public static int InstanceAssetAsync(
     string address,
     OnAssetProgress progressCallback,
     OnAssetComplete completeCallback,
     AsyncPriority priority = AsyncPriority.Default,
     SystemObject userdata  = null)
 {
     if (loader != null)
     {
         return(loader.InstanceAssetAsync(address, progressCallback, completeCallback, priority, userdata));
     }
     return(-1);
 }