AVAssetExportSession在由选择器调用时失败

Question

我正在尝试使用AVFoundation导出资产。我有exportSession设置的代码，如果放在viewDidLoad()中它可以正常工作。但是，如果我将逻辑放在按下按钮时调用的函数中，会话将失败，并显示未知错误。

以下是我正在使用的代码：

@objc func uploadVideo() {
    guard let url = self.videoUrl else { return }
    var asset = AVURLAsset(url: url)

    let compatiblePresets = AVAssetExportSession.exportPresets(compatibleWith: asset)

    if compatiblePresets.contains(AVAssetExportPresetLowQuality) {
        let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetLowQuality)
        let outputUrl = getDocumentsDirectory().appendingPathComponent("testing.mp4")
        exportSession?.outputURL = outputUrl
        exportSession?.outputFileType = AVFileType.mp4
        exportSession?.shouldOptimizeForNetworkUse = true
        exportSession?.exportAsynchronously(completionHandler: {
            guard let status = exportSession?.status else { return }
            switch status {
            case AVAssetExportSessionStatus.failed:
                print(exportSession?.error)
                break
            case AVAssetExportSessionStatus.cancelled:
                break
            case AVAssetExportSessionStatus.completed:
                print("Exported succesfully...")
                break
            default:
                break
            }
        })
    }
}

调用此方法时，我在控制台中收到以下错误：

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSUnderlyingError=0x10b41b400 {Error Domain=NSOSStatusErrorDomain Code=-16979 "(null)"}, NSLocalizedFailureReason=An unknown error occurred (-16979)

我似乎无法弄清问题是什么。非常感谢任何帮助。