Observing progress while using URLSessionDataTask and URLSessionDownloadTask both for same remote url

Question

I'm using URLSessionDataTask to show an image, and using URLSessionDownloadTask to download the image on my app. Also I needed to observe download progress, so I use KVO at Progress's fractionCompleted.

The thing is fractionCompleted value just has 0.05 and 1.0. I think this issue happened for using same remote url. Because when I don't use URLSessionDataTask(I can't show the image though), I get right progress update.

Why is it happened? Is there any reference?

Edited

First, I don't want to use URLSession delegate to get progress. I'm using two VCs(first one is RemoteGridViewController using URLSessionDataTask to show image and second one is MediaViewerController). MediaViewerController delegate to first VC for downloading the image on camera roll. I know I can use URLSession delegate method to notify progress by using NotificationCenter or something like that, but I don't want. This way makes strong coupling(?) between the VCs(I guess...)

Second, RemoteGridViewController use URLSession to show an image by URLSessionDataTask and cache it.

guard let url = URL(string: url) else {
    completionHandler(.failure(NetworkManagerError.urlError))
    return
}

var req = URLRequest(url: url)
req.cachePolicy = .returnCacheDataElseLoad

if let cachedResponse = sharedCache?.cachedResponse(for: req) {
    completionHandler(.success(cachedResponse.data))
} else {
    sessionDataTask = session.dataTask(with: req) { [weak self] (data, response, error) in
        if let error = error {
            completionHandler(.failure(error))
            print(error.localizedDescription)
            return
        }
        
        guard let response = response as? HTTPURLResponse, let data = data else {
            completionHandler(.failure(NetworkManagerError.dataError))
            return
        }
        self?.sharedCache?.storeCachedResponse(CachedURLResponse(response: response, data: data), for: req)
        completionHandler(.success(data))
    }
    
    sessionDataTask?.resume()
}

Third, MediaViewerController call MediaViewerDelegate method, which is downloadImage(itemAt:for:) to download an image.

extension RemoteGridViewController: MediaViewerDelegate {
    func downloadImage(itemAt indexPath: IndexPath, for mediaViewer: MediaViewerController) {
        let data = unsplashData[indexPath.item]
        let urlString = data.regular
        
        guard let url = URL(string: urlString) else { return }
        
        mediaViewer.presentProgressView()

        networkManager.downloadTaskForImage(with: url, progressHandler: mediaViewer.observe(_:)) { result in
            switch result {
            case .success(let image):
                DispatchQueue.main.async {
                    mediaViewer.toastView(with: true)
                }
                UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil)
            case .failure(let error):
                DispatchQueue.main.async {
                    mediaViewer.toastView(with: false)
                }
                print(error.localizedDescription)
            }
        }
    }
}

class NetworkManager {
    private let session: URLSession!
    private var sessionDownloadTask: URLSessionDownloadTask?
    
    func downloadTaskForImage(with url: URL, progressHandler: (Progress?) -> (), completionHandler: @escaping (Result<UIImage, Error>) -> ()) {
        sessionDownloadTask = session.downloadTask(with: url) { (location, response, error) in
            if let error = error {
                completionHandler(.failure(error))
                return
            }
            
            guard let response = response as? HTTPURLResponse, (200...299).contains(response.statusCode) else {
                completionHandler(.failure(NetworkManagerError.responseError))
                return
            }
            
            guard let location = location else {
                completionHandler(.failure(NetworkManagerError.urlError))
                return
            }
            
            do {
                let data = try Data(contentsOf: location)
                if let image = UIImage(data: data) {
                    completionHandler(.success(image))
                } else {
                    completionHandler(.failure(NetworkManagerError.dataError))
                }
            } catch let error {
                completionHandler(.failure(error))
            }
        }
        
        progressHandler(sessionDownloadTask?.progress)
        sessionDownloadTask?.resume()
    }
}

Answer 1

You can track for the downloaded bytes to total expected bytes to track the progress of downloading image.

Use following delegate method:

URLSession(_:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite:)

and following snippet:

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        // println("download task did write data")

        let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
        
        //Your code to download and save image to camera role
    }
}

I hope this may have resolved your query to check for exact progress with downloading.