KVO on singleton property?

Question

Hi all I'm trying to implement KVO on one of the string properties within a Singleton class. I'm currently running into some errors when trying to add an observer and was hoping that someone could tell me what I'm doing wrong. The below shows my singleton class.

class User: NSObject {

    static let currentUser = User()
    private override init() {}

    var pictureString: String?
}

In a separate ViewController's viewDidLoad, I've tried to add an observer to the pictureString variable like so.

    self.addObserver(self, forKeyPath: #keyPath(User.currentUser.pictureString), options: [.old, .new, .initial], context: nil)

the problem I'm dealing with right now is that it is stating that currentUser is not KVC-compliant. I'm currently getting the below error.

    addObserver: forKeyPath:@"currentUser.pictureString" options:7 context:0x0] was sent to an object that is not KVC-compliant for the "currentUser" property.'

I'd appreciate any help thanks.

Is it because `profilePic` property doesn't exist in `User` class? — Rishab, May 31 '17 at 04:53

score 8 · Accepted Answer · answered May 31 '17 at 16:50

8

See this answer:

As for now, Swift cannot have observable class properties.

If you really want to use a singleton, you could convert it to a non-static singleton with a normal property, which then also can be KVO'd... something like this:

class UserManager: NSObject {
  private override init() {}
  static var instance = UserManager()
  var currentUser = User()
}

class User: NSObject {
  dynamic var pictureString: String?
}

Note the dynamic keyword in the variable declaration -- this is needed to make KVO work in Swift (dynamic dispatch is explained here).

Then in your VC's viewDidLoad you can register for changes with this:

UserManager.instance.addObserver(self, forKeyPath: #keyPath(UserManager.currentUser.pictureString), options: [.old, .new, .initial], context: nil)

answered May 31 '17 at 16:50

dr_barto

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Didn't know about #keyPath(), very helpful thank you! – nickromano Dec 09 '17 at 23:18
Is it safe using KVO on singleton's property? – Paresh. P Apr 20 '20 at 03:50
@Paresh.P the singleton in the example is just a normal Swift object; so all you can do with other objects, you also can do with the singleton instance. – dr_barto Apr 21 '20 at 12:07

score 0 · Answer 2 · answered Feb 26 '18 at 22:22

class Viewcontroller: UIViewController {

  let user = User.currentUser

  override viewDidLoad() {
    super.viewDidLoad()
    addObserver(self, forKeyPath: #keyPath(user.pictureString), options: [.old, .new], context: nil)
  }
}

You can declare your singleton and use that for #keyPath. Otherwise, you will raise an exception.

KVO on singleton property?

2 Answers2