String, NotificationName {
        case myNotification
    }
}

And use it like

NotificationCenter.default.post(name: Notifications.myNotification.name, object: nil)

This way the notification names will be decoupled from the Foundation Notification.Name. And you will only have to modify your protocol in case the implementation for Notification.Name changes.

Gorse

This is exactly they way I originally thought it should work - notifications should be enums. Thanks for the trick! – Colobus

No problem! I edited the code to include conformation of the extension to NotificationName so the name property is only added to the enums that conform to the protocol. – Gorse

Strictly equivalent but more logical IMO, you can define the extension on NotificationName (instead of RawRepresentable) like this: extension NotificationName where Self: RawRepresentable, Self.RawValue == String { – Endorsement

Answer 2

There is a cleaner (I think) way to achieve it

extension Notification.Name {

    static let onSelectedSkin = Notification.Name("on-selected-skin")
}

And then you can use it like this

NotificationCenter.default.post(name: .onSelectedSkin, object: selectedSkin)

Chiliad

I'm using the code above. This is a static property. – Chiliad

Very clean, I like it alot – Agist

extension NSNotification.Name instead of extension Notification.Name . Otherwise Swift 3 complaints with 'Notification' is ambiguous for type lookup in this context – Nathanialnathaniel

You get my upvote for making a typo in the string and thus demonstrating the value of typed notification names :P – Tergal

It might be worth noting that this is the method suggested by Apple in WWDC 2016 Session 207 developer.apple.com/videos/play/wwdc2016/207 – Inhaler

If using Notification.Name as opposed to the Objective-C NSNotification.Name you should note that NotificationCenter.post now expects an object of type Notification. Posting looks like this: NotificationCenter.default.post(Notification(.onSelectedSkin, object: selectedSkin)). May appear more verbose, but it does make more sense verb-wise to post a Notification directly rather than a name of a notification. Not sure what version of Swift this started in, but I'm still in 3.2. – Salchunas

@Nathanialnathaniel no extra errors in Swift4, extension Notification.Name is enough – Proptosis

Nice, although I guess the string names have the risk of redundant values. – Englut

I had to extend NSNotification.Name in order to achieve this with Swift 5. – Ambary

Answer 3

Notification.post is defined as:

public func post(name aName: NSNotification.Name, object anObject: AnyObject?)

In Objective-C, the notification name is a plain NSString. In Swift, it's defined as NSNotification.Name.

NSNotification.Name is defined as:

public struct Name : RawRepresentable, Equatable, Hashable, Comparable {
    public init(_ rawValue: String)
    public init(rawValue: String)
}

This is kind of weird, since I would expect it to be an Enum, and not some custom struct with seemingly no more benefit.

There is a typealias in Notification for NSNotification.Name:

public typealias Name = NSNotification.Name

The confusing part is that both Notification and NSNotification exist in Swift

So in order to define your own custom notification, do somethine like:

public class MyClass {
    static let myNotification = Notification.Name("myNotification")
}

Then to call it:

NotificationCenter.default().post(name: MyClass.myNotification, object: self)

Colobus

Good answer. Some comments: This is kind of weird, since I would expect it to be an Enum — An enum is a closed set. If Notification.Name were an enum, nobody would be able to define new notifications. We use structs for otherwise-enum-like types that need to allow adding new members. (See the swift-evolution proposal.) – Solvable

The confusing part is that both Notification and NSNotification exist in Swift — Notification is a value type (a struct), so that it can benefit from Swift's semantics for value (im)mutability. Generally, Foundation types are dropping their "NS" in Swift 3, but where one of the new Foundation Value Types exists to supplant it, the old reference type sticks around (keeping the "NS" name) so that you can still use it when you need reference semantics or to subclass it. See the proposal. – Solvable

Let me clarify: I expect notification names to be enums, like Errors are. You can define your own Error enums, and make them conform to ErrorType. – Colobus

True — Apple could at least theoretically have made NotoficationName (or some such) a protocol, to which you create conforming types. I dunno, but there's likely a reason they didn't... Probably something to do with ObjC bridging? File a bug (to open source, Foundation Swift is in the open) if you've got a better solution worked out. – Solvable

In your solution you capitalize the declaration of MyNotification. Apple's rewrite of their notifications does the same thing. Any idea why this is? As far as I can tell the Swift 3 style guide only suggests capitalizing for protocols and classes. But these are static properties. – Chu

You are probably correct in that it should begin with lowercase. – Colobus

Answer 4

Easier way:

let name:NSNotification.Name = NSNotification.Name("notificationName")
NotificationCenter.default.post(name: name, object: nil)

Canara

Answer 5

I may suggest another option which is similar to what @CesarVarela suggested.

extension Notification.Name {
    static var notificationName: Notification.Name {
        return .init("notificationName")
    }
}

This will let you post and subscribe on notifications easily.

