How to automate factory design pattern in swift?

I want to automate this piece of code:

    let objectType = json["object"]["type"].stringValue;

    switch objectType {
        case "Message":
            activity.item = MessageLib.make(json["object"]) as! MessageItem;
        default:
        ()
    }

I want to make an object based on its type. But I don't want to write a new case for each new ObjectType. I just want to make a new object class. OBJECTTYPEItem