Swift 协议中的弱属性可能只是一个类或类绑定的协议类型

问题描述：

我想定义一个在 Viper 架构中使用的协议，以使用具有弱属性的协议在 Viper 组件之间建立连接，但我收到以下错误消息:

I would like to define a protocol which is used in a Viper architecture to establish a connection between a Viper components using a protocol with a weak property but I get the following error message:

'weak' 只能应用于类和类绑定的协议类型，不是'Self.ViperViewClass'

'weak' may only be applied to class and class-bound protocol types, not 'Self.ViperViewClass'

protocol ViperPresenter: class {

    associatedtype ViperViewClass
    weak var view: ViperViewClass! { get set }

}

答

协议目前不能要求将属性实现为 weak 存储属性.

Protocols can't currently require properties to be implemented as weak stored properties.

虽然 weak 和 unowned 关键字目前在属性要求中是允许的，但它们没有任何作用.以下是完全合法的:

Although the weak and unowned keywords are currently allowed on property requirements, they have no effect. The following is perfectly legal:

class C {}

protocol P {
  weak var c: C? { get set }
}

struct S : P {
  var c: C? // strong reference to a C instance, not weak.
}

这是作为错误提交，并且SE-0186 将使用weak 和 unowned 在协议中的属性要求 Swift 4.1 中的警告(在 Swift 3 和 4 模式下)，以及 Swift 5 中的错误.

This was filed as a bug, and SE-0186 will make the use of weak and unowned on property requirements in a protocol a warning in Swift 4.1 (in both Swift 3 and 4 modes), and an error in Swift 5.

但即使协议可以要求将属性实现为weak或unowned存储属性，编译器也需要知道ViperViewClass 是一个类类型(也就是说，
associatedtype ViperViewClass : AnyObject).

But even if protocols could require properties to be implemented as weak or unowned stored properties, the compiler would need to know that ViperViewClass is a class type (i.e by saying
associatedtype ViperViewClass : AnyObject).

Swift 协议中的弱属性可能只是一个类或类绑定的协议类型

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