运行 在后台时的 URLSession downloadTask 行为?
URLSession downloadTask behavior when running in the background?
我有一个应用程序需要下载一个可能相当大的文件(可能大到 20 MB)。我一直在阅读 URLSession downloadTasks 以及当应用程序进入后台或被 iOS 终止时它们如何工作。我希望继续下载,根据我的阅读,这是可能的。我找到了一个博客 post here,它详细讨论了这个主题。
根据我所读到的内容,我首先创建了一个下载管理器 class,如下所示:
class DownloadManager : NSObject, URLSessionDownloadDelegate, URLSessionTaskDelegate {
static var shared = DownloadManager()
var backgroundSessionCompletionHandler: (() -> Void)?
var session : URLSession {
get {
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
}
}
private override init() {
}
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
DispatchQueue.main.async {
if let completionHandler = self.backgroundSessionCompletionHandler {
self.backgroundSessionCompletionHandler = nil
completionHandler()
}
}
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
if let sessionId = session.configuration.identifier {
log.info("Download task finished for session ID: \(sessionId), task ID: \(downloadTask.taskIdentifier); file was downloaded to \(location)")
do {
// just for testing purposes
try FileManager.default.removeItem(at: location)
print("Deleted downloaded file from \(location)")
} catch {
print(error)
}
}
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
if totalBytesExpectedToWrite > 0 {
let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
let progressPercentage = progress * 100
print("Download with task identifier: \(downloadTask.taskIdentifier) is \(progressPercentage)% complete...")
}
}
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
if let error = error {
print("Task failed with error: \(error)")
} else {
print("Task completed successfully.")
}
}
}
我也在我的 AppDelegate 中添加了这个方法:
func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
DownloadManager.shared.backgroundSessionCompletionHandler = completionHandler
// if the app gets terminated, I need to reconstruct the URLSessionConfiguration and the URLSession in order to "re-connect" to the previous URLSession instance and process the completed download tasks
// for now, I'm just putting the app in the background (not terminating it) so I've commented out the lines below
//let config = URLSessionConfiguration.background(withIdentifier: identifier)
//let session = URLSession(configuration: config, delegate: DownloadManager.shared, delegateQueue: OperationQueue.main)
// since my app hasn't been terminated, my existing URLSession should still be around and doesn't need to be re-created
let session = DownloadManager.shared.session
session.getTasksWithCompletionHandler { (dataTasks, uploadTasks, downloadTasks) -> Void in
// downloadTasks = [URLSessionDownloadTask]
print("There are \(downloadTasks.count) download tasks associated with this session.")
for downloadTask in downloadTasks {
print("downloadTask.taskIdentifier = \(downloadTask.taskIdentifier)")
}
}
}
最后,我开始这样的测试下载:
let session = DownloadManager.shared.session
// this is a 100MB PDF file that I'm using for testing
let testUrl = URL(string: "https://scholar.princeton.edu/sites/default/files/oversize_pdf_test_0.pdf")!
let task = session.downloadTask(with: testUrl)
// I think I'll ultimately need to persist the session ID, task ID and a file path for use in the delegate methods once the download has completed
task.resume()
当我 运行 此代码并开始下载时,我看到正在调用委托方法,但我也看到一条消息:
A background URLSession with identifier com.example.testapp.background already exists!
我 认为 这是因为 application:handleEventsForBackgroundURLSession:completionHandler:
中的以下调用而发生的
let session = DownloadManager.shared.session
我的 DownloadManager class 中会话 属性 的 getter(我直接从之前引用的博客 post 中获取)总是试图创建一个新的URLSession 使用后台配置。据我了解,如果我的应用程序已终止,那么这将是 "reconnect" 到原始 URLSession 的适当行为。但是由于可能应用程序没有被终止而只是进入后台,当调用 application:handleEventsForBackgroundURLSession:completionHandler: 时,我应该引用 URLSession 的现有实例。至少我 认为 这就是问题所在。谁能为我澄清这种行为?谢谢!
