The common way of using the Builder Pattern in Swift

OldUserBuilder.swift

struct UserSignUp {
    var firstName: String
    var lastName: String?
    var username: String
    var password: String
    
    class Builder {
        var firstName: String?
        var lastName: String?
        var username: String?
        var password: String?
        var confirmPassword: String?
        
        func set(firstName: String) -> Builder {
            self.firstName = firstName
            return self
        }
        
        func set(lastName: String) -> Builder {
            self.lastName = lastName
            return self
        }
        
        func set(username: String) -> Builder {
            self.username = username
            return self
        }
        
        func set(password: String) -> Builder {
            self.password = password
            return self
        }
        
        func set(confirmPassword: String) -> Builder {
            self.confirmPassword = confirmPassword
            return self
        }
                
        func build() -> UserSignUp? {
            guard let firstName = firstName, !firstName.isEmpty else {
                return nil
            }
            // last name not required, so don't unwrap
            guard lastName?.isEmpty ?? true else {
                return nil
            }
            guard let username = username, !username.isEmpty else {
                return nil
            }
            guard let password = password, password.characters.count >= 4 else {
                return nil
            }
            guard let confirmPassword = confirmPassword, confirmPassword == password else {
                return nil
            }
            return UserSignUp(firstName: firstName, lastName: lastName, username: username, password: password)
        }
    }
}

// USAGE
let user: User? = User.Builder()
    .set(firstName: "Matt")
    .set(lastName: "Hoffman")
    .set(username: "muuhoffman")
    .set(password: "foo")
    .set(confirmPassword: "foo")
    .build()