如何使与网络的TableViewDataSourcePrefetching顺利进行？

Question

我似乎无法通过网络和Operation使tableViewDataSourcePrefetching顺利运行。难道我做错了什么？下面是表格视图方法。我也使用自定义单元格。在我要从Web的图像url转换数据（使用url会话）并将其从数据转换为UIImage的操作中，并在完成闭包中将其返回以及将其分配给变量。

n >= 3

我正在尝试使用下面的缓存：

        let cell = tableView.dequeueReusableCell(withIdentifier: "ImageCell", for: indexPath) as! ImageTableViewCell

         if let imagesModel = imageController.viewModel(at: indexPath.row) {

            cell.images = imagesModel
            if let imageLoadOperation = imageLoadOperations[indexPath],
                let image = imageLoadOperation.image {

                    cell.picImageView.image = image

            } else {
                let imageLoadOperation = ImageLoadOperation(url: imagesModel.downloadUrl, completionHandler: { [weak self] (image) in
                    guard let strongSelf = self else {
                        return
                    }

                    DispatchQueue.main.async {
                        cell.picImageView.image = image
                    }
                    strongSelf.imageLoadOperations.removeValue(forKey: indexPath)
                })
                imageLoadQueue.addOperation(imageLoadOperation)
                imageLoadOperations[indexPath] = imageLoadOperation
            }
        }

        return cell
    }


    override func tableView(_ tableView: UITableView, didEndDisplaying cell: UITableViewCell, forRowAt indexPath: IndexPath) {
        guard let imageLoadOperation = imageLoadOperations[indexPath] else {
            return
        }
        imageLoadOperation.cancel()
        imageLoadOperations.removeValue(forKey: indexPath)

    }
}

extension ImagesTableViewController: UITableViewDataSourcePrefetching {
    func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {

        for indexPath in indexPaths {
            if let _ = imageLoadOperations[indexPath] {
                return
            }
            if let viewModel = imageController.viewModel(at: (indexPath as NSIndexPath).row) {
                let imageLoadOperation = ImageLoadOperation(url: viewModel.downloadUrl, completionHandler: {(image) in

                    })
                imageLoadQueue.addOperation(imageLoadOperation)
                imageLoadOperations[indexPath] = imageLoadOperation
            }
        }
    }

    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {

        for indexPath in indexPaths {
            guard let imageLoadOperation = imageLoadOperations[indexPath] else {
                return
            }
            imageLoadOperation.cancel()
            imageLoadOperations.removeValue(forKey: indexPath)

        }
    }

这是我的项目中使用的Operation im：

    static func downloadImageFromUrl(_ url: String, completionHandler: @escaping (UIImage?) -> Void) {


        guard let url = URL(string: url) else {
            completionHandler(nil)
            return
        }

        let cache =  URLCache.shared
        let request = URLRequest(url: url)

        DispatchQueue.global(qos: .userInitiated).async {
            if let data = cache.cachedResponse(for: request)?.data, let image = UIImage(data: data) {
                DispatchQueue.main.async {
                    completionHandler(image)
                }
            } else {




        let task: URLSessionDataTask = URLSession.shared.dataTask(with: request, completionHandler: { (data, response, error) -> Void in
            guard let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
                let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
                let data = data, error == nil,
                let image = UIImage(data: data) else {
                    completionHandler(nil)
                    return
            }
            let cachedData = CachedURLResponse(response: response!, data: data)
            cache.storeCachedResponse(cachedData, for: request)

            completionHandler(image)
        })
        task.resume()
            }}

    }
}