Merge two videos with audio and video together in iOS

Question

I am trying to merge two videos together in AVFoundation. I am using AVMutableComposition and I add both tracks to the composition, resulting in a final video where I have the first video with its audio, and after that the 2nd audio but no video.

How can I get the audio and video of both tracks?

Thank you

let composition = AVMutableComposition()
        let audioTrack: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)!
        let videoTrack: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)!
        
        let audioTrack2: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)!
        let videoTrack2: AVMutableCompositionTrack = composition.addMutableTrack(withMediaType: AVMediaType.video, preferredTrackID: kCMPersistentTrackID_Invalid)!
        var outputURL = documentDirectory.appendingPathComponent("output-temp")
        
        do {
            try! audioTrack.insertTimeRange(CMTimeRangeFromTimeToTime(start: startTime, end: endTime), of: asset.tracks(withMediaType: AVMediaType.audio)[0], at: CMTime.zero)
            
            try! videoTrack.insertTimeRange(CMTimeRangeFromTimeToTime(start: startTime, end: endTime), of: asset.tracks(withMediaType: AVMediaType.video)[0], at: CMTime.zero)
            
            try! audioTrack2.insertTimeRange(CMTimeRangeFromTimeToTime(start: startTime, end: asset2.duration), of: asset2.tracks(withMediaType: AVMediaType.audio)[0], at: CMTime.invalid)
            
            try! videoTrack2.insertTimeRange(CMTimeRangeFromTimeToTime(start: startTime, end: asset2.duration), of: asset2.tracks(withMediaType: AVMediaType.video)[0], at: CMTime.invalid)
            
            try manager.createDirectory(at: outputURL, withIntermediateDirectories: true, attributes: nil)
        
            let id = "id-\(Int.random(in: 0...199))"
            let mediaType = "mp4"
            outputURL = outputURL.appendingPathComponent("preVideo-\(id).\(mediaType)")
            
        } catch let error {
            print(error)
        }

Answer 1

The problem is that you are adding a second video track to the composition. You need to insert both videos into the same video track. Just delete your let videoTrack2 and go from there.