how_unit_testing_can_help_write_better_designs_1.swift

final class MovieManager {

    /// Fetches popular movies from the server, then stores them.
    func getPopularMovies(_ completion: @escaping (Result<[Movie], MovieManagerError>) -> Void) {

        HTTPClient.shared.get(path: "/movies/popular") { httpResponse, data, error in
            // handles error if any

            do {
                // parses data from the server
                let movies = try JSONDecoder().decode([Movie].self, from: data)

                // saves movies in core data
                let moc = // instanciate a managed object context...
                for movie in movies {
                    ManagedMovie(movie).createOrUpdateIfExists(in: moc)
                }

                do {
                    try moc.save()
                } catch {
                    completion(.failure(.coreData(error)))
                    return
                }

                completion(.success(movies))
            } catch {
                completion(.failure(.parsing(error)))
            }
        }
    }

    /// Fetches a movie from the store if it exists, from the server otherwise.
    func getMovie(id movieID: String, completion: @escaping (Result<Movie, MovieManagerError>) -> Void) {

        let moc = // instanciates a managed object context...
        let storedMovieFetch = NSFetchRequest(entityName: "Movie")
        storedMovieFetch.predicate = NSPredicate(format: "id == %@", movieID)

        do {
            // get potential stored movie
            guard let storedMovies = try moc.executeFetchRequest(storedMovieFetch) as? [Movie] else {
                completion(.failure(.coreData(.inconsistentTable(entityName: "Movie"))))
                return
            }

            if let storedMovie = storedMovies.first {
                completion(.success(Movie(storedMovie)))
            }
        } catch {
            completion(.failure(.coreData(error)))
            return
        }

        HTTPClient.shared.get(path: "/movies/\(movieID)") { httpResponse, data, error in
            // handles error if any

            do {
                // parses data from the server
                let movie = try JSONDecoder().decode(Movie.self, from: data)

                // saves movie in core data
                ManagedMovie(movie).createOrUpdateIfExists(in: moc)
                
                do {
                    try moc.save()
                } catch {
                    completion(.failure(.coreData(error)))
                    return
                }

                completion(.success(movie))
            } catch {
                completion(.failure(.parsing(error)))
            }
        }
    }
}