protocol_extension.swift

protocol CardType {
    var holderName: String { get set }
    var number: String { get set }
    var expiration: String? { get set }
    var month: String? { get }
    var year: String? { get }
}

struct Card: CardType {
    var holderName: String
    var number: String
    var expiration: String?
    var month: String? { return "" }
    var year: String? { return "" }
}

protocol Mockable {
    static func mock(_ option: Int) -> Self
}

extension Card: Mockable {
    
    static func mock(_ option: Int) -> Card {
        switch option {
        case 1:
            return Card(holderName: "John Malkovich", number: "4111 1111 1111 1111", expiration: "09/12")
        default:
            return Card(holderName: "Mock", number: "0000 0000 0000 0000", expiration: "00/00")
        }
    }
    
}

let mockCard = Card.mock(1)