Exporting mp4 through AVAssetExportSession fails

Question

I start saying that I spent a lot of time searching through documentation, posts here and somewhere else, but I can't figure out the solution for this problem.

I'm using AVAssetExportSession for exporting an .mp4 file stored in a AVAsset instance. What I do is:

• I check the isExportable property of AVAsset
• I then get an array of exportPresets compatible with the AVAsset instance
• I take the AVAssetExportPreset1920x1080, or, if not existing I try to export the media with AVAssetExportPresetPassthrough (FYI, 100% of times, the preset I need is always contained in the list, but I tried also the passthrough option and it doesn't work anyway)

The outputFileType is AVFileTypeMPEG4 and I tried also by assigning the .mp4 extension to the file, but nothing makes it work. I always receive this error

Error Domain=AVFoundationErrorDomain Code=-11838 "Operation Stopped" UserInfo={NSUnderlyingError=0x600000658c30 {Error Domain=NSOSStatusErrorDomain Code=-12109 "(null)"}, NSLocalizedFailureReason=The operation is not supported for this media., NSLocalizedDescription=Operation Stopped}

Below is the code I'm using

func _getDataFor(_ item: AVPlayerItem, completion: @escaping (Data?) -> ()) {
    guard item.asset.isExportable else {
        completion(nil)
        return
    }

    let compatiblePresets = AVAssetExportSession.exportPresets(compatibleWith: item.asset)
    var preset: String = AVAssetExportPresetPassthrough
    if compatiblePresets.contains(AVAssetExportPreset1920x1080) { preset = AVAssetExportPreset1920x1080 }

    guard
        let exportSession = AVAssetExportSession(asset: item.asset, presetName: preset),
        exportSession.supportedFileTypes.contains(AVFileTypeMPEG4) else {
        completion(nil)
        return
    }

    var tempFileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].appendingPathComponent("temp_video_data.mp4", isDirectory: false)
    tempFileUrl = URL(fileURLWithPath: tempFileUrl.path)

    exportSession.outputURL = tempFileUrl
    exportSession.outputFileType = AVFileTypeMPEG4
    let startTime = CMTimeMake(0, 1)
    let timeRange = CMTimeRangeMake(startTime, item.duration)
    exportSession.timeRange = timeRange

    exportSession.exportAsynchronously {
        print("\(exportSession.error)")
        let data = try? Data(contentsOf: tempFileUrl)
        _ = try? FileManager.default.removeItem(at: tempFileUrl)
        completion(data)
    }
}

Answer 1

Seems like converting the AVAsset instance in a AVMutableComposition did the trick. If, please, anyone knows the reason why this works let me know.

This is the new _getDataFor(_:completion:) method implementation

func _getDataFor(_ item: AVPlayerItem, completion: @escaping (Data?) -> ()) {
    guard item.asset.isExportable else {
        completion(nil)
        return
    }

    let composition = AVMutableComposition()
    let compositionVideoTrack = composition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
    let compositionAudioTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))

    let sourceVideoTrack = item.asset.tracks(withMediaType: AVMediaTypeVideo).first!
    let sourceAudioTrack = item.asset.tracks(withMediaType: AVMediaTypeAudio).first!
    do {
        try compositionVideoTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, item.duration), of: sourceVideoTrack, at: kCMTimeZero)
        try compositionAudioTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, item.duration), of: sourceAudioTrack, at: kCMTimeZero)
    } catch(_) {
        completion(nil)
        return
    }

    let compatiblePresets = AVAssetExportSession.exportPresets(compatibleWith: composition)
    var preset: String = AVAssetExportPresetPassthrough
    if compatiblePresets.contains(AVAssetExportPreset1920x1080) { preset = AVAssetExportPreset1920x1080 }

    guard
        let exportSession = AVAssetExportSession(asset: composition, presetName: preset),
        exportSession.supportedFileTypes.contains(AVFileTypeMPEG4) else {
        completion(nil)
        return
    }

    var tempFileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].appendingPathComponent("temp_video_data.mp4", isDirectory: false)
    tempFileUrl = URL(fileURLWithPath: tempFileUrl.path)

    exportSession.outputURL = tempFileUrl
    exportSession.outputFileType = AVFileTypeMPEG4
    let startTime = CMTimeMake(0, 1)
    let timeRange = CMTimeRangeMake(startTime, item.duration)
    exportSession.timeRange = timeRange

    exportSession.exportAsynchronously {
        print("\(tempFileUrl)")
        print("\(exportSession.error)")
        let data = try? Data(contentsOf: tempFileUrl)
        _ = try? FileManager.default.removeItem(at: tempFileUrl)
        completion(data)
    }
}