Swift KVO 在符合协议(protocol)的对象上

Question

我有一个协议(protocol) (X) 和一个实现协议(protocol) X 的类 (A):

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

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

在另一个类中我想观察变量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
       ...
       }
    } 
}

这里等式的每一部分都是或符合 NSObject，所以我希望能够 KVO toBeObserved 但我却遇到了运行时崩溃:

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

谢谢。

Answer 1

确保将观察到的属性标记为@objc 和dynamic。作为Using Key-Value Observing in Swift说:

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

协议(protocol)和参与类也需要标记为@objc。例如:

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)
        }
    }
}