MergeVideosUsingExoplayer

private fun mergeVideos(videoFiles: List<File>, outputFile: File) {
    val extractorFactory = DefaultExtractorsFactory()
    val mediaMuxer = MediaMuxer(outputFile.absolutePath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4)

    var videoTrackIndex = -1
    var audioTrackIndex = -1

    videoFiles.forEachIndexed { index, videoFile ->
        val dataSourceFactory = DefaultDataSourceFactory(context, "exoplayer")
        val mediaSource = ProgressiveMediaSource.Factory(dataSourceFactory, extractorFactory).createMediaSource(Uri.fromFile(videoFile))

        val videoTrack = mediaMuxer.addTrack(mediaSource.getTrackInfo(0))
        val audioTrack = mediaMuxer.addTrack(mediaSource.getTrackInfo(1))

        if (index == 0) {
            mediaMuxer.start()
        }

        val bufferInfo = MediaCodec.BufferInfo()
        var isFinished = false

        var currentSampleTime: Long = 0
        while (!isFinished) {
            val sampleData = ByteArray(1024 * 1024)
            val sampleSize = mediaSource.readData(videoTrack, currentSampleTime, bufferInfo)
            if (sampleSize < 0) {
                isFinished = true
                break
            }
            bufferInfo.presentationTimeUs += 1000000L / 30
            bufferInfo.flags = mediaSource.getSampleFlags(videoTrack, bufferInfo.presentationTimeUs)
            mediaMuxer.writeSampleData(videoTrack, ByteBuffer.wrap(sampleData, 0, sampleSize), bufferInfo)
            currentSampleTime = bufferInfo.presentationTimeUs
        }

        isFinished = false
        currentSampleTime = 0
        while (!isFinished) {
            val sampleData = ByteArray(1024 * 1024)
            val sampleSize = mediaSource.readData(audioTrack, currentSampleTime, bufferInfo)
            if (sampleSize < 0) {
                isFinished = true
                break
            }
            bufferInfo.presentationTimeUs += 1000000L / 30
            bufferInfo.flags = mediaSource.getSampleFlags(audioTrack, bufferInfo.presentationTimeUs)
            mediaMuxer.writeSampleData(audioTrack, ByteBuffer.wrap(sampleData, 0, sampleSize), bufferInfo)
            currentSampleTime = bufferInfo.presentationTimeUs
        }

        mediaSource.release()
    }

    mediaMuxer.stop()
    mediaMuxer.release()
}