`：class`究竟在协议声明中做了什么？ [英] What exactly does `: class` do in a protocol declaration?
问题描述
此 SO帖子很好地解释了如何解决创建委托的问题，即弱。
This SO post explains pretty well how to solve the issue of creating a delegate that is weak.
基本上，有两种方法：
使用 @objc keyword：
Using the @objc keyword:
@objc protocol MyClassDelegate {
}
class MyClass {
weak var delegate: MyClassDelegate?
}
使用：class keyword：
Using the :class keyword:
protocol MyClassDelegate: class {
}
class MyClass {
weak var delegate: MyClassDelegate?
}
我正在尝试做一些研究，以了解两者之间究竟有什么区别方法是。使用< docs 非常明确code> @objc ：
I am trying to do some research to understand what exactly the differences between the two approaches are. The docs are pretty clear about using @objc:
在Objective-C中可以访问和使用Swift类必须是Objective-C类的后代，或者必须标记为
@objc。
但是，我找不到有关：class 实际上是什么的一些信息。从细节上考虑整个概念，它实际上并没有多大意义。我的理解是 class 是Swift中用来声明类的关键字。所以，在这里似乎我们使用关键字 class 本身作为协议（因为我们在之后附加它：协议声明后）。
However, nowhere I found some information about what :class actually does. Considering the whole notion in detail, it actually doesn't make a lot of sense to. My understanding is that class is a keyword in Swift to declare classes. So, here it seems like we are using the keyword class itself as a protocol (as we're appending it after the : after the protocol declaration).
那么，为什么这甚至在语法上有效呢？究竟是什么呢？
So, why does this even work syntactically and what exactly does it do?
推荐答案
：class 确保只有类可以实现协议。那是任何类，而不仅仅是 NSObject 的子类。另一方面， @objc 告诉编译器使用Objective-C样式的消息传递来调用方法，而不是使用vtable来查找函数。
:class ensures that only classes can implement the protocol. And that's any class, not just subclasses of NSObject. @objc, on the other hand, tells the compiler to use Objective-C-style message passing to call methods, instead of using a vtable to look up functions.
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