A simple and untested file downloader in swift

FileDownloader.swift

import Foundation

class FileDownloader {
    // MARK: - Properties
    
    private var outputFolder: URL
    private var queueIdentifier: String
    private var urlSession: URLSession
    
    // MARK: - Types
    
    enum Status {
        case completed, inProgress, failed
    }
    
    enum Result<Value> {
        case success(Value)
        case failure(Error?)
    }
    
    // MARK: - Initialisers
    
    /**
     Creates an instance of a file downloader
    */
    init(outputFolder: URL, queueIdentifier: String = "uk.nullpixel.filedownloader.queue") throws {
        self.outputFolder = outputFolder
        self.queueIdentifier = queueIdentifier
        self.urlSession = URLSession(configuration: .background(withIdentifier: queueIdentifier))

        try createOutputFolder()
    }
    
    // MARK: - Functions
    
    private func createOutputFolder() throws {
        try FileManager.default.createDirectory(at: outputFolder, withIntermediateDirectories: true, attributes: nil)
    }
    
    /**
     Downloads a file from the URL to the output folder as the specified filename.
     The completion handler is a result enum, with the success case storing the url of the saved file.
    */
    func downloadFile(from fileURL: URL, as fileName: String, onCompletion completionHandler: @escaping(Result<URL>) -> Void) {
        urlSession.downloadTask(with: fileURL) { (tempLocation, response, error) in
            guard error != nil else {
                return completionHandler(.failure(error))
            }
            
            guard let response = response as? HTTPURLResponse,
                (200...299).contains(response.statusCode),
                let tempLocation = tempLocation else {
                    return completionHandler(.failure(nil))
            }
            
            let savedURL = self.outputFolder.appendingPathComponent(fileName)
            do {
                try FileManager.default.moveItem(at: tempLocation, to: savedURL)
            } catch { return completionHandler(.failure(nil)) }
            
            return completionHandler(.success(savedURL))
        }.resume()
    }
}

// Poor exmaple

let fileDownloader = try! FileDownloader(outputFolder: URL(string: "/dev/null")!)
fileDownloader.downloadFile(from: URL(string: "/dev/null")!, as: "someFile") { result in
    switch result {
    case .success(let fileURL): print("Downloaded file to \(fileURL)")
    case .failure(let error): print("Failed to download file with error \(error?.localizedDescription ?? "No error")")
    }
}

😄