Swift KVO on an object conforming to a protocol

Question

I have a protocol (X) and a class (A) that implements protocol X:

protocol X, NSObjectProtocol {
   var toBeObserved: MyCustomClass? { get}
}

class A: NSObject, X {
   var toBeObserved: MyCustomClass?
   ...
}

in another class I want to observe the variable toBeObserved:

class B {
    ...
    var instanceConformingToX: X <-note: not A but simply the protocol X
    ...
    func someFunc() {
       self.observation = self.observe(\.instanceConformingToX.toBeObserved) { (observed, change) in
       ...
       }
    } 
}

}

Everything piece of the equation here is or conform to NSObject, so I expect to be able to KVO toBeObserved but instead I get a runtime crash:

Fatal error: Could not extract a String from KeyPath Swift.KeyPath<MyAppName.B, MyFramework.A>

Thanks.

Answer 1

Make sure to mark the observed property as @objc and dynamic. As Using Key-Value Observing in Swift says:

Mark properties that you want to observe through key-value observing with both the @objc attribute and the dynamic modifier.

The protocol and participating classes will need to be marked @objc, too. E.g.:

class MyCustomClass: NSObject { ... }

@objc protocol X: NSObjectProtocol {
    @objc dynamic var toBeObserved: MyCustomClass? { get }
}

class A: NSObject, X {
    var toBeObserved: MyCustomClass? = MyCustomClass()
}

class B: NSObject {
    @objc var x: X = A()

    var token: NSKeyValueObservation?

    func addObserver() {
        token = observe(\.x.toBeObserved) { object, _ in
            print(object)
        }
    }
}