Question

这是代码。这是我的UICollectionViewDataSource课程。

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let identifier = "UICollectionViewCell"
    let photo = photos[indexPath.row]
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: identifier, for: indexPath) as! PhotoCollectionViewCell
    let url = ImageUploaderAPI.urlForPhotoPath(photoTitle: photo.remoteURL)

    if (photo.image == nil) {
        Alamofire.download(url).downloadProgress { progress in
                print("Download Progress: \(progress.fractionCompleted)")
            }
            .responseData { response in
            if let data = response.result.value {
                let image = UIImage(data: data)
                photo.image = image!
                cell.updateWithImage(image: image)
                print("Downloaded: " + url.absoluteString)
                collectionView.reloadData()
            }
        }
    } else {
        cell.updateWithImage(image: photo.image)
    }
    return cell
}

progress.fractionCompleted显示正在下载图像，但我不确定为什么没有图像正在更新。是因为Alamofire如何异步工作？任何建议将不胜感激。

Answer 1

以下是3种可能的解决方案。第一个是因为后台线程问题。而不仅仅是collectionView.reloadData，请尝试使用：

DispatchQueue.main.async { 
collectionView.reloadData
}

另一个可能的解决方案是.resume问题？您可能想尝试添加.resume，如下例所示：

if (photo.image == nil) {
        Alamofire.download(url).downloadProgress { progress in
                print("Download Progress: \(progress.fractionCompleted)")
            }
            .responseData { response in
            if let data = response.result.value {
                let image = UIImage(data: data)
                photo.image = image!
                cell.updateWithImage(image: image)
                print("Downloaded: " + url.absoluteString)
                collectionView.reloadData()
            }
        }.resume
    } else {
        cell.updateWithImage(image: photo.image)
    }
    return cell
}

我的第三个也是最后一个解决方案是简单地摆脱if (photo.image == nil) {

希望这有帮助

Answer 2

我最终这样做了：

cell.activityIndicator.startAnimating()
    if (photo.image == nil) {
        let dataTask = URLSession.shared.dataTask(with: url) {
            data, response, error in
                if error == nil {
                    if let data = data {
                        let image = UIImage(data: data)

                        print("Downloaded: " + url.absoluteString)

                        DispatchQueue.main.async {
                            photo.image = image!
                            collectionView.reloadItems(at: [indexPath])

                        }
                    }
                } else {
                    print(error)
                }
        }
        dataTask.resume()
    } else {
        cell.updateWithImage(image: photo.image)
    }
    cell.activityIndicator.stopAnimating()
    cell.activityIndicator.isHidden = true

    return cell

collectionview只在cellForItemAtIndexPath中下载一个图像

2 个答案: