通过AVAssetExportSession将资产导出到文件系统总是失败 - Swift 3

Question

我想将存储在AVPlayerItem中的.mp4-video导出到文件系统.如果成功,我想把它变成一个NSData变量(但这不是问题的一部分,只是想告诉你为什么我想把它存储到文件系统中).

//This is the AVPlayerItem-Object
let item = self.talentView.getCurrentItem()

//They return all true, so the item itself is not the issue
print(item.asset.isPlayable)
print(item.asset.isReadable)
print(item.asset.isExportable)

//Creating Path, where the file will be stored, also tried "video.mp4" at the appending string
var exportPath: NSString = NSTemporaryDirectory().appendingFormat("/video.mp4")
var exportUrl: NSURL = NSURL.fileURL(withPath: exportPath as String) as NSURL

let session = AVAssetExportSession(asset: item.asset, presetName: AVAssetExportPresetPassthrough)             
session?.outputFileType = AVFileTypeMPEG4
session?.outputURL = exportUrl as URL

session?.exportAsynchronously(completionHandler: { 

    //case is always .failed
    switch session!.status {
           case .failed:
                 print("failed")
                 print(session!.error!)
           default:
                 print("something else")
         }
 })

错误输出:

错误域= AVFoundationErrorDomain代码= -11838"操作已停止"UserInfo = {NSUnderlyingError = 0x170441230 {错误域= NSOSStatusErrorDomain代码= -12109"(null)"},NSLocalizedFailureReason =此媒体不支持该操作.,NSLocalizedDescription =操作已停止}

这很奇怪,因为print(item.asset.isExportable)返回true.

我已经研究了很多并尝试了各种类型的exportUrl,但导出总是失败.我不明白这个问题,也许有人有想法？