我有一个应用程序必须下载多个大文件.我想让它逐个下载每个文件,而不是并发.当它同时运行时,应用程序会重载并崩溃.
所以.我试图将一个downloadTaskWithURL包装在NSBlockOperation内,然后在队列中设置maxConcurrentOperationCount = 1.我在下面写了这段代码,但是由于两个文件同时被下载,所以没有工作.
import UIKit
class ViewController: UIViewController,NSURLSessionDelegate,NSURLSessionDownloadDelegate {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view,typically from a nib.
processURLs()
}
func download(url: NSURL){
let sessionConfiguration = NSURLSessionConfiguration.defaultSessionConfiguration()
let session = NSURLSession(configuration: sessionConfiguration,delegate: self,delegateQueue: nil)
let downloadTask = session.downloadTaskWithURL(url)
downloadTask.resume()
}
func processURLs(){
//setup queue and set max conncurrent to 1
var queue = NSOperationQueue()
queue.name = "Download queue"
queue.maxConcurrentOperationCount = 1
let url = NSURL(string: "http://azspeastus.blob.core.windows.net/azurespeed/100MB.bin?sv=2014-02-14&sr=b&sig=%2FZNzdvvzwYO%2BQUbrLBQTalz%2F8zByvrUWD%2BDfLmkpZuQ%3D&se=2015-09-01T01%3A48%3A51Z&sp=r")
let url2 = NSURL(string: "http://azspwestus.blob.core.windows.net/azurespeed/100MB.bin?sv=2014-02-14&sr=b&sig=ufnzd4x9h1FKmLsODfnbiszXd4EyMDUJgWhj48QfQ9A%3D&se=2015-09-01T01%3A48%3A51Z&sp=r")
let urls = [url,url2]
for url in urls {
let operation = NSBlockOperation { () -> Void in
println("starting download")
self.download(url!)
}
queue.addOperation(operation)
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// dispose of any resources that can be recreated.
}
func URLSession(session: NSURLSession,downloadTask: NSURLSessionDownloadTask,didFinishDownloadingToURL location: NSURL) {
//code
}
func URLSession(session: NSURLSession,didResumeAtOffset fileOffset: Int64,expectedTotalBytes: Int64) {
//
}
func URLSession(session: NSURLSession,didWriteData bytesWritten: Int64,totalBytesWritten: Int64,totalBytesExpectedToWrite: Int64) {
var progress = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite)
println(progress)
}
}
如何正确地写这个来实现我的目标,一次只能下载一个文件.
解决方法
您的代码将无法正常工作,因为NSURLSessionDownloadTask异步运行.因此,NSBlockOperation在下载完成之前完成,因此在操作顺序触发时,下载任务将以异步方式并行进行.
为了解决这个问题,您可以将请求包装在异步NSOperation子类中.有关更多信息,请参阅并发编程指南中的Configuring Operations for Concurrent Execution.
但是,在我说明如何在您的情况(基于委托的NSURLSession)之前,让我首先向您展示使用完成处理程序转换时更简单的解决方案.我们稍后会为您提出更复杂的问题.所以,在Swift 3：
class DownloadOperation : AsynchronousOperation {
var task: URLSessionTask!
init(session: URLSession,url: URL) {
super.init()
task = session.downloadTask(with: url) { temporaryURL,response,error in
defer { self.completeOperation() }
guard error == nil && temporaryURL != nil else {
print("\(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()
}
}
/// Asynchronous operation base class
///
/// This is abstract to class performs all of the necessary KVN 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 `completeOperation()` is called; or
/// override `cancel` method,calling `super.cancel()` and then cleaning-up
/// and ensuring `completeOperation()` is called.
public class AsynchronousOperation : Operation {
override public var isAsynchronous: Bool { return true }
private let stateLock = NSLock()
private var _executing: Bool = false
override private(set) public var isExecuting: Bool {
get {
return stateLock.withCriticalScope { _executing }
}
set {
willChangeValue(forKey: "isExecuting")
stateLock.withCriticalScope { _executing = newValue }
didChangeValue(forKey: "isExecuting")
}
}
private var _finished: Bool = false
override private(set) public var isFinished: Bool {
get {
return stateLock.withCriticalScope { _finished }
}
set {
willChangeValue(forKey: "isFinished")
stateLock.withCriticalScope { _finished = newValue }
didChangeValue(forKey: "isFinished")
}
}
/// Complete the operation
///
/// This will result in the appropriate KVN of isFinished and isExecuting
public func completeOperation() {
if isExecuting {
isExecuting = false
}
if !isFinished {
isFinished = true
}
}
override public func start() {
if isCancelled {
isFinished = true
return
}
isExecuting = true
main()
}
}
/*
copyright (C) 2015 Apple Inc. All Rights Reserved.
