Swift 4 - Notification Center addObserver issue

Question

I'm crashing and getting an unrecognized selector error every time a Notification arrives and the App tries to execute its associated method. Here's my code - which is in viewDidLoad:

let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: Selector(("sayHello")), name:NSNotification.Name(rawValue: "dataDownloadCompleted"), object: nil)

The sayHello() method is quite simple - looks like this:

func sayHello() {
    print("Hello")
}

I've verified that the Notification is posted successfully and that it arrives successfully - so that's not the issue. The crash happens when the App looks to act upon the arrival of the Notification - by executing the sayHello() method. It keeps giving me that unrecognized selector error.

Any ideas what I'm doing wrong? (By the way, this worked perfectly with Swift 3 & Xcode 8, but now with Swift 4 and Xcode 9 the syntax has changed [Xcode walked me through the necessary code fixes/updates] - but the crashes keep happening.)

Answer 1

You can improve your code with these steps:

extension Notification.Name {
    static let dataDownloadCompleted = Notification.Name(
       rawValue: "dataDownloadCompleted")
}

And use it like this:

let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self,
                               selector: #selector(YourClass.sayHello),
                               name: .dataDownloadCompleted,
                               object: nil)

But as was already pointed out, issue is solved by changing to #selector