nburk nburk - 1 month ago 7
Swift Question

What exactly does `: class` do in a protocol declaration?

This SO post explains pretty well how to solve the issue of creating a

delegate
that is
weak
.

Essentially, there are 2 approaches:

Using the
@objc
keyword:


@objc protocol MyClassDelegate {
}

class MyClass {
weak var delegate: MyClassDelegate?
}


Using the
:class
keyword:


protocol MyClassDelegate: class {
}

class MyClass {
weak var delegate: MyClassDelegate?
}


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
:


To be accessible and usable in Objective-C, a Swift class must be a descendant of an Objective-C class or it must be marked
@objc
.


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).

So, why does this even work syntactically and what exactly does it do?

Answer

: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.