合并视频，但AVAssetExportSession从未完成

Question

Trying to merge some videos together and export them as a single file, everything seems to be correct from looking at tutorials/examples however my AVAssetExportSession never seems to be complete, and my video file is never exported, any help as to a blindingly obvious error I'm missing would be very appreciated.

Below is the function where i merge the videos

note 'videos' in the loop is a member variable var videos = [AVAsset]() which gets populated (and it does i checked) before merge is called.

private func merge()
{
    // Create AVMutableComposition to contain all AVMutableComposition tracks
    var mix_composition = AVMutableComposition()
    var total_time_seconds  = 0.0
    var tracks = [AVCompositionTrack]()

    // Loop over videos and create tracks, keep incrementing total duration
    for video in videos
    {
        // Create the composition track for this video
        let track = mix_composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))

        // Add video duration to total time
        total_time_seconds = total_time_seconds + video.duration.seconds

        // Add track to array of tracks
        tracks.append(track)

        // Add time range to track
        do
        {
            try track.insertTimeRange(CMTimeRangeMake(kCMTimeZero, video.duration), ofTrack: video.tracksWithMediaType(AVMediaTypeVideo)[0], atTime: video.duration)
        }
        catch _
        {
        }
    }

    // Set total time
    let preferred_time_scale: Int32 = 600;
    let total_time = CMTimeMakeWithSeconds(total_time_seconds, preferred_time_scale)

    // Create main instrcution for video composition
    let main_instruction = AVMutableVideoCompositionInstruction()
    main_instruction.timeRange = CMTimeRangeMake(kCMTimeZero, total_time)

    // Create array to hold instructions
    var layer_instructions = [AVVideoCompositionLayerInstruction]()

    // Ensure we have the same number of tracks as videos
    if videos.count == tracks.count
    {
        // Loop number of videos and tracks
        for var index = 0; index < videos.count; ++index
        {
            // Create compositioninstruction for each track
            let instruction = videoCompositionInstructionForTrack(tracks[index], asset: videos[index])

            if(index == 0)
            {
                instruction.setOpacity(0.0, atTime: videos[index].duration)
            }

            // Add instruction to instructions array
            layer_instructions.append(instruction)
        }
    }

    // Set tack instructions to main instruction
    main_instruction.layerInstructions = layer_instructions
    let main_composition = AVMutableVideoComposition()
    main_composition.instructions = [main_instruction]
    main_composition.frameDuration = CMTimeMake(1, 30)
    main_composition.renderSize = CGSize(width: UIScreen.mainScreen().bounds.width, height: UIScreen.mainScreen().bounds.height)

    // Get path for Final video in the current project directory
    let documents_url = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0]
    let final_url = documents_url.URLByAppendingPathComponent("TEST.mp4")

    // Create AV Export Session
    let exporter = AVAssetExportSession(asset: mix_composition, presetName: AVAssetExportPresetHighestQuality)
    exporter!.outputURL = final_url
    exporter!.outputFileType = AVFileTypeMPEG4
    exporter!.shouldOptimizeForNetworkUse = true
    exporter!.videoComposition = main_composition

    // Perform the Export
    exporter!.exportAsynchronouslyWithCompletionHandler() {
        dispatch_async(dispatch_get_main_queue(), { () -> Void in
            self.exportDidFinish(exporter!)
        })
    }
}

Below shows the exportDidFinished function which is called when exportAsynchronouslyWithCompletionHandler is called. I get inside this function but nothing ever happens because the session status is never completed.

func exportDidFinish(session: AVAssetExportSession)
{
    if session.status == AVAssetExportSessionStatus.Completed
    {
        let outputURL = session.outputURL
        let library = ALAssetsLibrary()
        if library.videoAtPathIsCompatibleWithSavedPhotosAlbum(outputURL)
        {
            library.writeVideoAtPathToSavedPhotosAlbum(outputURL,
                completionBlock: { (assetURL:NSURL!, error:NSError!) -> Void in
                    if error != nil
                    {
                        let alert = UIAlertView(title: "Error", message: "Video Not Saved", delegate: nil, cancelButtonTitle: "OK")
                        alert.show()

                    }
                    else
                    {
                        let alert = UIAlertView(title: "Success", message: "Video Saved", delegate: nil, cancelButtonTitle: "OK")
                        alert.show()
                    }
            })
        }
    }
}

printed the session status showed that it was 4 which is failed, so i printed session.error and got this, but I'm unsure on what it means, any help would be great

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

Answer 1

If exportDidFinish is called but nothing happens, your session's status is not AVAssetExportSessionStatus.Completed . It may be AVAssetExportSessionStatus.Failed , or some other value. Check those values and if it did fail check the session.error property for more info.

EDIT : If you want one video to play after another, create only one AVMutableCompositionTrack . See below for relevant changes:

    ...
    var mix_composition = AVMutableComposition()

    // Create the composition track for the videos
    let track = mix_composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))

    //keep track of total time
    var totalTime = kCMTimeZero

    for video in videos
    {
        // Add time range to track
        do
        {
            let videoTrack = video.tracksWithMediaType(AVMediaTypeVideo)[0]
            let videoDuration = videoTrack.duration
            let timeRange = CMTimeRangeMake(kCMTimeZero,videoDuration)

            try track.insertTimeRange(timeRange, ofTrack: videoTrack, atTime: totalTime)

            totalTime = CMTimeAdd(totalTime,videoDuration)
        }
        catch _
        {
        }
    }

    // Create main instruction for video composition
    let main_instruction = AVMutableVideoCompositionInstruction()
    main_instruction.timeRange = CMTimeRangeMake(kCMTimeZero, totalTime)

    // Create array to hold instructions
    var layer_instructions = [AVVideoCompositionLayerInstruction]()

    // Create layer instruction
    let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)

    // Add it to the array
    layer_instructions.append(layerInstruction)

    ...

You'll also want to adjust your renderSize to something appropriate. The size of your videos may not be the size of your screen.