How To Download Multiple Files Sequentially using NSURLSession downloadTask in Swift
Your code won't work because URLSessionDownloadTask runs asynchronously. Thus the BlockOperation completes before the download is done and therefore while the operations fire off sequentially, the download tasks will continue asynchronously and in parallel.
While there are work-arounds one can contemplate (e.g., recursive patterns initiating one request after the prior one finishes, non-zero semaphore pattern on background thread, etc.), the elegant solution is one of the proven asynchronous frameworks. Historically if you wanted to control the degree of concurrency of a series of asynchronous tasks, we would reach for an asynchronous Operation subclass. Nowadays, in iOS 13 and later, we might consider Combine. (There are other third-party asynchronous programming frameworks, but I will restrict myself to Apple provided approaches.)
Operation
To address this, you can wrap the requests in asynchronous Operation subclass. See Configuring Operations for Concurrent Execution in the Concurrency Programming Guide for more information.
But before I illustrate how to do this in your situation (the delegate-based URLSession), let me first show you the simpler solution when using the completion handler rendition. We'll later build upon this for your more complicated question. So, in Swift 3 and later:
class DownloadOperation : AsynchronousOperation { var task: URLSessionTask! init(session: URLSession, url: URL) { super.init() task = session.downloadTask(with: url) { temporaryURL, response, error in defer { self.finish() } guard let httpResponse = response as? HTTPURLResponse, 200..<300 ~= httpResponse.statusCode else { // handle invalid return codes however you'd like return } guard let temporaryURL = temporaryURL, error == nil else { print(error ?? "Unknown error") return } do { let manager = FileManager.default let destinationURL = try manager.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false) .appendingPathComponent(url.lastPathComponent) try? manager.removeItem(at: destinationURL) // remove the old one, if any try manager.moveItem(at: temporaryURL, to: destinationURL) // move new one there } catch let moveError { print("\(moveError)") } } } override func cancel() { task.cancel() super.cancel() } override func main() { task.resume() } }Where
/// Asynchronous operation base class////// This is abstract to class emits all of the necessary KVO notifications of `isFinished`/// and `isExecuting` for a concurrent `Operation` subclass. You can subclass this and/// implement asynchronous operations. All you must do is:////// - override `main()` with the tasks that initiate the asynchronous task;////// - call `completeOperation()` function when the asynchronous task is done;////// - optionally, periodically check `self.cancelled` status, performing any clean-up/// necessary and then ensuring that `finish()` is called; or/// override `cancel` method, calling `super.cancel()` and then cleaning-up/// and ensuring `finish()` is called.class AsynchronousOperation: Operation { /// State for this operation. @objc private enum OperationState: Int { case ready case executing case finished } /// Concurrent queue for synchronizing access to `state`. private let stateQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".rw.state", attributes: .concurrent) /// Private backing stored property for `state`. private var rawState: OperationState = .ready /// The state of the operation @objc private dynamic var state: OperationState { get { return stateQueue.sync { rawState } } set { stateQueue.sync(flags: .barrier) { rawState = newValue } } } // MARK: - Various `Operation` properties open override var isReady: Bool { return state == .ready && super.isReady } public final override var isExecuting: Bool { return state == .executing } public final override var isFinished: Bool { return state == .finished } // KVO for dependent properties open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> { if ["isReady", "isFinished", "isExecuting"].contains(key) { return [#keyPath(state)] } return super.keyPathsForValuesAffectingValue(forKey: key) } // Start public final override func start() { if isCancelled { finish() return } state = .executing main() } /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception. open override func main() { fatalError("Subclasses must implement `main`.") } /// Call this function to finish an operation that is currently executing public final func finish() { if !isFinished { state = .finished } }}Then you can do:
for url in urls { queue.addOperation(DownloadOperation(session: session, url: url))}So that's one very easy way to wrap asynchronous URLSession/NSURLSession requests in asynchronous Operation/NSOperation subclass. More generally, this is a useful pattern, using AsynchronousOperation to wrap up some asynchronous task in an Operation/NSOperation object.
