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当我使用 avassetexportsession 时，有时会起作用，但有时会失败，并且出现以下错误。我查看了在线和苹果文档，但找不到有关此错误的任何内容。

错误：

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could
not be completed" UserInfo={NSUnderlyingError=0x14dcf3bf0 
{Error Domain=NSOSStatusErrorDomain Code=-12842 "(null)"},              
NSLocalizedFailureReason=An unknown error occurred (-12842)

代码：

        let url = NSURL.fileURLWithPath(savedFileTest)
        EditSongViewController.url.urlComplete = url
        let exporter = AVAssetExportSession(asset: mixedFile, presetName: AVAssetExportPresetMediumQuality)
        exporter!.audioMix = audioMixer
        exporter!.outputURL = url
        exporter!.outputFileType = AVFileTypeMPEG4
        exporter!.exportAsynchronouslyWithCompletionHandler( {
            // this handler gets called in a background thread!
            switch exporter!.status {
            case  AVAssetExportSessionStatus.Failed:
                self.indicator.stopAnimation(false, completion: nil)
                //println("failed url=\(exporter.outputURL), error=\(exporter.error)")
                return;
            case AVAssetExportSessionStatus.Cancelled:
                self.indicator.stopAnimation(false, completion: nil)
                return;
            default:
                self.indicator.stopAnimation(false, completion: nil) …