enum class ProgressMode {
  CURRENT_CHANGES,
  INDEFINETELY
}

data class Progress(val transferredBytes: Long, val transferableBytes: Long) {
  val fractionTransferred: Double
  val transferComplete: Boolean
}
typealias ProgressListener = (progress: Progress) -> Unit

class SyncSession {
  fun addDownloadProgressListener(mode: ProgressMode, listener: ProgressListener): Cancellable 
 fun addUploadProgressListener(mode: ProgressMode, listener: ProgressListener): Cancellable
  fun uploadAllLocalChanges(timeout: Long = Long.MAX_VALUE, unit = TimeUnit.SECONDS)  
 fun downloadAllServerChanges(timeout: Long = Long.MAX_VALUE, unit = TimeUnit.SECONDS) 
}