在 UI 集合视图中滚动时，单元格会重新加载错误的图像

Question

我正在使用 Dropbox 的 API 下载图像并将其显示在集合视图中。当用户滚动时，图像要么从单元格中消失并加载另一个图像，要么重新加载新图像并替换单元格中的图像。如何防止这种情况发生？我尝试使用SDWebImage，这使图像保持正确的顺序，但每次滚动屏幕时图像仍然会消失并重新加载。此外，我直接下载图像，而不是从 URL 下载图像，我宁愿不必编写一轮工作即可使用 SDWebImage。

我会发布一个 gif 作为示例，但我的声誉太低了。

欢迎任何帮助:)

var filenames = [String]()
var selectedFolder = ""
    // image cache
    var imageCache = NSCache<NSString, UIImage>()
override func viewDidLoad() {
    super.viewDidLoad()
    getFileNames { (names, error) in
        self.filenames = names
        if error == nil {
            self.collectionView?.reloadData()
            print("Gathered filenames")
        }
    }
    collectionView?.collectionViewLayout = gridLayout
    collectionView?.reloadData()
override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(true)
}

func getFileNames(completion: @escaping (_ names: [String], _ error: Error?) -> Void) {
    let client = DropboxClientsManager.authorizedClient!
    client.files.listFolder(path: "(selectedFolder)", recursive: false, includeMediaInfo: true, includeDeleted: false, includeHasExplicitSharedMembers: false).response { response, error in
        var names = [String]()
        if let result = response {
            for entry in result.entries {
                if entry.name.hasSuffix("jpg") {
                    names.append(entry.name)
                }
            }
        } else {
            print(error!)
        }
        completion(names, error as? Error)
    }
}

func checkForNewFiles() {
    getFileNames { (names, error) in
        if names.count != self.filenames.count {
            self.filenames = names
            self.collectionView?.reloadData()
        }
    }
}
func downloadFiles(fileName: String, completion:@escaping (_ image: UIImage?, _ error: Error?) -> Void) {
    if let cachedImage = imageCache.object(forKey: fileName as NSString) as UIImage? {
        print("using a cached image")
        completion(cachedImage, nil)
    } else {
        let client = DropboxClientsManager.authorizedClient!
        client.files.download(path: "(selectedFolder)(fileName)").response { response, error in
            if let theResponse = response {
                let fileContents = theResponse.1
                if let image = UIImage(data: fileContents) {
                    // resize the image here and setObject the resized Image to save it to cache.
                    // use resized image for completion as well
                    self.imageCache.setObject(image, forKey: fileName as NSString)
                    completion(image, nil) // completion(resizedImage, nil)
                }
                else {
                    completion(nil, error as! Error?)
                }
            } else if let error = error {
                completion(nil, error as? Error)
            }
            }
            .progress { progressData in
        }
    }

}

override func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
}

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return self.filenames.count
}

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! ImageCell
    cell.backgroundColor = UIColor.lightGray
    let fileName = self.filenames[indexPath.item]
    let cellIndex = indexPath.item
    self.downloadFiles(fileName: fileName) { (image, error) in
        if cellIndex == indexPath.item {
          cell.imageCellView.image = image
          print("image download complete")
        }
    }
    return cell
}
override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)
    gridLayout.invalidateLayout()
}
override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    imageCache.removeAllObjects()
}

Answer 1

因为 TableView 和 CollectionView 使用

dequeueReusableCell(withReuseIdentifier: for indexPath:)功能，当您配置新单元格时，swift 在桌子下所做的是使用屏幕外的单元格来帮助手机的内存，并且该单元格可能已经具有图像集，您必须处理这种情况。

我建议你看看方法" prepareCellForReuse "在这种情况下，我认为你要做的是将imageView.image atribute 设置为nil。

我很确定它会解决你的问题或给你正确的方向，但如果它不起作用，请告诉我，我会尽力帮助你。

最佳结果。

Answer 2

我修复了它。它需要在cellForItemAt func中设置单元格图像= nil，如果用户在完成下载之前将单元格滚动到屏幕外，则取消图像请求。下面是新的cellForItemAt代码：

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let fileId = indexPath.item
    let fileName = self.filenames[indexPath.item]
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! ImageCell
    cell.backgroundColor = UIColor.lightGray
    if cell.request != nil {
        print("request not nil; cancel ", fileName)
    }
    cell.request?.cancel()
    cell.request = nil
    cell.imageCellView.image = nil
    print ("clear image ", fileId)
    self.downloadFiles(fileId:fileId, fileName: fileName, cell:cell) { (image, error) in
        guard let image = image else {
            print("abort set image ", fileId)
            return
        }
        cell.imageCellView.image = image
        print ("download/cache: ", fileId)
    }
    return cell
}    

Answer 3

使用 SDWebImage 并添加占位符图像：

cell.imageView.sd_setImage(with: URL(string: "http://www.domain.com/path/to/image.jpg"), placeholderImage: UIImage(named: "placeholder.png"))

Answer 4

我发布这个以防它可以帮助某人。我有一个集合视图(显示为垂直列表(，其项目是集合视图(显示为水平单行网格(。滚动列表时，重复子集合视图中的图像。

我通过将其放置在父集合视图的单元格类中来解决它。

override func prepareForReuse() {
    collectionView.reloadData()
    super.prepareForReuse()
}