Ios AVAssetExportSession无法导出从iCloud下载的视频

我正在尝试创建从用户相册中选择的视频的缩小版本。输出的最大尺寸为720p。因此，在检索视频时，我使用

.mediumQualityFormat

作为

deliveryMode

。如果用户设备中不存在原始视频或其中等质量版本，这会导致iOS从iCloud检索720p视频

let videoRequestOptions = PHVideoRequestOptions()
videoRequestOptions.deliveryMode = .mediumQualityFormat
videoRequestOptions.isNetworkAccessAllowed = true

PHImageManager.default().requestAVAsset(forVideo: asset, options: videoRequestOptions) { (asset, audioMix, info) in
    // Proceess the asset
}

问题是，当我使用

AVAssetExportSession

创建资产的缩小版本时，如果资产是中等变体而不是原始版本，则导出过程会立即失败，并出现以下错误：

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSLocalizedFailureReason=An unknown error occurred (-17507), NSLocalizedDescription=İşlem tamamlanamadı, NSUnderlyingError=0x283bbcf60 {Error Domain=NSOSStatusErrorDomain Code=-17507 "(null)"}}

我在任何地方都找不到这个错误的含义

当我将

deliveryMode

属性设置为

.auto

或

.highQualityFormat

时，一切都正常工作

当我检查资产url时，我注意到当从iCloud检索视频时，其文件名有一个“.medium”后缀，如本例所示：

file:///var/mobile/Media/PhotoData/Metadata/PhotoData/CPLAssets/group338/191B2348-5E19-4A8E-B15C-A843F9F7B5A3.medium.MP4

奇怪的是，如果我使用

FileManager

将此url中的视频复制到另一个目录，从该文件创建一个新的

AVAsset

，并在创建

AVExportSession

实例时使用该资源，问题就会消失

如果有人能提供一些关于问题所在的见解，我将不胜感激

这就是我如何使用AVAssetExportSession创建原始视频的缩小版本

let originalVideoURL = // The url of the asset retrieved from requestAVAsset

let outputVideoPath = NSTemporaryDirectory() + "encodedVideo.mp4"
let outputVideoURL = URL(fileURLWithPath: outputVideoPath)

guard
        let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality),
        let videoTrack = asset.tracks(withMediaType: .video).first else {
        // Error handling
    return
}

let videoComposition = AVMutableVideoComposition()
videoComposition.renderSize = scaledSize // How this is calculated is irrelevant
videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)

let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)
let transform = videoTrack.preferredTransform

layerInstruction.setTransform(transform, at: .zero)

let instruction = AVMutableVideoCompositionInstruction()
instruction.timeRange = CMTimeRangeMake(start: .zero, duration: asset.duration)
instruction.layerInstructions = [layerInstruction]
videoComposition.instructions = [instruction]

exportSession.videoComposition = videoComposition
exportSession.outputURL = outputVideoURL
exportSession.outputFileType = .mp4
exportSession.shouldOptimizeForNetworkUse = true

exportSession.exportAsynchronously(completionHandler: {[weak self] in
    guard let self = self else { return }

    if let url = exportSession.outputURL, exportSession.status == .completed {
        // Works for local videos
    } else {
        // Fails with error code 17507 when loading videos with delivery size "Medium"
    }
})