GuestObjectUsingNullObjectDesignPattern.swift

import Foundation

protocol User {
    var firstName: String { get }
    var lastName: String { get }
    
    func fullName() -> String
}

class CurrentUser: User {
    let firstName: String
    let lastName: String
    
    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
    
    func fullName() -> String {
        return "\(firstName) \(lastName)"
    }
}

class GuestUser: User {
    var firstName = "Guest"
    
    var lastName = "User"
    
    func fullName() -> String {
        return "\(firstName) \(lastName)"
    }
}

class YourViewControllerOrSomeOtherPlace {
    private let user: User
    
    init(user: User) {
        self.user = user
    }
    
    func viewDidLoadOrSomeOtherMethod() {
        print("rendering UI for the user since we clearly have one, either current or guest")
        print("user's full name is: \(user.fullName())")
    }
}

let currentUser = CurrentUser(firstName: "Joe", lastName: "Dow")
let aVCWithAUser = YourViewControllerOrSomeOtherPlace(user: currentUser)
aVCWithAUser.viewDidLoadOrSomeOtherMethod()

print("=================")

let guestUser = GuestUser()
let aVCWithGuestUser = YourViewControllerOrSomeOtherPlace(user: guestUser)
aVCWithGuestUser.viewDidLoadOrSomeOtherMethod()