实现 AVAssetDownloadURLSession 下载 HLS 流时出错

我正在尝试为流应用程序实现离线模式。 目标是能够在用户的设备上下载 HLS 流，以便即使用户离线时也可以观看流。

我最近偶然发现本教程 https://developer.apple.com/library/content/documentation/AudioVideo/Conceptual/MediaPlaybackGuide/Contents/Resources/en.lproj/HTTPLiveStreaming/HTTPLiveStreaming.html。 它似乎满足了我试图实现的确切要求，但我在尝试使其工作时遇到了问题。

我创建了一个小 DownloadManager 来应用本教程的逻辑。 这是我的单例类：

import AVFoundation

class DownloadManager:NSObject {

static var shared = DownloadManager()
private var config: URLSessionConfiguration!
private var downloadSession: AVAssetDownloadURLSession!

override private init() {
    super.init()
    config = URLSessionConfiguration.background(withIdentifier: "\(Bundle.main.bundleIdentifier!).background")
    downloadSession = AVAssetDownloadURLSession(configuration: config, assetDownloadDelegate: self, delegateQueue: OperationQueue.main)
}

func setupAssetDownload(_ url: URL) {
    let options = [AVURLAssetAllowsCellularAccessKey: false]

    let asset = AVURLAsset(url: url, options: options)

    // Create new AVAssetDownloadTask for the desired asset
    let downloadTask = downloadSession.makeAssetDownloadTask(asset: asset,
                                                             assetTitle: "Test Download",
                                                             assetArtworkData: nil,
                                                             options: nil)
    // Start task and begin download
    downloadTask?.resume()
}

func restorePendingDownloads() {
    // Grab all the pending tasks associated with the downloadSession
    downloadSession.getAllTasks { tasksArray in
        // For each task, restore the state in the app
        for task in tasksArray {
            guard let downloadTask = task as? AVAssetDownloadTask else { break }
            // Restore asset, progress indicators, state, etc...
            let asset = downloadTask.urlAsset
            downloadTask.resume()
        }
    }
}

func playOfflineAsset() -> AVURLAsset? {
    guard let assetPath = UserDefaults.standard.value(forKey: "assetPath") as? String else {
        // Present Error: No offline version of this asset available
        return nil
    }
    let baseURL = URL(fileURLWithPath: NSHomeDirectory())
    let assetURL = baseURL.appendingPathComponent(assetPath)
    let asset = AVURLAsset(url: assetURL)
    if let cache = asset.assetCache, cache.isPlayableOffline {
        return asset
        // Set up player item and player and begin playback
    } else {
        return  nil
        // Present Error: No playable version of this asset exists offline
    }
}

func getPath() -> String {
    return UserDefaults.standard.value(forKey: "assetPath") as? String ?? ""
}

func deleteOfflineAsset() {
    do {
        let userDefaults = UserDefaults.standard
        if let assetPath = userDefaults.value(forKey: "assetPath") as? String {
            let baseURL = URL(fileURLWithPath: NSHomeDirectory())
            let assetURL = baseURL.appendingPathComponent(assetPath)
            try FileManager.default.removeItem(at: assetURL)
            userDefaults.removeObject(forKey: "assetPath")
        }
    } catch {
        print("An error occured deleting offline asset: \(error)")
    }
}
}

extension DownloadManager: AVAssetDownloadDelegate {
    func urlSession(_ session: URLSession, assetDownloadTask: AVAssetDownloadTask, didLoad timeRange: CMTimeRange, totalTimeRangesLoaded loadedTimeRanges: [NSValue], timeRangeExpectedToLoad: CMTimeRange) {
        var percentComplete = 0.0
        // Iterate through the loaded time ranges
        for value in loadedTimeRanges {
        // Unwrap the CMTimeRange from the NSValue
        let loadedTimeRange = value.timeRangeValue
        // Calculate the percentage of the total expected asset duration
        percentComplete += loadedTimeRange.duration.seconds / timeRangeExpectedToLoad.duration.seconds
    }
        percentComplete *= 100

    debugPrint("Progress \( assetDownloadTask) \(percentComplete)")

    let params = ["percent": percentComplete]
    NotificationCenter.default.post(name: NSNotification.Name(rawValue: "completion"), object: nil, userInfo: params)
    // Update UI state: post notification, update KVO state, invoke callback, etc.
}

func urlSession(_ session: URLSession, assetDownloadTask: AVAssetDownloadTask, didFinishDownloadingTo location: URL) {
    // Do not move the asset from the download location
    UserDefaults.standard.set(location.relativePath, forKey: "assetPath")
}

func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
    debugPrint("Download finished: \(location)")
}

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
    debugPrint("Task completed: \(task), error: \(String(describing: error))")

    guard error == nil else { return }
    guard let task = task as? AVAssetDownloadTask else { return }

    print("DOWNLOAD: FINISHED")
}
}

当我尝试打电话给我的时候，我的问题就出现了setupAssetDownload功能。 每次我尝试恢复 downloadTask 时，我都会在urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)委托功能。

消息的日志是：

任务完成：<__nscfbackgroundavassetdownloadtask>{任务标识符：1}，错误：可选（错误 Domain=AVFoundationErrorDomain Code=-11800 \"操作无法 已完成\" UserInfo={NSLocalizedFailureReason=未知错误 发生（-12780），NSLocalizedDescription=操作无法进行 完全的}）

为了向您提供我通过的 URL 的所有相关信息setupAssetDownload函数的类型是URL(string: "https://bitdash-a.akamaihd.net/content/MI201109210084_1/m3u8s/f08e80da-bf1d-4e3d-8899-f0f6155f6efa.m3u8")!

我一直在寻找这个错误的原因和解决方案，但我似乎暂时无法找到。 我将非常感谢有关如何解决此问题的任何提示或任何线索，或者我的单例实现中可以解释此行为的任何错误指示。

先感谢您。

Martin

EDIT:

看来这个bug是发生在模拟器上的。我在真实设备上启动我的应用程序，下载开始没有任何问题。希望这可以帮助。仍然不明白为什么我不能在模拟器上尝试这种行为。