Unfortunately, in your question, you wanted to use delegate-based URLSession/NSURLSession so you could monitor the progress of the downloads. This is more complicated.
This is because the "task complete" NSURLSession delegate methods are called at the session object's delegate. This is an infuriating design feature of NSURLSession (but Apple did it to simplify background sessions, which isn't relevant here, but we're stuck with that design limitation).
But we have to asynchronously complete the operations as the tasks finish. So we need some way for the session to figure out which operation to complete when didCompleteWithError is called. Now you could have each operation have its own NSURLSession object, but it turns out that this is pretty inefficient.
So, to handle that, I maintain a dictionary, keyed by the task's taskIdentifier, which identifies the appropriate operation. That way, when the download finishes, you can "complete" the correct asynchronous operation. Thus:
/// Manager of asynchronous download `Operation` objectsclass DownloadManager: NSObject { /// Dictionary of operations, keyed by the `taskIdentifier` of the `URLSessionTask` fileprivate var operations = [Int: DownloadOperation]() /// Serial OperationQueue for downloads private let queue: OperationQueue = { let _queue = OperationQueue() _queue.name = "download" _queue.maxConcurrentOperationCount = 1 // I'd usually use values like 3 or 4 for performance reasons, but OP asked about downloading one at a time return _queue }() /// Delegate-based `URLSession` for DownloadManager lazy var session: URLSession = { let configuration = URLSessionConfiguration.default return URLSession(configuration: configuration, delegate: self, delegateQueue: nil) }() /// Add download /// /// - parameter URL: The URL of the file to be downloaded /// /// - returns: The DownloadOperation of the operation that was queued @discardableResult func queueDownload(_ url: URL) -> DownloadOperation { let operation = DownloadOperation(session: session, url: url) operations[operation.task.taskIdentifier] = operation queue.addOperation(operation) return operation } /// Cancel all queued operations func cancelAll() { queue.cancelAllOperations() } }// MARK: URLSessionDownloadDelegate methodsextension DownloadManager: URLSessionDownloadDelegate { func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) { operations[downloadTask.taskIdentifier]?.urlSession(session, downloadTask: downloadTask, didFinishDownloadingTo: location) } func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) { operations[downloadTask.taskIdentifier]?.urlSession(session, downloadTask: downloadTask, didWriteData: bytesWritten, totalBytesWritten: totalBytesWritten, totalBytesExpectedToWrite: totalBytesExpectedToWrite) }}// MARK: URLSessionTaskDelegate methodsextension DownloadManager: URLSessionTaskDelegate { func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { let key = task.taskIdentifier operations[key]?.urlSession(session, task: task, didCompleteWithError: error) operations.removeValue(forKey: key) } }/// Asynchronous Operation subclass for downloadingclass DownloadOperation : AsynchronousOperation { let task: URLSessionTask init(session: URLSession, url: URL) { task = session.downloadTask(with: url) super.init() } override func cancel() { task.cancel() super.cancel() } override func main() { task.resume() }}// MARK: NSURLSessionDownloadDelegate methodsextension DownloadOperation: URLSessionDownloadDelegate { func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) { guard let httpResponse = downloadTask.response as? HTTPURLResponse, 200..<300 ~= httpResponse.statusCode else { // handle invalid return codes however you'd like return } do { let manager = FileManager.default let destinationURL = try manager .url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: true) .appendingPathComponent(downloadTask.originalRequest!.url!.lastPathComponent) try? manager.removeItem(at: destinationURL) try manager.moveItem(at: location, to: destinationURL) } catch { print(error) } } func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) { let progress = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite) print("\(downloadTask.originalRequest!.url!.absoluteString) \(progress)") }}// MARK: URLSessionTaskDelegate methodsextension DownloadOperation: URLSessionTaskDelegate { func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { defer { finish() } if let error = error { print(error) return } // do whatever you want upon success } }And then use it like so:
let downloadManager = DownloadManager()override func viewDidLoad() { super.viewDidLoad() let urlStrings = [ "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/s72-55482.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo10/hires/as10-34-5162.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo-soyuz/apollo-soyuz/hires/s75-33375.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-134-20380.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-140-21497.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-148-22727.jpg" ] let urls = urlStrings.compactMap { URL(string: $0) } let completion = BlockOperation { print("all done") } for url in urls { let operation = downloadManager.queueDownload(url) completion.addDependency(operation) } OperationQueue.main.addOperation(completion)}See revision history for Swift 2 implementation.
