swift - Swift Haneke 只缓存 UITableView 前三行的图片

Question

在我TableViewCell使用这种方法从我的服务器中检索 URL，然后我创建一个视频的缩略图，最后将它插入到UIImageView.

var URL: String? {
    didSet {

        if gotThumb == false {
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
                let videoUrl = NSURL(string: Somelink )

                let asset = AVAsset(URL: videoUrl!)
                let assetImgGenerate = AVAssetImageGenerator(asset: asset)
                assetImgGenerate.appliesPreferredTrackTransform = true
                let time = CMTimeMake(asset.duration.value / 3, asset.duration.timescale)
                dispatch_async(dispatch_get_main_queue(), {
                    if let cgImage = try? assetImgGenerate.copyCGImageAtTime(time, actualTime: nil) {

                        self.thumbImageView.hnk_setImage(UIImage(CGImage: cgImage), key: "\(self.thumbImageView.tag)", placeholder: nil, format: nil, success: { (image) -> () in
                            print("Got Thumb")

                            self.thumbImageView.image = image
                            self.gotThumb = true
                        })
                    }
                })
            })
        } else {

            print("Already have an image")
        }
        print(URL!)
    }
}

正如我的标题所述。Haneke 仅缓存由上述方法创建的前三个图像。发生的事情是，无论ImageViews我有多少，TableView它们都被前三个图像填充

我很纠结如何解决这个问题。