Swift Using Self in Generics & Protocol (w/o Init at method Level)

Question

Update: The question voted to close this as duplicate deals with self.init() which can be solved by marking class final or requiring init. But How does that work on Repositories or return from any other method other than init (My Implementation is @NSManaged Object)

Here is what I want to do. Get a JSON Via Request, then parse it into an object.

var req = Request("http://somewhere/story/get/1")
var story = req.parseJsonResponse() as Story

The Request class somehow looks like this:

class Request {

    public var httpJsonResponse : String

    init(_ url : String) {
        //This comes from HTTP
        httpJsonResponse = "{}"
    }

    public func parseJsonResponse<T : IParse>() -> T {

        return T.parse(json: httpJsonResponse)

    }

}

Now, I want to make sure that IParse implementation has parse method:

protocol IParse {
    static func parse(json : String) -> Self
}

So far so good, here's the problem with implementation:

class Story : IParse {
    required init() 
    {
    }
    public static func parse(json: String) -> Self
    {
        //return self.init() works thanks to @tomahh
        return StoryRepo.ById(1)
    }

}

class StoryRepo {
    public static func ById(_ id : Int) -> Story {
        return Story()
    }
}

This does not work. Neither of these work:

1. Returning Story
2. Casting Story() as! Self

See updated playground gist.

Answer 1

class Story : IParse {
    required init() 
    {
    }

    public static func parse(json: String) -> Self
    {
        return self.init()
    }
}

This will work. This makes sure that you actually build an instance of Self, so that a Tale subclass of Story would not create a boring Story, but a wonderful tale indeed.

The required keyword forces subclasses to override this init, so that we are sure it is valid to actually call self.init() within parse.

---

Another solution, if you don't want to struggle with inheritance, would have been to make the Story class final

final Story: IParse {
  public static func parse(json: String) -> Story {
    return Story()
  }
}

In this example, because Story will never be inherited from, it is safe to call Story() and return a Story type from the parse function.