Combine
For Combine, the idea would be to create a Publisher for URLSessionDownloadTask. Then you can do something like:
var downloadRequests: AnyCancellable?/// Download a series of assetsfunc downloadAssets() { downloadRequests = downloadsPublisher(for: urls, maxConcurrent: 1).sink { completion in switch completion { case .finished: print("done") case .failure(let error): print("failed", error) } } receiveValue: { destinationUrl in print(destinationUrl) }}/// Publisher for single download////// Copy downloaded resource to caches folder.////// - Parameter url: `URL` being downloaded./// - Returns: Publisher for the URL with final destination of the downloaded asset.func downloadPublisher(for url: URL) -> AnyPublisher<URL, Error> { URLSession.shared.downloadTaskPublisher(for: url) .tryCompactMap { let destination = try FileManager.default .url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: true) .appendingPathComponent(url.lastPathComponent) try FileManager.default.moveItem(at: $0.location, to: destination) return destination } .receive(on: RunLoop.main) .eraseToAnyPublisher()}/// Publisher for a series of downloads////// This downloads not more than `maxConcurrent` assets at a given time.////// - Parameters:/// - urls: Array of `URL`s of assets to be downloaded./// - maxConcurrent: The maximum number of downloads to run at any given time (default 4)./// - Returns: Publisher for the URLs with final destination of the downloaded assets.func downloadsPublisher(for urls: [URL], maxConcurrent: Int = 4) -> AnyPublisher<URL, Error> { Publishers.Sequence(sequence: urls.map { downloadPublisher(for: $0) }) .flatMap(maxPublishers: .max(maxConcurrent)) { $0 } .eraseToAnyPublisher()}Now, unfortunately, Apple supplies a DataTaskPublisher (which loads the full asset into memory which is not acceptable solution for large assets), but one can refer to their source code and adapt it to create a DownloadTaskPublisher:
// DownloadTaskPublisher.swift//// Created by Robert Ryan on 9/28/20.//// Adapted from Apple's `DataTaskPublisher` at:// https://github.com/apple/swift/blob/88b093e9d77d6201935a2c2fb13f27d961836777/stdlib/public/Darwin/Foundation/Publishers%2BURLSession.swiftimport Foundationimport Combine// MARK: Download Tasks@available(macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)extension URLSession { /// Returns a publisher that wraps a URL session download task for a given URL. /// /// The publisher publishes temporary when the task completes, or terminates if the task fails with an error. /// /// - Parameter url: The URL for which to create a download task. /// - Returns: A publisher that wraps a download task for the URL. public func downloadTaskPublisher(for url: URL) -> DownloadTaskPublisher { let request = URLRequest(url: url) return DownloadTaskPublisher(request: request, session: self) } /// Returns a publisher that wraps a URL session download task for a given URL request. /// /// The publisher publishes download when the task completes, or terminates if the task fails with an error. /// /// - Parameter request: The URL request for which to create a download task. /// - Returns: A publisher that wraps a download task for the URL request. public func downloadTaskPublisher(for request: URLRequest) -> DownloadTaskPublisher { return DownloadTaskPublisher(request: request, session: self) } public struct DownloadTaskPublisher: Publisher { public typealias Output = (location: URL, response: URLResponse) public typealias Failure = URLError public let request: URLRequest public let session: URLSession public init(request: URLRequest, session: URLSession) { self.request = request self.session = session } public func receive<S: Subscriber>(subscriber: S) where Failure == S.Failure, Output == S.Input { subscriber.receive(subscription: Inner(self, subscriber)) } private typealias Parent = DownloadTaskPublisher private final class Inner<Downstream: Subscriber>: Subscription, CustomStringConvertible, CustomReflectable, CustomPlaygroundDisplayConvertible where Downstream.Input == Parent.Output, Downstream.Failure == Parent.Failure { typealias Input = Downstream.Input typealias Failure = Downstream.Failure private let lock: NSLocking private var parent: Parent? // GuardedBy(lock) private var downstream: Downstream? // GuardedBy(lock) private var demand: Subscribers.Demand // GuardedBy(lock) private var task: URLSessionDownloadTask! // GuardedBy(lock) var description: String { return "DownloadTaskPublisher" } var customMirror: Mirror { lock.lock() defer { lock.unlock() } return Mirror(self, children: [ "task": task as Any, "downstream": downstream as Any, "parent": parent as Any, "demand": demand, ]) } var playgroundDescription: Any { return description } init(_ parent: Parent, _ downstream: Downstream) { self.lock = NSLock() self.parent = parent self.downstream = downstream self.demand = .max(0) } // MARK: - Upward Signals func request(_ d: Subscribers.Demand) { precondition(d > 0, "Invalid request of zero demand") lock.lock() guard let p = parent else { // We've already been cancelled so bail lock.unlock() return } // Avoid issues around `self` before init by setting up only once here if self.task == nil { let task = p.session.downloadTask( with: p.request, completionHandler: handleResponse(location:response:error:) ) self.task = task } self.demand += d let task = self.task! lock.unlock() task.resume() } private func handleResponse(location: URL?, response: URLResponse?, error: Error?) { lock.lock() guard demand > 0, parent != nil, let ds = downstream else { lock.unlock() return } parent = nil downstream = nil // We clear demand since this is a single shot shape demand = .max(0) task = nil lock.unlock() if let location = location, let response = response, error == nil { _ = ds.receive((location, response)) ds.receive(completion: .finished) } else { let urlError = error as? URLError ?? URLError(.unknown) ds.receive(completion: .failure(urlError)) } } func cancel() { lock.lock() guard parent != nil else { lock.unlock() return } parent = nil downstream = nil demand = .max(0) let task = self.task self.task = nil lock.unlock() task?.cancel() } } }}Now, unfortunately, that isn’t using URLSession delegate pattern, but rather the completion handler rendition. But one could conceivably adapt it for delegate pattern.
Also, this will stop downloads when one fails. If you don't want it to stop just because one fails, you could conceivably define it to Never fail, and instead replaceError with nil:
/// Publisher for single download////// Copy downloaded resource to caches folder.////// - Parameter url: `URL` being downloaded./// - Returns: Publisher for the URL with final destination of the downloaded asset. Returns `nil` if request failed.func downloadPublisher(for url: URL) -> AnyPublisher<URL?, Never> { URLSession.shared.downloadTaskPublisher(for: url) .tryCompactMap { let destination = try FileManager.default .url(for: .cachesDirectory, in: .userDomainMask, appropriateFor: nil, create: true) .appendingPathComponent(url.lastPathComponent) try FileManager.default.moveItem(at: $0.location, to: destination) return destination } .replaceError(with: nil) .receive(on: RunLoop.main) .eraseToAnyPublisher()}/// Publisher for a series of downloads////// This downloads not more than `maxConcurrent` assets at a given time.////// - Parameters:/// - urls: Array of `URL`s of assets to be downloaded./// - maxConcurrent: The maximum number of downloads to run at any given time (default 4)./// - Returns: Publisher for the URLs with final destination of the downloaded assets.func downloadsPublisher(for urls: [URL], maxConcurrent: Int = 4) -> AnyPublisher<URL?, Never> { Publishers.Sequence(sequence: urls.map { downloadPublisher(for: $0) }) .flatMap(maxPublishers: .max(maxConcurrent)) { $0 } .eraseToAnyPublisher()}
Here is rather minimalistic and pure-swift approach. With no NSOperationQueue(), just didSet-observer
