Swift 对象上的 KVO 符合协议

Question

我有一个协议 (X) 和一个实现协议 X 的 class (A):

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

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

在另一个 class 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
       ...
       }
    } 
}

}

这里等式的所有部分都是或符合 NSObject，所以我希望能够 KVO toBeObserved 但我遇到了运行时崩溃：

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。正如 Using Key-Value Observing in Swift 所说：

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

协议和参与类也需要标记 @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)
        }
    }
}

Swift 对象上的 KVO 符合协议

Swift KVO on an object conforming to a protocol

protocols

key-value-observing

swift