[Final] #dependencyInversion

Final.swift

class FriendsService: FriendsWorker {
    func fetch() -> [Friend] {
        return []
    }
}

class FriendsDatabase: FriendsWorker {
    func fetch() -> [Friend] {
        return []
    }
}

class Profile {
    private let worker: FriendsWorker
    
    init(worker: FriendsWorker) {
        self.worker = worker
        fetch()
    }
    
    private func fetch() {
        worker.fetch()
    }
}

let friendsService = FriendsService()
let profileAPI = Profile(worker: friendsService)

let friendsDatabase = FriendsDatabase()
let profileDatabase = Profile(worker: friendsDatabase)