协议一致性检查

Question

如何使用AssociatedType对协议执行一致性检查。Xcode显示错误：

协议'MyListener‘只能用作泛型约束，因为它具有自或关联的类型要求

我的最终目标是从一个weakObjects数组中提取“weakObjects”，其中处理程序与函数参数匹配。

请注意。NSPointerArray of weakObjects应该捕获不同类型的MyListeners。

public class MyHandler<O,E> {
    var source = [O]()
    var dest = [E]()
}
public protocol MyListener:class {
    var section: Int {get}
    associatedtype O
    associatedtype E
    var handler: MyHandler<O,E>? { get }
}

public class MyAnnouncer {
    private let mapWeakObjects: NSPointerArray = NSPointerArray.weakObjects()
    public func add<L: MyListener>(listener: L) {
        let pointer = Unmanaged.passUnretained(listener).toOpaque()
        mapWeakObjects.addPointer(pointer)
    }
    public func search<O, E> (h:MyHandler<O,E>) -> [Int] {
        _ = mapWeakObjects.allObjects.filter { listener in
            if listener is MyListener { // Compilation failed
            }
            if let _ = listener as? MyListener { //Compilation error
            }
            if listener is MyListener.Type { //Compilation failed
            }
        }
        return [] // ultimate goal is to extract corresponding [MyListener.section].
    }
}

Answer 1

不幸的是，Swift不支持与AssociatedType一致的协议。

您应该尝试使用类型擦除。其中一种方法是通过创建新的AnyType类来实现类型删除。下面是另一种发布类型擦除的方法(来自internet的示例)

protocol SpecialValue { /* some code*/ }

protocol TypeErasedSpecialController {
  var typeErasedCurrentValue: SpecialValue? { get }
}

protocol SpecialController : TypeErasedSpecialController {
  associatedtype SpecialValueType : SpecialValue
  var currentValue: SpecialValueType? { get }
}

extension SpecialController {
  var typeErasedCurrentValue: SpecialValue? { return currentValue }
}

extension String : SpecialValue {}

struct S : SpecialController {
  var currentValue: String?
}

var x: Any = S(currentValue: "Hello World!")
if let sc = x as? TypeErasedSpecialController { // Now we can perform conformance
  print(sc.typeErasedCurrentValue) 
}