How to define initializers in a protocol extension?

Question

protocol Car {
     var wheels : Int { get set}

     init(wheels: Int)

}

extension Car {
    init(wheels: Int) {
        self.wheels = wheels
    }
}

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Answer 1

May not be the same but in my case instead of using init I used a static func to return the object of the class.

protocol Serializable {
   static func object(fromJSON json:JSON) -> AnyObject?
}

class User {
    let name:String

    init(name:String) {
        self.name = name
    }
}

extension User:Serializable {

    static func object(fromJSON json:JSON) -> AnyObject? {
        guard let name = json["name"] else {
            return nil
        }

        return User(name:name)
     }

}

Then to create the object I do something like:

let user = User.object(fromJSON:json) as? User

I know its not the best thing ever but its the best solution I could find to not couple business model with the data layer.

NOTE: I'm lazy and I coded everything directly in the comment so if something doesn't work let me know.