swift 4 : generic conforming to AnyObject does not accept a protocol conforming to AnyObject

Question

我需要创建一个类型安全的弱引用数组

1. 一个包含“类型安全”弱引用并且可以是数组元素的结构:

   public struct WeakRef<T: AnyObject>: Hashable {
       public weak var ref: T?
       public let hashValue: Int
   
       init(_ ref: T) {
           self.ref = ref
           self.hashValue = ObjectIdentifier(ref).hashValue
       }
   }
   
   extension WeakRef: Equatable {
       public static func ==(lhs: WeakRef<T>, rhs: WeakRef<T>) -> Bool {
           return lhs.hashValue == rhs.hashValue
       }
   }
   
   extension WeakRef: Comparable {
       static public func < (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue < rhs.hashValue
       }
       static public func <= (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue <= rhs.hashValue
       }
       static public func >= (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue >= rhs.hashValue
       }
       static public func > (lhs:WeakRef<T>, rhs:WeakRef<T>) -> Bool {
           return lhs.hashValue > rhs.hashValue
       }
   }
   

2. 我有一个需要与这个弱引用一起使用的协议(protocol):

   protocol LemmingTrackingProtocol: AnyObject {
       func onLemmingZPositionChanged()
       func onLemmingDrop()
   }
   

3. 这是可能的:

   var trackers = [WeakRef<LemmingTrackingProtocol>]()
   

4. 但这不是:

   func addTracker(_ tracker: LemmingTrackingProtocol) {
       let tracker = WeakRef(tracker) // <-- Cannot invoke initializer...
   
       ...
   }
   

请提示我做错了什么。谢谢!

Answer 1

长话短说:

只需将 @objc 添加到所需的协议(protocol)(在我的例子中是 LemmingTrackingProtocol)

警告

虽然这有效，you'll probably get stuck with not being able to create an extension with your @objc protocol

详细

根据@Hamish 提供的链接:

1. 这是一个(有点)错误:https://bugs.swift.org/browse/SR-55 , 它仍然存在于 Swift 4 中
2. protocols do not (should not) always conform to themselves