See LICENSE.txt for this sample’s licensing information
Abstract:
An extension to `NSLock` to simplify executing critical code.
From Advanced NSOperations sample code in WWDC 2015 https://developer.apple.com/videos/play/wwdc2015/226/
From https://developer.apple.com/sample-code/wwdc/2015/downloads/Advanced-NSOperations.zip
*/
extension NSLock {
/// Perform closure within lock.
///
/// An extension to `NSLock` to simplify executing critical code.
///
/// - parameter block: The closure to be performed.
func withCriticalScope<T>(block: () -> T) -> T {
lock()
let value = block()
unlock()
return value
}
}
或在Swift 2：
/// Asynchronous NSOperation subclass for downloading
class DownloadOperation : AsynchronousOperation {
var task: NSURLSessionTask!
init(session: NSURLSession,URL: NSURL) {
super.init()
task = session.downloadTaskWithURL(URL) { temporaryURL,error in
defer {
self.completeOperation()
}
print(URL.lastPathComponent)
guard error == nil && temporaryURL != nil else {
print(error)
return
}
do {
let manager = NSFileManager.defaultManager()
let documents = try manager.URLForDirectory(.DocumentDirectory,inDomain: .UserDomainMask,appropriateForURL: nil,create: false)
let destinationURL = documents.URLByAppendingPathComponent(URL.lastPathComponent!)
if manager.fileExistsAtPath(destinationURL.path!) {
try manager.removeItemAtURL(destinationURL)
}
try manager.moveItemAtURL(temporaryURL!,toURL: destinationURL)
} catch let moveError {
print(moveError)
}
}
}
override func cancel() {
task.cancel()
super.cancel()
}
override func main() {
task.resume()
}
}
//
// AsynchronousOperation.swift
//
// Created by Robert Ryan on 9/20/14.
// copyright (c) 2014 Robert Ryan. All rights reserved.
//
import Foundation
/// Asynchronous Operation base class
///
/// This class performs all of the necessary KVN of `isFinished` and
/// `isExecuting` for a concurrent `NSOperation` subclass. So,to developer
/// a concurrent NSOperation subclass,you instead subclass this class which:
///
/// - must override `main()` with the tasks that initiate the asynchronous task;
///
/// - must call `completeOperation()` function when the asynchronous task is done;
///
/// - optionally,calling `super.cancel()` and then cleaning-up
/// and ensuring `completeOperation()` is called.
public class AsynchronousOperation : NSOperation {
override public var asynchronous: Bool { return true }
private let stateLock = NSLock()
private var _executing: Bool = false
override private(set) public var executing: Bool {
get {
return stateLock.withCriticalScope { _executing }
}
set {
willChangeValueForKey("isExecuting")
stateLock.withCriticalScope { _executing = newValue }
didChangeValueForKey("isExecuting")
}
}
private var _finished: Bool = false
override private(set) public var finished: Bool {
get {
return stateLock.withCriticalScope { _finished }
}
set {
willChangeValueForKey("isFinished")
stateLock.withCriticalScope { _finished = newValue }
didChangeValueForKey("isFinished")
}
}
/// Complete the operation
///
/// This will result in the appropriate KVN of isFinished and isExecuting
public func completeOperation() {
if executing {
executing = false
}
if !finished {
finished = true
}
}
override public func start() {
if cancelled {
finished = true
return
}
executing = true
main()
}
override public func main() {
fatalError("subclasses must override `main`")
}
}
/*
copyright (C) 2015 Apple Inc. All Rights Reserved.
See LICENSE.txt for this sample’s licensing information
Abstract:
An extension to `NSLock` to simplify executing critical code.
From Advanced NSOperations sample code in WWDC 2015 https://developer.apple.com/videos/play/wwdc2015/226/
From https://developer.apple.com/sample-code/wwdc/2015/downloads/Advanced-NSOperations.zip
*/
import Foundation
extension NSLock {
/// Perform closure within lock.
///
/// An extension to `NSLock` to simplify executing critical code.
///
/// - parameter block: The closure to be performed.
func withCriticalScope<T>(@noescape block: Void -> T) -> T {
lock()
let value = block()
unlock()
return value
}
}
那你可以做：
for url in urls {
queue.addOperation(DownloadOperation(session: session,url: url))
}
所以这是在异步操作/ NSOperation子类中包装异步URLSession / NSURLSession请求的一种非常简单的方法.更一般地说,这是一个有用的模式,使用AsynchronousOperation来在Operation / NSOperation对象中包装一些异步任务.
