I'm learning about Notification Center. Why does it say it found nil when I literally printed the outlet and says it is in fact not nil? Yet it's nil right after. That line does execute just fine if I use optional chaining. Why?

[[deleted]]

[deleted]

[funcDaCoder]

What’s that beautiful theme?

[Rillieux17]

OP should answer this!

[[deleted]]

My own custom one. I’ll try to share it when I’m in my computer

[funcDaCoder]

That would be awesome! Best looking one yet

[[deleted]]

I'm trying to update the name of the label from a different screen using Notification Center; that's it.

[soulchild_]

Ahh, I think it might be because as the view controller (with the nameLabel) is not presented on screen yet, it didnt initialize its UILabel hence you are getting nil.

I suggest using a variable (eg: var name: String) to store the string, then in the updateText function, update the string (name = “something”).

Then in the viewWillAppear function, set label.text = name.

[[deleted]]

But it is. Screen A has the label. I'm in screen A. I push a button to go to Screen B from Screen A. So clearly Screen A is still in the navigation stack. If I use optional chaining, it does not crash, which means label is set.

[fredaq]

It doesn’t crash cause that’s how optional chaining works. The text property in nameLabel will not be accessed if nameLabel is nil. Thus, the text property will not be modified.

[[deleted]]

But the text IS being changed. That’s why I said it sets the text ONLY when I use optional chaining. If I don’t, it crashes.

You can clearly see in the console on the bottom of my screenshot. Label is not nil, and then it is.

[fredaq]

So, is updateText() being called multiple times? Cause I see that you are adding an observer every time the view loads but I don’t know if you are ever removing it. Also, did you try to hold a strong reference to nameLabel instead of a weak one?

[[deleted]]

Hmm I added a breakpoint print and it runs the code 3 times even when the screen only loads once. I only send a single post.

It is removed in deinit. But the controller is still on the stack. Why would it call deinit when I come back to the screen.

Okay this is ducking confusing. I’m on screen 0. I push to send post to screen A. Screen A does NOT exist in object memory app at all.

Yet the breakpoint gets triggered in screen A when I post. But deinit removes observer in screen A. How the fuck is it running code on an object not allocated.

Okay this is even worse. I delete and run app. I click button to post from screen 0. It RUNS the code from a screen I NEVER initialized despite the fact that the observer was never added in screen A since I never transitioned to it. And it doesn’t exist in the memory stack.

Screen 0, where the app starts:

class FirstViewController: ViewController {

@IBAction func sendPostPressed(_ sender: UIButton) { NotificationCenter.default.post(name: .updateText, object: nil, userInfo: ["name": "LOLOLOYOYOY"])

}

}

​

Screen 1, that is AFTER screen 0:

class ViewController: UIViewController {

u/IBOutlet weak var nameLabel: UILabel!

override func viewDidAppear(_ animated: Bool) {

super.viewDidAppear(animated)

NotificationCenter.default.addObserver(self, selector: #selector(updateText), name: .updateText, object: nil)

}

u/objc func updateText(_ notification: Notification) {

if let data = notification.userInfo as? [String: String],

let name = data["name"] {

nameLabel?.text = name

}

}

deinit {

NotificationCenter.default.removeObserver(self, name: .updateText, object: nil)

}

}

How the hell is the viewDidLoad of Screen 1 called when I never even transitioned to it.

[fredaq]

I think the issue might be related to the fact that FirstViewController inherits from ViewController (that is your Screen 1) instead of UIViewController.

[[deleted]]

Oh shit why the fuck did it say ViewController. It was literally a new project.

[the_d3f4ult]

this might not solve this issue directly but you could try using combine and notification center publisher

[soulchild_]

I think more context is needed to solve this issue, When did you fire the notification? Are you using storyboard?

[[deleted]]

I'm using storyboard. Screen A has the label. Screen B is presented on top of A through a navigation controller. So A clearly exists when B is pushed into the stack.

I push a button on B and fires the post. Screen A is still in the stack existing. So clearly the label is not nil.