arch.swift

class ViewModel {
    private let repository: Repository
    private(set) var data: Data?
    var updateUi: (() -> Void)?

    func viewDidLoad() {
        repository.getData(cachePolicy: .cacheThenNetwork) { [weak self] data in
            self?.data = data
            self?.updateUi?()
        }
    }
}

enum CachePolicy {
    case cacheOnly, cacheThenNetwork, networkOnly
}

protocol Repository: AnyObject {
    func getData(cachePolicy: CachePolicy, completion: @escaping (Result<Data, Error>) -> Void)
}

enum RepositoryError: Error {
    case emptyCacheError
}

class DefaultRepository: Repository {
    private let localDataSource: LocalDataSource
    private let remoteDataSource: RemoteDataSource

    func getData(cachePolicy: CachePolicy, completion: @escaping (Result<Data, Error>) -> Void) {
        switch cachePolicy {
            case .cacheOnly:
                if let data = localDataSource.getData() {
                    completion(.success(data))
                } else {
                    completion(.failure(RepositoryError.emptyCacheError))
                }
            case .cacheThenNetwork:
                if let data = localDataSource.getData() {
                    completion(.success(data))
                }
                remoteDataSource.getData(completion: completion)
            case .networkOnly:
                remoteDataSource.getData(completion: completion)
        }
    }
}

protocol LocalDataSource {
    func getData() -> Data?
}

protocol RemoteDataSource {
    func getData(completion: @escaping (Result<Data, Error>) -> Void)
}