foo.swift

func outlineViewItemWillExpand(_ notification: Notification) {
    if let entity = notification.userInfo?["NSObject"] as? DesktopFolderEntity {
      guard let sortDescriptor = self.outlineView.sortDescriptors.first else { return }

      entity.isExpanding = true

      let childrenFetch = BlockOperation {
        entity.buildChildren()
        entity.sortChildren(sortDescriptor: sortDescriptor)

        OperationQueue.main.addOperation {
          entity.isExpanding = false
          self.outlineView.reloadData()
        }
      }

      self.childrenFetchQueue.addOperation(childrenFetch)
    }
}