asynchronous.swift

/// Manager of asynchronous download `Operation` objects

class DownloadManager: NSObject {

    /// Dictionary of operations, keyed by the `taskIdentifier` of the `URLSessionTask`

    fileprivate var operations = [Int: DownloadOperation]()

    /// Serial NSOperationQueue 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 NSURLSession 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 addDownload(_ 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 methods

extension 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 methods

extension 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 downloading

class 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 methods

extension DownloadOperation: URLSessionDownloadDelegate {

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        do {
            let manager = FileManager.default
            let destinationURL = try manager.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
                .appendingPathComponent(downloadTask.originalRequest!.url!.lastPathComponent)
            if manager.fileExists(atPath: destinationURL.path) {
                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: NSURLSessionTaskDelegate methods

extension DownloadOperation: URLSessionTaskDelegate {

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)  {
        completeOperation()
        if error != nil {
            print("\(error)")
        }
    }

}