NotificationCenter.default.post(Notification(name: .notificationName))

Hope this will help you.

Ductile

Answer 6

You can add a custom initializer to NSNotification.Name

extension NSNotification.Name {
    enum Notifications: String {
        case foo, bar
    }
    init(_ value: Notifications) {
        self = NSNotification.Name(value.rawValue)
    }
}

Usage:

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

Beardsley

Lower case 'enum type' and 'init(_ type: type)' for Swift 3.0.2 – Abjure

@Abjure Only the cases in an enum should be lowercased, not the enum itself. Type names are uppercased, and enums are types. – Coplanar

Answer 7

NSNotification.Name(rawValue: "myNotificationName")

Superordinate

Notification.Name("myNotificationName") – Bedesman

Answer 8

I did my own implementation mixing things from there and there, and find this as the most convenient. Sharing for who any that might be interested:

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

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(self.onSomethingChange(notification:)),
                                               name: Notification.MyApp.Something,
                                               object: nil)
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }

    @IBAction func btnTapped(_ sender: UIButton) {
        NotificationCenter.default.post(name: Notification.MyApp.Something,
                                      object: self,
                                    userInfo: [Notification.MyApp.Something:"foo"])
    }

    func onSomethingChange(notification:NSNotification) {
        print("notification received")
        let userInfo = notification.userInfo!
        let key = Notification.MyApp.Something 
        let something = userInfo[key]! as! String //Yes, this works :)
        print(something)
    }
}

Decasyllable

Answer 9

This is just reference

// Add observer:
NotificationCenter.default.addObserver(self,
    selector: #selector(notificationCallback),
    name: MyClass.myNotification,
    object: nil)

    // Post notification:
    let userInfo = ["foo": 1, "bar": "baz"] as [String: Any]
    NotificationCenter.default.post(name: MyClass.myNotification,
        object: nil,
        userInfo: userInfo)

Swedish

Answer 10

The advantage of using enums is that we get the compiler to check that the name is correct. Reduces potential issues and makes refactoring easier.

For those who like using enums instead of quoted strings for notification names, this code does the trick:

enum MyNotification: String {
    case somethingHappened
    case somethingElseHappened
    case anotherNotification
    case oneMore
}

extension NotificationCenter {
    func add(observer: Any, selector: Selector, 
             notification: MyNotification, object: Any? = nil) {
        addObserver(observer, selector: selector, 
                    name: Notification.Name(notification.rawValue),
                    object: object)
    }
    func post(notification: MyNotification, 
              object: Any? = nil, userInfo: [AnyHashable: Any]? = nil) {
        post(name: NSNotification.Name(rawValue: notification.rawValue), 
             object: object, userInfo: userInfo)
    }
}

Then you can use it like this:

NotificationCenter.default.post(.somethingHappened)

Though unrelated to the question, the same can be done with storyboard segues, to avoid typing quoted strings:

enum StoryboardSegue: String {
    case toHere
    case toThere
    case unwindToX
}

extension UIViewController {
    func perform(segue: StoryboardSegue) {
        performSegue(withIdentifier: segue.rawValue, sender: self)
    }
}

Then, on your view controller, call it like:

perform(segue: .unwindToX)

Fouquet

> NotificationCenter.default.post(.somethingHappened) This throws an error; the methods you added in your extension accept more arguments. – Gwenngwenneth

Answer 11

@CesarVarela's answer is good, but to make the code slightly cleaner, you can do the following:

extension Notification.Name {
    typealias Name = Notification.Name

    static let onSelectedSkin = Name("on-selected-skin")
    static let onFoo = Name("on-foo")
}

Lacerta

Answer 12

If you want this to work cleanly in a project that uses both Objective-C and Swift at the same time, I found it to be easier to create the notifications in Objective-C.

Create an .m/.h file:

//CustomNotifications.h
#import <Foundation/Foundation.h>

// Add all notifications here
extern const NSNotificationName yourNotificationName;

//CustomNotifications.m
#import "CustomNotifications.h"

// Add their string values here
const NSNotificationName yourNotificationName = @"your_notification_as_string";

In your MyProject-Bridging-Header.h (named after your project) to expose them to Swift.

#import "CustomNotifications.h"

Use your notifications in Objective-C like this:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(yourMethod:) name:yourNotificationName:nil];

And in Swift (5) like this:

NotificationCenter.default.addObserver(self, selector: #selector(yourMethod(sender:)), name: .yourNotificationName, object: nil)

Ambary

Answer 13

if you use string-only custom notifications, there's no reason to extend any classes but String

    extension String {
        var notificationName : Notification.Name{
            return Notification.Name.init(self)
        }
    }

Selfpollination