Swift AVFoundation instructions don't set the opacity - swift

Question

I have a function that merges videos. All videos merge properly and the first two videos will play perfectly but then only the audio for the third video plays. I am assuming the video is there, but it is just blocked by the second video. I am confused though because I am using instructions to set the opacity of each asset to 0 when the video is done, but it doesn't work. Also even if I don't set any instructions, the first video disappears to allow the second video to play, but the second video never disappears. What is going on!?
func mergeVideo(completion: #escaping (_ url: URL?, _ error: Error?) -> Void) {
let mixComposition = AVMutableComposition()
let videoComposition = AVMutableVideoComposition()
guard let documentDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
completion(nil, nil)
return
}
let outputURL = documentDirectory.appendingPathComponent("\(id).mov")
do {
if FileManager.default.fileExists(atPath: outputURL.path) {
try FileManager.default.removeItem(at: outputURL)
}
} catch {
print(error.localizedDescription)
}
// If there is only one video, save export time.
if let video = videos.first, videos.count == 1 {
do {
if let url = URL(string: video.videoURL) {
try FileManager().copyItem(at: url, to: outputURL)
completion(outputURL, nil)
mergedVideoURL = outputURL.lastPathComponent
}
} catch let error {
completion(nil, error)
}
return
}
var currentTime = CMTime.zero
let renderSize = CGSize(width: 1280.0, height: 720.0)
let mainInstruction = AVMutableVideoCompositionInstruction()
videos.enumerated().forEach { index, video in
if let vidURL = URL(string: video.videoURL)?.lastPathComponent {
let url = documentDirectory.appendingPathComponent(vidURL)
let asset = AVAsset(url: url)
guard let assetTrack = asset.tracks.first else { return }
mainInstruction.timeRange = CMTimeRangeMake(start: .zero, duration: CMTimeAdd(mixComposition.duration, asset.duration))
let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: assetTrack)
instruction.setOpacity(0.0, at: asset.duration)
mainInstruction.layerInstructions.append(instruction)
do {
let timeRange = CMTimeRangeMake(start: .zero, duration: asset.duration)
try mixComposition.insertTimeRange(timeRange, of: asset, at: currentTime)
currentTime = CMTimeAdd(currentTime, asset.duration)
} catch let error {
completion(nil, error)
}
}
}//forEach
videoComposition.instructions = [mainInstruction]
videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
videoComposition.renderSize = renderSize
guard let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetPassthrough) else {
completion(nil, nil)
return
}
exporter.outputURL = outputURL
exporter.outputFileType = .mov
// Pass Video Composition to the Exporter.
exporter.videoComposition = videoComposition
exporter.exportAsynchronously {
DispatchQueue.main.async {
switch exporter.status {
case .completed:
completion(exporter.outputURL, nil)
case .failed:
completion(exporter.outputURL, exporter.error)
case.cancelled:
completion(exporter.outputURL, exporter.error)
case .unknown:
completion(exporter.outputURL, exporter.error)
case .waiting:
print("waiting")
case .exporting:
print("exporting")
#unknown default:
completion(exporter.outputURL, exporter.error)
}
}
}
}