Progress returned by itemProvider.loadFileRepresentation always 100%

Code Block 
var progress = itemProvider.loadFileRepresentation(
    forTypeIdentifier: id, 
    completionHandler: { [self] (u, error) in
    //...
    })
 var timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [self] timer in
      print("\(progress?.completedUnitCount)/\(progress?.totalUnitCount)")
 }