export-loop.swift

override func viewDidLoad() {
    super.viewDidLoad()

    let data = try! Data(contentsOf: URL(string: "https://i.imgur.com/dXxP7a9.mp4")!)

    let fileName = String(format: "%@_%@", ProcessInfo.processInfo.globallyUniqueString, "html5gif.mov")
    let fileURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(fileName)

    try! data.write(to: fileURL, options: [.atomic])

    print("Download completed")
    tweakMetaData(forVideoAtURL: fileURL)
}

func tweakMetaData(forVideoAtURL url: URL) {
    let asset = AVURLAsset(url: url)

    let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetPassthrough)!

    let exportFileName = String(format: "%@_%@", ProcessInfo.processInfo.globallyUniqueString, "html5gif-redone.mov")
    let exportFileURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(exportFileName)

    exportSession.outputURL = exportFileURL
    exportSession.outputFileType = .mov

    var metaData = asset.metadata

    let mutableMetaDataItem = AVMutableMetadataItem()
    mutableMetaDataItem.dataType = kCMMetadataBaseDataType_RawData as String
    mutableMetaDataItem.keySpace = .quickTimeUserData
    mutableMetaDataItem.key = NSString("loop")

    var loopVal = Int32(0)
    mutableMetaDataItem.value = NSData(bytes: &loopVal, length: MemoryLayout.size(ofValue: loopVal))

    metaData.append(mutableMetaDataItem)

    exportSession.metadata = metaData

    exportSession.exportAsynchronously {
        print("asset export finished")
        if exportSession.status == .completed {
            print("completed successfully")
            UISaveVideoAtPathToSavedPhotosAlbum(exportFileURL.path, nil, nil, nil)
        } else {
            print("oh no")
        }
    }
}