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Xcode – Swift – How to use KVO's 'observe' method on protocol

I need an observable property on certain view controllers in my application, I’m trying to standardize this through a protocol.

protocol MyProtocol: UIViewController {
    dynamic var observeMe: Bool { get set }
}

But when trying to observe the property like so:

let viewController = UIViewController()
if let observableViewController = viewController as? MyProtocol {
    observableViewController.observe(.observeMe, options: [.old, .new], changeHandler: { object, change in
        // Do something
    })
}

Xcode throws the compiler error:

"Member ‘observe’ cannot be used on value of protocol type
‘MyProtocol’; use a generic constraint instead"

Tags:

2

Answers


  1. 0

    You can try this –

    import Foundation

protocol MyProtocol {
    dynamic var observeMe: Bool { get set }
}

class Test : NSObject, MyProtocol {
    @objc var observeMe: Bool = true
    private var observeMeObservation: NSKeyValueObservation?
    
    override init() {
        super.init()
        
        observeMeObservation = self.observe(.observeMe, changeHandler: { [weak self] (_, _) in
            guard let self = self else { return }
            print("observeMe : (self.observeMe)")
        })
    }
    
    func doSomeChanges() {
        self.observeMe = false
        self.observeMe = true
    }
    
    deinit {
        observeMeObservation?.invalidate()
    }
}

let test = Test()
test.doSomeChanges()
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  2. 0

    You can declare a function and apply the generic constraint to it.

    In Swift 5.7 you can simply do:

    func observeViewController(_ vc: some MyProtocol) {
  vc.observe(.observeMe, options: [.old, .new], changeHandler: { object, change in
  })
}

    Then you can call it:

    let viewController = UIViewController()
if let observableViewController = viewController as? MyProtocol {
  observeViewController(viewController)
}

    Prior to Swift 5.7, some MyProtocol is not supported and you need to use type erasure:

    class AnyMyProtocol: MyProtocol & UIViewController {
  var observeMe: Bool

  private var base: MyProtocol

  init(_ base: MyProtocol) {
    self.base = base
  }

  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
}

func observeViewController(_ vc: MyProtocol) {
  vc.observe(.observeMe, options: [.old, .new], changeHandler: { object, change in
  })
}

    And call it like so:

    let viewController = UIViewController()
observeViewController(vc: AnyMyProtocol(base: observableViewController))

    By the way, you will receive a warning reading:

    Passing reference to non-'@objc dynamic' property 'observeMe' to KVO method 'observe(_:options:changeHandler:)' may lead to unexpected behavior or runtime trap

    But in your case you can’t declare it as @objc because an @objc protocol can’t inherit from a non-protocol type.

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