PersonViewModel.swift

class DetailedPersonViewModel: PersonViewModel {
    var title: String
    var birthday: String
    var dateFormatter = DateFormatter()

    init(person: Person) {
        title = person.firstName + " " + person.lastName

        dateFormatter.dateStyle = .short        
        birthday = dateFormatter.string(from: person.birthday)
    }

    func doSomeWork() {
        // Some complicated logic
    }
}

class LuckyPersonViewModel: PersonViewModel {
    var title: String
    var birthday: String
    var dateFormatter = DateFormatter()

    init(person: Person) {
        title = "Lucky " + person.firstName

        dateFormatter.dateStyle = .long        
        birthday = "Birthday: " + dateFormatter.string(from: person.birthday)
    }

    func doSomeWork() {
        // Any other logic
    }
}