NotificationCenter.default.addObserver is not calling the objective C function

Question

I am using swift 5 on Xcode 11.0, trying to pass a userInfo from view-controller-A to view-controller-B and I already made a notification center post in view-controller-A and here is the code:

 let uid = response["session-id"] as? String

                NotificationCenter.default.post(
                    name: NSNotification.Name("didReceiveSession"),
                    object: nil,
                    userInfo: ["uid" : uid!]
                )

And in view-controller-B I registered the notification and added an observer like so:

  NotificationCenter.default.addObserver(
            self,
            selector: #selector(self.getSessionId),
            name: NSNotification.Name("didReceiveSession"),
            object: nil
        )

And here is getSessionId:

 @objc func getSessionId(_ notification: Notification)
    {
        if let data = notification.userInfo as? [String: String]
        {
            self.session_Id = data["uid"]
        }
    }

I tried a lot of workarounds like:

1- Change name: NSNotification.Name("didReceiveSession") to name: NSNotification.Name(rawValue: "didReceiveSession") in both view controllers

2- Create an instance in view-controller-A:

2.1- share = view-controller-A()

2.2- Change object: nil to object: view-controller-A.share in the addObserver (view-controller-B)

3- Change the selector in view-controller-B from selector: #selector(self.getSessionId) to selector: #selector(getSessionId(_:))

But unfortunately, all attempts did not work. I can't figure out what's wrong.

How do you know it's not being called? Set a breakpoint on the if let ... in your handler function. Does that trigger? Also, set breakpoints on both the post and the addObserver call. Is addObservertriggered before post? — Gereon
@Gereon I did set a breakpoint on the handler function and that's how I know it's not being called. I also set a breakpoint on both the post and addObserver, the post is being triggered before the addObserver so nothing suspicious in that. — S.Hasan
On the contrary. If post is triggered before addObserver, who observes the notification? — Gereon
Absolutely. If it doesn't, there is no place for the OS to deliver the notifiaction to, and it is immediately discarded. — Gereon

DonMag DonMag · Accepted Answer · 2020-04-27T14:42:06

This is a very incorrect way to use Notifications.

What you want to do is pass your "sessionID" value to from VC-A to VC-B when VC-A is moving to / presenting VC-B.

Assuming you are using a segue...

Based on what you posted, you already have a variable / property in VC-B, probably:

var session_ID: [String] = ""

So, in VC-A, you would do something like this:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let nextVC = segue.destination as? VCB {
        nextVC.session_ID = self.uid
    }
}

NotificationCenter.default.addObserver is not calling the objective C function

2 Answers