CurrentUser Swift implementation for iOS

CurrentUer.swift

//: Playground - noun: a place where people can play

import Foundation

// MARK: Infrastructure

/// Contract for remote user service
protocol UserService {
    func signIn(completion handler: (User?) -> ())
}

class DefaultUserService: UserService {
    func signIn(completion handler: (User?) -> ()) {
        let user = UserEntity(id: "userId", email: "mail@example.com", firstName: "John", lastName: "Smith")
        handler(user)
    }
}

/// Contract for local user storage
protocol UserStorage {
    func save(user: User)
    func read() -> User?
}

class DefaultUserStorage: UserStorage {

    func save(user: User) {
    }

    func read() -> User? {
        return nil
    }

}

// MARK: Model

/// User contract that used everywhere in API
protocol User {
    var id: String { get }
    var email: String { get }
    var firstName: String { get }
    var lastName: String { get }
}

/// User routines extensions
extension User {
    var fullName: String {
        return "\(firstName) \(lastName)"
    }
}

/// User implementation, this struct could be considered as Value Object
struct UserEntity: User {
    let id: String
    let email: String
    let firstName: String
    let lastName: String
}

// MARK: Current User Contracts

/// Constract for current user
protocol CurrentUser: User {
    func signIn(completion handler: (User?) -> ())
}

extension CurrentUser where Self == Current.User {
    func signIn(completion handler: (User?) -> ()) {
        service.signIn(completion: handler)
    }
}

/// Contract for user that could be saved/restored
protocol StorableUser {
    func save()
    func read()
    func fill(from user: User?)
}

extension StorableUser where Self == Current.User {
    func save() {
        storage.save(user: self)
    }
    func read() {

    }
    func fill(from user: User?) {
        self.id        = user?.id ?? ""
        self.email     = user?.email ?? ""
        self.firstName = user?.firstName ?? ""
        self.lastName  = user?.lastName ?? ""
    }
}

// MARK: Current User Implementation

/// Access point for current user
class Current {

}

/// Defines implementation for CurrentUser and StorableUser contracts, and holds its instance as singleton
extension Current {
    static let user: CurrentUser = User(service: DefaultUserService(), storage: DefaultUserStorage())

    class User: CurrentUser, StorableUser {

        init(service: UserService, storage: UserStorage) {
            self.service = service
            self.storage = storage

            let stored = self.storage.read()

            self.id         = stored?.id ?? ""
            self.email      = stored?.email ?? ""
            self.firstName  = stored?.firstName ?? ""
            self.lastName   = stored?.lastName ?? ""
        }

        let service: UserService
        let storage: UserStorage

        var id: String
        var email: String
        var firstName: String
        var lastName: String
    }
}

// MARK: Usage

Current.user.signIn { user in
    if let user = user {
        print("Logged in as \(user.fullName)")
    }
}