weak引用在Swift中似乎无法正常工作，除非声明一个@objc的protocol，但在纯Swift应用程序中我不想这样做。
这段代码会产生编译错误（weak不能应用于非类类型MyClassDelegate）：
class MyClass {
weak var delegate: MyClassDelegate?
}
protocol MyClassDelegate {
}
我需要在协议前加上@objc，然后它就可以工作了。
问题：如何用“纯”的 Swift 实现弱引用代理？
AnyObject。protocol ProtocolNameDelegate: AnyObject {
// Protocol stuff goes here
}
class SomeClass {
weak var delegate: ProtocolNameDelegate?
}
使用 AnyObject，你可以表达只有类才能符合此协议，而结构体或枚举则不能。
protocol ProtocolNameDelegate: AnyObject，但无所谓。 - hnhweak var delegate: DiceGameDelegate?即可。 - Mykhailo Lysenko我一直困惑委托是否应该是弱引用。最近我学到了更多关于委托和何时使用弱引用的知识，所以让我在这里补充一些内容，以便未来的读者能够了解。
The purpose of using the weak keyword is to avoid strong reference cycles (retain cycles). Strong reference cycles happen when two class instances have strong references to each other. Their reference counts never go to zero so they never get deallocated.
You only need to use weak if the delegate is a class. Swift structs and enums are value types (their values are copied when a new instance is made), not reference types, so they don't make strong reference cycles.
weak references are always optional (otherwise you would used unowned) and always use var (not let) so that the optional can be set to nil when it is deallocated.
A parent class should naturally have a strong reference to its child classes and thus not use the weak keyword. When a child wants a reference to its parent, though, it should make it a weak reference by using the weak keyword.
weak should be used when you want a reference to a class that you don't own, not just for a child referencing its parent. When two non-hierarchical classes need to reference each other, choose one to be weak. The one you choose depends on the situation. See the answers to this question for more on this.
As a general rule, delegates should be marked as weak because most delegates are referencing classes that they do not own. This is definitely true when a child is using a delegate to communicate with a parent. Using a weak reference for the delegate is what the documentation recommends. (But see this, too.)
Protocols can be used for both reference types (classes) and value types (structs, enums). So in the likely case that you need to make a delegate weak, you have to make it an object-only protocol. The way to do that is to add AnyObject to the protocol's inheritance list. (In the past you did this using the class keyword, but AnyObject is preferred now.)
protocol MyClassDelegate: AnyObject {
// ...
}
class SomeClass {
weak var delegate: MyClassDelegate?
}
阅读以下文章有助于我更好地理解这个问题。它们还讨论了相关问题，如unowned关键字和在闭包中发生的强引用循环。
AnyObject 是在 Swift 中使用弱引用的官方方式。
class MyClass {
weak var delegate: MyClassDelegate?
}
protocol MyClassDelegate: AnyObject {
}
来自苹果公司:
为了避免强引用循环，代理应该声明为弱引用。有关弱引用的更多信息，请参阅类实例之间的强引用循环。将协议标记为仅限类将允许您稍后声明代理必须使用弱引用。您可以通过继承 AnyObject 来将协议标记为仅限类，如“仅限类协议”中所述。
class已经被弃用了吗？ - hnh更新: 看起来手册已经更新了，我所提到的示例已被删除。请参见上面@flainez答案的编辑。
原文: 即使您不与Obj-C进行交互，使用@objc也是正确的方法。它确保您的协议应用于类而不是枚举或结构体。请参见手册中的“检查协议一致性”。
@objc、AnyObject或class限定符，否则符合协议的对象可能不是引用对象。AnyObject，因为class有望被弃用）。@objc添加到您的类和属性中。这与您的开发语言无关。它会导致编译器以与Objective-C运行时兼容的方式构建您的代码，这对于某些操作系统接口（例如目标/动作和旧样式键路径）是必需的。protocol MyProtocol {
func doSomething()
}
class MyClass: MyProtocol {
func doSomething() {
print("Doing something")
}
}
var weakProtocol: Weak<MyProtocol>?
let myObject = MyClass()
weakProtocol = Weak(myObject)
weakProtocol?.doSomething() // Will print "Doing something"
以下是示例
protocol NameOfProtocol: class {
// member of protocol
}
class ClassName: UIViewController {
weak var delegate: NameOfProtocol?
}
苹果使用“NSObjectProtocol”而非“class”。
public protocol UIScrollViewDelegate : NSObjectProtocol {
...
}
这对我也起作用了，并且消除了在尝试实现自己的委托模式时遇到的错误。