AVMutableComposition overlay one video on another - swift

Question

I am trying to overlay one video on another but exported video contains only the first video. Here is my code.
If I change the places of the firstAsset and second asset, then the second video would be the only one to be displayed, so the problem is not in assets.
`func newoverlay(video firstAsset: AVURLAsset, withSecondVideo secondAsset: AVURLAsset) {
let mixComposition = AVMutableComposition()
// 2 - Create two video tracks
guard
let firstTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
else {
return
}
do {
try firstTrack.insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: firstAsset.duration), of: firstAsset.tracks(withMediaType: AVMediaType.video)[0], at: CMTime.zero)
} catch {
print("Failed to load first track")
return
}
guard
let secondTrack = mixComposition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
else {
return
}
do {
try secondTrack.insertTimeRange(CMTimeRangeMake(start: CMTime.zero, duration: secondAsset.duration), of: secondAsset.tracks(withMediaType: AVMediaType.video)[0], at: CMTime.zero)
} catch {
print("Failed to load second track")
return
}
// 4 - Get path
guard let documentDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
return
}
let dateFormatter = DateFormatter()
dateFormatter.dateStyle = .long
dateFormatter.timeStyle = .short
let date = dateFormatter.string(from: Date())
let url = documentDirectory.appendingPathComponent("mergeVideo-\(date).mov")
// 5 - Create Exporter
guard let exporter = AVAssetExportSession(asset: mixComposition, presetName: AVAssetExportPresetPassthrough) else {
return
}
exporter.outputURL = url
exporter.outputFileType = AVFileType.mp4
exporter.shouldOptimizeForNetworkUse = true
// 6 - Perform the Export
exporter.exportAsynchronously() {
DispatchQueue.main.async {
// self.exportDidFinish(exporter)
}
}
}