Candidate is not '@objc' but protocol requires it

Question

I have been following this tutorial to learn swift & iOS app development. In the Protocol section, the tutorial defined the following protocol:

@objc protocol Speaker {
  func Speak()
  optional func TellJoke()
}

It says If you want to have a protocol with optional methods, you must prefix the protocol with the @objc tag (even if your class is not interoperating with objective-C).

Then, it shows the sample to implement the protocol:

class Vicki: Speaker {
  func Speak() {
    println("Hello, I am Vicki!")
  }
  func TellJoke() {
    println("Q: What did Sushi A say to Sushi B?")
  }
}

I tried the above code in my xcode playground, but I got the compiler error "Type Vicki doesn't conform to protocol Speaker".

Xcode also pops up an fix-it text which says "Candidate is not '@objc' but protocol requires it".

I get completely confused now, the tutorial doesn't mention this error at all. Could someone please explain to me all this to make me clear? Thanks!

Answer 1

From what I can tell, marking your protocol as @objc means that any classes implementing it also have to be exposed to Objective-C. This can be done either by making Vicki a subclass of NSObject:

class Vicki: NSObject, Speaker {

Or by marking each implemented method as @objc:

class Vicki: Speaker {
    @objc func Speak() {
        print("Hello, I am Vicki!")
    }
    @objc func TellJoke() {
        print("Q: What did Sushi A say to Sushi B?")
    }
}

Update: From Apple's Swift Language Documentation

Optional protocol requirements can only be specified if your protocol is marked with the @objc attribute.

...

Note also that @objc protocols can be adopted only by classes, and not by structures or enumerations. If you mark your protocol as @objc in order to specify optional requirements, you will only be able to apply that protocol to class types.

Answer 2

In order for a type to conform to a protocol marked @objc, that type must also marked with @objc or the methods being used to satisfy the protocol must be.

Using @objc on a declaration tells the compiler to expose it to the Objective-C runtime. In order for that to be possible, the type that was declared @objc must be able to be represented in Objective-C.

For classes this means they must inherit from NSObject. For methods, this means they become backed by dynamic Objective-C message passing.