dependencyInversion.swift

protocol UserType {
    var id: UInt { get set }
    var name: String { get set }
}

struct User: UserType {
    
    var id: UInt
    var name: String
    
    init(id: UInt, name: String) {
        self.id = id
        self.name = name
    }
}

protocol Storage {
    func add(_ user: UserType)
    func delete(_ user: UserType)
}

final class RealmStorage: Storage {
    
    func add(_ user: UserType) {}
    
    func delete(_ user: UserType) {}
    
}

final class KeychainStorage: Storage {
    
    func add(_ user: UserType) {}
    
    func delete(_ user: UserType) {}
    
}

protocol UsersProtocol {
    func didLoad(_ users: [UserType])
    func didRemove(_ user: UserType)
}

final class UsersInteractor: UsersProtocol {
    
    let storage: Storage
    
    init(storage: Storage) {
        self.storage = storage
    }
    
    func didLoad(_ users: [UserType]) {
        users.forEach { storage.add($0) }
    }
    
    func didRemove(_ user: UserType) {
        storage.delete(user)
    }
    
}

let author = User(id: 999, name: "Maxim Vialykh")
let guest = User(id: 1, name: "Guest")
let users = [author, guest]

let usersInteractor = UsersInteractor(storage: RealmStorage())
usersInteractor.didLoad(users)

let spyInteractor = UsersInteractor(storage: KeychainStorage())
spyInteractor.didRemove(guest)