Annotating Swift function declarations that support conformance to a protocol?

Question

Is there a standard mechanism for annotating function declarations in Swift to indicate that they are present because a class conforms to some protocol?

For instance, this declaration might be present because a class conforms to NSCoding. (Marking it with override would result in a syntax error, so it's not the kind of annotation I am looking for.) Ideally I am looking for a code-level annotation (e.g. override instead of /*! ... */).

// ... annotation such as "conform to NSCoding", if possible
func encodeWithCoder(encoder: NSCoder) {
   // ...
}

@Aderstedt Annotating in the code would be best (see update to Q). — Drux
– Drux, Commented Aug 20, 2015 at 7:38
AFAIK there's no way to annotate protocol conformance in the code. Use comments. — zrzka
– zrzka, Commented Aug 20, 2015 at 7:51
@robertvojta Too bad. (I am using comments now but wish for an alternative.) — Drux
– Drux, Commented Aug 20, 2015 at 7:56

Community · Accepted Answer · 2017-05-23 12:24:20Z

You can use extension. for example:

protocol SomeProtocol {
    func doIt() -> Int
}


class ConcreteClass {
    ....
}

extension ConcreteClass: SomeProtocol {
    func doIt() -> Int {
       // ...
       return 1
    }
}

But you cannot define required initializer in extension, for example:

// THIS DOES NOT WORK!!!

class Foo: NSObject {
}
extension Foo: NSCoding {
    required convenience init(coder aDecoder: NSCoder) {
        self.init()
    }

    func encodeWithCoder(aCoder: NSCoder) {
        // ...
    }
}

emits an error:

error: 'required' initializer must be declared directly in class 'Foo' (not in an extension)
    required convenience init(coder aDecoder: NSCoder) {
    ~~~~~~~~             ^

In this case, you should use // MARK: comments:

class Foo: NSObject {

    // ...


    // MARK: NSCoding

    required init(coder aDecoder: NSCoder) {
        super.init()
    }

    func encodeWithCoder(aCoder: NSCoder) {
        // ... 
    }
}

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