不幸的是,在您的问题中,您希望使用基于委托的URLSession / NSURLSession,以便您可以监控下载的进度.这更复杂
这是因为在会话对象的委托中调用了“任务完成”NSURLSession委托方法.这是NSURLSession的一个令人激动的设计功能(但是苹果做了简化后台会话,这在这里是不相关的,但我们坚持设计限制).
但是,随着任务的完成,我们必须异步地完成操作.所以我们需要一些方法来解决当didCompleteWithError被调用时用操作来完成.现在你可以让每个操作都有自己的NSURLSession对象,但事实证明这是非常低效的.
所以,为了处理这一点,我维护一个字典,由任务的taskIdentifier键入,它标识适当的操作.这样,当下载完成后,您可以“完成”正确的异步操作.所以,在Swift 3：
/// 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,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,totalBytesExpectedToWrite: Int64) {
operations[downloadTask.taskIdentifier]?.urlSession(session,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,didFinishDownloadingTo location: URL) {
do {
let manager = FileManager.default
let destinationURL = try manager.url(for: .documentDirectory,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,totalBytesExpectedToWrite: Int64) {
let progress = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite)
print("\(downloadTask.originalRequest!.url!.absoluteString) \(progress)")
}
}
// MARK: NSURLSessionTaskDelegate methods
extension DownloadOperation: URLSessionTaskDelegate {
func urlSession(_ session: URLSession,didCompleteWithError error: Error?) {
completeOperation()
if error != nil {
print("\(error)")
}
}
}
或在Swift 2：
/// Manager of asynchronous NSOperation objects
class DownloadManager: NSObject,NSURLSessionTaskDelegate,NSURLSessionDownloadDelegate {
/// Dictionary of operations,keyed by the `taskIdentifier` of the `NSURLSessionTask`
private var operations = [Int: DownloadOperation]()
/// Serial NSOperationQueue for downloads
let queue: NSOperationQueue = {
let _queue = NSOperationQueue()
_queue.name = "download"
_queue.maxConcurrentOperationCount = 1
return _queue
}()
/// Delegate-based NSURLSession for DownloadManager
lazy var session: NSURLSession = {
let sessionConfiguration = NSURLSessionConfiguration.defaultSessionConfiguration()
return NSURLSession(configuration: sessionConfiguration,delegateQueue: nil)
}()
/// Add download
///
/// - parameter URL: The URL of the file to be downloaded
///
/// - returns: The DownloadOperation of the operation that was queued
func addDownload(url: NSURL) -> 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: NSURLSessionDownloadDelegate methods
func URLSession(session: NSURLSession,didFinishDownloadingToURL location: NSURL) {
operations[downloadTask.taskIdentifier]?.URLSession(session,didFinishDownloadingToURL: location)
}
func URLSession(session: NSURLSession,totalBytesExpectedToWrite: Int64) {
operations[downloadTask.taskIdentifier]?.URLSession(session,totalBytesExpectedToWrite: totalBytesExpectedToWrite)
}
// MARK: NSURLSessionTaskDelegate methods
func URLSession(session: NSURLSession,task: NSURLSessionTask,didCompleteWithError error: NSError?) {
let key = task.taskIdentifier
operations[key]?.URLSession(session,didCompleteWithError: error)
operations.removeValueForKey(key)
}
}
/// Asynchronous NSOperation subclass for downloading
class DownloadOperation : AsynchronousOperation {
let task: NSURLSessionTask
init(session: NSURLSession,URL: NSURL) {
task = session.downloadTaskWithURL(URL)
super.init()
}
override func cancel() {
task.cancel()
super.cancel()
}
override func main() {
task.resume()
}
// MARK: NSURLSessionDownloadDelegate methods
func URLSession(session: NSURLSession,didFinishDownloadingToURL location: NSURL) {
do {
let manager = NSFileManager.defaultManager()
let documents = try manager.URLForDirectory(.DocumentDirectory,create: false)
let destinationURL = documents.URLByAppendingPathComponent(downloadTask.originalRequest!.URL!.lastPathComponent!)
if manager.fileExistsAtPath(destinationURL.path!) {
try manager.removeItemAtURL(destinationURL)
}
try manager.moveItemAtURL(location,toURL: destinationURL)
} catch {
print(error)
}
}
func URLSession(session: NSURLSession,totalBytesExpectedToWrite: Int64) {
let progress = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite)
print("\(downloadTask.originalRequest!.URL!.absoluteString) \(progress)")
}
// MARK: NSURLSessionTaskDelegate methods
func URLSession(session: NSURLSession,didCompleteWithError error: NSError?) {
completeOperation()
if error != nil {
print(error)
}
}
}
然后我就这样使用它：
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.flatMap { URL(string: $0) } // use NSURL in Swift 2
for url in urls {
downloadManager.addDownload(url)
}
}