SDWebImageDownloader返回nil值

Question

SDWebImageDownloader返回nil而不是SDWebImageOperation对象。下面是一些代码。

class PictureProvider: UIActivityItemProvider {
    private var originalLoadingOperation: SDWebImageOperation?

    fileprivate func loadOriginalImage(with link: String) {

        let imageDownloader = SDWebImageDownloader.shared()
        originalLoadingOperation = imageDownloader!.downloadImage(with: URL(string: link),
                                                               options: .continueInBackground,
                                                              progress: nil,
                                                             completed: { (image, data, error, finished) in 

                                                                 if image != nil
                                                                 {
                                                                     SDImageCache.shared().store(image, forKey: link)
                                                                 }
        })
    }
}

originalLoadingOperation总是＆＃39; nil＆＃39;。