Often I need to declare a protocol and need a weak reference to it. It is quite common in the delegation pattern.
In order to store a weak reference, the implementing type needs to be a reference type. Therefore, the protocol must limit the type of its implementation so that only classes can implement it.
Swift 4 now introduced a new, preferred way to declare a class-only protocol:
However, the old way, where you have to use the
class keyword still works fine. Prior to Swift 4, there were some subtle differences between these two keywords, which has been removed in SE-0156 - Class and Subytpe existentials.
So now you can declare a protocol like this:
Only objects can implement it, so the compiler will prevent the following code form beeing compiled:
In addition to that, the protocol can still be used by a weak reference, like
The old way, the
class keyword is not yet deprecated, but you can start accustom yourself to using
The information in this article are taken from here.