import Foundation class DownloadManager { var delegate: HavingWebView? var gotFirstAndEnough = true var finalURL: NSURL?{ didSet{ if finalURL != nil { if let s = self.contentOfURL{ self.delegate?.webView.loadHTMLString(s, baseURL: nil) } } } } var lastRequestBeginning: NSDate? var myLinks = [String](){ didSet{ self.handledLink = self.myLinks.count } } var contentOfURL: String? var handledLink = 0 { didSet{ if handledLink == 0 { self.finalURL = nil print("🔴🔶🔴🔶🔶🔴🔶🔴🔶🔴🔶🔴") } else { if self.finalURL == nil { if let nextURL = NSURL(string: self.myLinks[self.handledLink-1]) { self.loadAsync(nextURL) } } } } } func loadAsync(url: NSURL) { let sessionConfig = NSURLSessionConfiguration.ephemeralSessionConfiguration() let session = NSURLSession(configuration: sessionConfig, delegate: nil, delegateQueue: NSOperationQueue.mainQueue()) let request = NSMutableURLRequest(URL: url, cachePolicy: NSURLRequestCachePolicy.ReloadIgnoringCacheData, timeoutInterval: 15.0) request.HTTPMethod = "GET" print("🚀") self.lastRequestBeginning = NSDate() print("Requet began: \(self.lastRequestBeginning )") let task = session.dataTaskWithRequest(request, completionHandler: { (data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in if (error == nil) { if let response = response as? NSHTTPURLResponse { print("\(response)") if response.statusCode == 200 { if let content = String(data: data!, encoding: NSUTF8StringEncoding) { self.contentOfURL = content } self.finalURL = url } } } else { print("Failure: \(error!.localizedDescription)"); } let elapsed = NSDate().timeIntervalSinceDate(self.lastRequestBeginning!) print("trying \(url) takes \(elapsed)") print("🏁 Request finished") print("____________________________________________") self.handledLink -= 1 }) task.resume() } }In ViewController:
protocol HavingWebView { var webView: UIWebView! {get set}}class ViewController: UIViewController, HavingWebView { @IBOutlet weak var webView: UIWebView! override func viewDidLoad() { super.viewDidLoad() // Do any additional setup after loading the view, typically from a nib. let dm = DownloadManager() dm.delegate = self dm.myLinks = ["https://medium.com/the-mission/consider-the-present-and-future-value-of-your-decisions-b20fb72f5e#.a12uiiz11", "https://medium.com/@prianka.kariat/ios-10-notifications-with-attachments-and-much-more-169a7405ddaf#.svymi6230", "https://blog.medium.com/39-reasons-we-wont-soon-forget-2016-154ac95683af#.cmb37i58b", "https://backchannel.com/in-2017-your-coworkers-will-live-everywhere-ae14979b5255#.wmi6hxk9p"] }}
More than one code in the background situation. I can learn by used global variable and NSTimer. You can try too.
Define 'indexDownloaded' global variable.
import UIKitimport Foundationprivate let _sharedUpdateStatus = UpdateStatus()class UpdateStatus : NSObject {// MARK: - SHARED INSTANCEclass var shared : UpdateStatus { return _sharedUpdateStatus} var indexDownloaded = 0}This code add in the DownloadOperation class.
print("⬇️" + URL.lastPathComponent! + " downloaded") UpdateStatus.shared.indexDownloaded += 1 print(String(UpdateStatus.shared.indexDownloaded) + "\\" + String(UpdateStatus.shared.count))This function in your viewController.
func startTimeAction () { let urlStrings = [ "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/s72-55482.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo10/hires/as10-34-5162.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo-soyuz/apollo-soyuz/hires/s75-33375.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-134-20380.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-140-21497.jpg", "http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-148-22727.jpg" ] let urls = urlStrings.flatMap { URL(string: $0) } for url in urls { queue.addOperation(DownloadOperation(session: session, url: url)) } UpdateStatus.shared.count = urls.count progressView.setProgress(0.0, animated: false) timer.invalidate() timer = NSTimer.scheduledTimerWithTimeInterval(0.2, target: self, selector: #selector(timeAction), userInfo: nil, repeats: true)}func timeAction() { if UpdateStatus.shared.count != 0 { let set: Float = Float(UpdateStatus.shared.indexDownloaded) / Float(UpdateStatus.shared.count) progressView.setProgress(set, animated: true) }in this way, by updating the progressview will look at the number of downloaded each time the timer runs.