ios - 泛型 'is not identical to' 错误

Question

所以我正在尝试在 Swift 中使用泛型来创建类似 KVO 的功能，但是我收到了这个错误，我只是想不通原因:

这是全部代码:

struct Observable<T> {
    typealias ValueType = T
    typealias SubscriptionType = Subscription<T>

    var value: ValueType {
        didSet {
            let event = ValueChangeEvent(oldValue, value)
            self.subscriptions.notify(event)
        }
    }

    var subscriptions = Subscriptions<SubscriptionType>()

    init(_ value: ValueType) {
        self.value = value
    }
}

struct ValueChangeEvent<T> {
    typealias ValueType = T

    let oldValue: ValueType
    let newValue: ValueType

    init(_ o: ValueType, _ n: ValueType) {
        oldValue = o
        newValue = n
    }
}

class Subscription<T> {
    typealias ValueType = T
    typealias HandlerType = (oldValue: ValueType, newValue: ValueType) -> ()

    let handler: HandlerType

    init(handler: HandlerType) {
        self.handler = handler
    }

    func notify(event: ValueChangeEvent<ValueType>) {
        self.handler(oldValue: event.oldValue, newValue: event.newValue)
    }
}

struct Subscriptions<T> {
    typealias ValueType = T
    typealias SubscriptionType = Subscription<T>

    private var subscriptions = Array<SubscriptionType>()

    mutating func add(handler: SubscriptionType.HandlerType) -> SubscriptionType {
        let subscription = Subscription(handler: handler)
        self.subscriptions.append(subscription)
        return subscription
    }

    mutating func remove(subscription: SubscriptionType) {
        var index = NSNotFound
        var i = 0
        for element in self.subscriptions as [SubscriptionType] {
            if element === subscription {
                index = i
                break
            }
            i++
        }

        if index != NSNotFound {
            self.subscriptions.removeAtIndex(index)
        }
    }

    func notify(event: ValueChangeEvent<ValueType>) {
        for subscription in subscriptions {
           subscription.notify(event)
        }
    }
}

如有任何帮助，我们将不胜感激。

Answer 1

根据我所看到的，它看起来像 subscriptions Observable<T>的属性(property)定义不正确:

var subscriptions = Subscriptions<SubscriptionType>()

而它应该是:

var subscriptions = Subscriptions<ValueType>()

我没有深入研究逻辑，所以我不能说经过这些更改后您的代码是否仍然有效 - 如果不能，这是您找出问题所在的一个很好的起点。