您的问题是每次引用会话变量时都在创建一个新会话:
var session : URLSession {
get {
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
}
}
相反,将会话保留为实例变量,然后直接获取它:
class DownloadManager:NSObject {
static var shared = DownloadManager()
var delegate = DownloadManagerSessionDelegate()
var session:URLSession
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
override init() {
session = URLSession(configuration: config, delegate: delegate, delegateQueue: OperationQueue())
super.init()
}
}
class DownloadManagerSessionDelegate: NSObject, URLSessionDelegate {
// implement here
}
当我在操场上这样做时,它显示重复调用给出相同的会话,并且没有错误:
会话不在进程中,它是 OS 的一部分。每次访问写入的会话变量时,您都会增加引用计数,这会导致错误。
我有一个应用程序需要下载一个可能相当大的文件(可能大到 20 MB)。我一直在阅读 URLSession downloadTasks 以及当应用程序进入后台或被 iOS 终止时它们如何工作。我希望继续下载,根据我的阅读,这是可能的。我找到了一个博客 post here,它详细讨论了这个主题。
根据我所读到的内容,我首先创建了一个下载管理器 class,如下所示:
class DownloadManager : NSObject, URLSessionDownloadDelegate, URLSessionTaskDelegate {
static var shared = DownloadManager()
var backgroundSessionCompletionHandler: (() -> Void)?
var session : URLSession {
get {
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
}
}
private override init() {
}
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
DispatchQueue.main.async {
if let completionHandler = self.backgroundSessionCompletionHandler {
self.backgroundSessionCompletionHandler = nil
completionHandler()
}
}
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
if let sessionId = session.configuration.identifier {
log.info("Download task finished for session ID: \(sessionId), task ID: \(downloadTask.taskIdentifier); file was downloaded to \(location)")
do {
// just for testing purposes
try FileManager.default.removeItem(at: location)
print("Deleted downloaded file from \(location)")
} catch {
print(error)
}
}
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
if totalBytesExpectedToWrite > 0 {
let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
let progressPercentage = progress * 100
print("Download with task identifier: \(downloadTask.taskIdentifier) is \(progressPercentage)% complete...")
}
}
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
if let error = error {
print("Task failed with error: \(error)")
} else {
print("Task completed successfully.")
}
}
}
我也在我的 AppDelegate 中添加了这个方法:
func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
DownloadManager.shared.backgroundSessionCompletionHandler = completionHandler
// if the app gets terminated, I need to reconstruct the URLSessionConfiguration and the URLSession in order to "re-connect" to the previous URLSession instance and process the completed download tasks
// for now, I'm just putting the app in the background (not terminating it) so I've commented out the lines below
//let config = URLSessionConfiguration.background(withIdentifier: identifier)
//let session = URLSession(configuration: config, delegate: DownloadManager.shared, delegateQueue: OperationQueue.main)
// since my app hasn't been terminated, my existing URLSession should still be around and doesn't need to be re-created
let session = DownloadManager.shared.session
session.getTasksWithCompletionHandler { (dataTasks, uploadTasks, downloadTasks) -> Void in
// downloadTasks = [URLSessionDownloadTask]
print("There are \(downloadTasks.count) download tasks associated with this session.")
for downloadTask in downloadTasks {
print("downloadTask.taskIdentifier = \(downloadTask.taskIdentifier)")
}
}
}
最后,我开始这样的测试下载:
let session = DownloadManager.shared.session
// this is a 100MB PDF file that I'm using for testing
let testUrl = URL(string: "https://scholar.princeton.edu/sites/default/files/oversize_pdf_test_0.pdf")!
let task = session.downloadTask(with: testUrl)
// I think I'll ultimately need to persist the session ID, task ID and a file path for use in the delegate methods once the download has completed
task.resume()
当我 运行 此代码并开始下载时,我看到正在调用委托方法,但我也看到一条消息:
A background URLSession with identifier com.example.testapp.background already exists!
我 认为 这是因为 application:handleEventsForBackgroundURLSession:completionHandler:
中的以下调用而发生的let session = DownloadManager.shared.session
我的 DownloadManager class 中会话 属性 的 getter(我直接从之前引用的博客 post 中获取)总是试图创建一个新的URLSession 使用后台配置。据我了解,如果我的应用程序已终止,那么这将是 "reconnect" 到原始 URLSession 的适当行为。但是由于可能应用程序没有被终止而只是进入后台,当调用 application:handleEventsForBackgroundURLSession:completionHandler: 时,我应该引用 URLSession 的现有实例。至少我 认为 这就是问题所在。谁能为我澄清这种行为?谢谢!
您的问题是每次引用会话变量时都在创建一个新会话:
var session : URLSession {
get {
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue())
}
}
相反,将会话保留为实例变量,然后直接获取它:
class DownloadManager:NSObject {
static var shared = DownloadManager()
var delegate = DownloadManagerSessionDelegate()
var session:URLSession
let config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
override init() {
session = URLSession(configuration: config, delegate: delegate, delegateQueue: OperationQueue())
super.init()
}
}
class DownloadManagerSessionDelegate: NSObject, URLSessionDelegate {
// implement here
}
当我在操场上这样做时,它显示重复调用给出相同的会话,并且没有错误:
会话不在进程中,它是 OS 的一部分。每次访问写入的会话变量时,您都会增加引用计数,这会导致错误。