我们将字幕添加到用户录制的视频中，但是我们的 AVAssetExportSession 对象的导出不确定性地失败了:有时可以，有时则不能。甚至不清楚如何重现错误。

我们注意到 Assets 轨道似乎在导出过程中丢失了。

在导出之前，如预期的那样有两个轨道(一个用于音频，一个用于视频)。但是在 exportDidFinish 中检查同一文件 URL 的轨道数显示 0 条轨道。所以导出过程似乎出了点问题。

更新:注释掉 exporter.videoComposition = mutableComposition 可修复错误，但当然不会对视频应用任何转换。所以问题似乎出在创建 AVMutableVideoComposition 上，这会在导出过程中导致下游出现问题。关于 AVMutableVideoComposition 的文档和教程很少，因此即使您没有解决方案但可以推荐 Apple 以外的引用资源，这也会有所帮助。

错误:

Error Domain=AVFoundationErrorDomain Code=-11841 "Operation Stopped" UserInfo=0x170676e80 {NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The video could not be composed.}

代码:

    let videoAsset = AVURLAsset(URL: fileUrl, options: nil)
    let mixComposition = AVMutableComposition()
    let videoTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
    let audioTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))

    let sourceVideoTrack = videoAsset.tracksWithMediaType(AVMediaTypeVideo)[0] as! AVAssetTrack
    let sourceAudioTrack = videoAsset.tracksWithMediaType(AVMediaTypeAudio)[0] as! AVAssetTrack
    videoTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoAsset.duration), ofTrack: sourceVideoTrack, atTime: kCMTimeZero, error: nil)
    audioTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, videoAsset.duration), ofTrack: sourceAudioTrack, atTime: kCMTimeZero, error: nil)

    // Create something mutable???
    // -- Create instruction
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
    let videoLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: sourceVideoTrack)
    instruction.layerInstructions = [videoLayerInstruction]

    let mutableComposition = AVMutableVideoComposition()
    //mutableComposition.renderSize = videoTrack.naturalSize
    mutableComposition.renderSize = CGSize(width: 320, height: 320)
    mutableComposition.frameDuration = CMTimeMake(1, 60)
    mutableComposition.instructions = [instruction]

    // Animate
    mutableComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, inLayer: parentLayer)

    // -- Get path
    let fileName = "/editedVideo-\(arc4random() % 10000).mp4"
    let allPaths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
    let docsPath = allPaths[0] as! NSString
    let exportPath = docsPath.stringByAppendingFormat(fileName)
    let exportUrl = NSURL.fileURLWithPath(exportPath as String)!

    println("Tracks before export: \(mixComposition.tracks.count). File URL: \(exportUrl)")

    // -- Remove old video?
    if NSFileManager.defaultManager().fileExistsAtPath(exportPath as String) {
        println("Deleting existing file\n")
        NSFileManager.defaultManager().removeItemAtPath(exportPath as String, error: nil)
    }

    // -- Create exporter
    let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetHighestQuality)
    exporter.videoComposition = mutableComposition
    exporter.outputFileType = AVFileTypeMPEG4
    exporter.outputURL = exportUrl
    exporter.shouldOptimizeForNetworkUse = true

    // -- Export video
    exporter.exportAsynchronouslyWithCompletionHandler({
        self.exportDidFinish(exporter)
    })


func exportDidFinish(exporter: AVAssetExportSession) {
    println("Exported video with status: \(getExportStatus(exporter))")

    // Save video to photo album
    let assetLibrary = ALAssetsLibrary()
    assetLibrary.writeVideoAtPathToSavedPhotosAlbum(exporter.outputURL, completionBlock: {(url: NSURL!, error: NSError!) in
        println("Saved video to album \(exporter.outputURL)")
        if (error != nil) {
            println("Error saving video")
        }
    })

    // Check asset tracks
    let asset = AVAsset.assetWithURL(exporter.outputURL) as? AVAsset
    println("Tracks after export: \(asset!.tracks.count). File URL: \(exporter.outputURL)")
}

问题:

1) 是什么导致了问题，解决方案是什么？

2) 关于如何一致地重现错误的建议，希望有助于调试问题？

最佳答案

似乎解决方法是确保 AVMutableVideoCompositionLayerInstruction 中的 assetTrack 参数不是来自 AVURLAsset 对象，而是来自视频对象由 addMutableTrackWithMediaType 返回。

换句话说，这一行:

let videoLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: sourceVideoTrack)

应该是:

let videoLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)

唉。数小时无尽的挫败感，因为有时第一行有效，有时却无效。

仍然想将赏金奖励给某人。

如果你能解释为什么第一行非确定性地失败，而不是每次都失败，或者提供更深入的 AVMutableComposition 及其相关类的教程——为了向用户录制的视频添加文本覆盖——悬赏都是你的。 :)

关于ios - AVAssetExportSession 导出不确定地失败，错误为 : "Operation Stopped, NSLocalizedFailureReason=The video could not be composed."，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31092455/