类型 A 要求类型 B 是一个类类型 swift 4 [英] type A requires that type B be a class type swift 4

问题描述

下面的代码给了我一个编译错误

<块引用>

'WeakReference' 要求 'ServiceDelegate' 是一个类类型

protocol ServiceDelegate: AnyObject {func doIt()}类 SomeClass() {//编译错误出现在这一行私有变量观察者 = [WeakReference]()}

弱引用代码:

final class WeakReference{私有(设置)弱变量值:T?初始化(值:T?){self.value = 价值}}

我该如何解决这个错误?应按照此网站正确声明委托.>

到目前为止我尝试过的:

• 将委托协议一致性从 AnyObject 更改为class 没有解决问题.
• 在干净的操场上试试上面的代码.

解决方案

你不能有 WeakReference.ServiceDelegate 本身不是 AnyObject，它只要求符合它的任何东西都是 AnyObject.

您需要使 SomeClass 泛型并使用泛型类型作为 WeakReference 的类型:

class SomeClass{私有变量观察者 = [WeakReference<T>]()}

如果 SomeClass 上的泛型过于紧凑，并且您希望能够将多个不相关类的实例作为观察者，那么我会通过放弃 WeakReference:

final class WeakServiceDelegate {私有(设置)弱变量值:ServiceDelegate?初始化(值:ServiceDelegate?){self.value = 价值}}类 SomeClass {私有变量观察者 = [WeakServiceDelegate]()}

或者，您可以使 WeakReference 有条件地符合 ServiceDelegate:

extension WeakReference: ServiceDelegate where T: ServiceDelegate {func doIt() {价值?.doIt()}}

然后在SomeClass中使用ServiceDelegate的数组:

class SomeClass {私有变量观察者 = [ServiceDelegate]()func addObserver(_观察者:T){观察者.追加(弱参考(值:观察者))}}

the following code gives me a compile error

'WeakReference' requires that 'ServiceDelegate' be a class type

protocol ServiceDelegate: AnyObject {
    func doIt()
}

class SomeClass() {
    // compile error occurs in this line
    private var observers = [WeakReference<ServiceDelegate>]()
}

WeakReference code:

final class WeakReference<T: AnyObject> {

    private(set) weak var value: T?

    init(value: T?) {
        self.value = value
    }
}

How can I fix this error? Delegate should be declared correctly as per this site.

What I have tried so far:

• Changing the delegate protocol conformance from AnyObject to class does not solve the problem.
• Try the above code in a clean playground.

解决方案

You can't have a WeakReference<ServiceDelegate>. ServiceDelegate itself is not an AnyObject, it just requires that anything that conforms to it be an AnyObject.

You would need to make SomeClass generic and use the generic type as the type for the WeakReference:

class SomeClass<T: ServiceDelegate> {
    private var observers = [WeakReference<T>]()
}

If the generic on SomeClass is too constricting and you want to be able to have instances of multiple unrelated classes as observers then I would do it by abandoning the generic parameter on WeakReference:

final class WeakServiceDelegate {
    private(set) weak var value: ServiceDelegate?

    init(value: ServiceDelegate?) {
        self.value = value
    }
}

class SomeClass {
    private var observers = [WeakServiceDelegate]()
}

Alternatively you could make WeakReference conditionally conform to ServiceDelegate:

extension WeakReference: ServiceDelegate where T: ServiceDelegate {
    func doIt() {
        value?.doIt()
    }
}

And then use an array of ServiceDelegate in SomeClass:

class SomeClass {
    private var observers = [ServiceDelegate]()

    func addObserver<T: ServiceDelegate>(_ observer: T) {
        observers.append(WeakReference(value: observer))
    }
}

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