Showing the file download progress with NSURLSessionDataTask

Question

I want to display file download progress (how many bytes are received) of particular file. It works fine with the NSURLSessionDownloadTask .My question is I want to achieve

Answer 1

Since iOS 11.0 and macOS 10.13, URLSessionTask (former NSURLSessionTask) adopted the ProgressReporting protocol. That means you can use the progress property to track the progress of a session task.

Expecting you already know how to use KVO observers, you can do something like:

task = session.downloadTask(with: url)
task.resume()
task.progress.addObserver(self, forKeyPath: "fractionCompleted", options: .new, context: &self.progressKVOContext)

and observe the value with:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if context == &self.progressKVOContext, let keyPath = keyPath {
        switch keyPath {
        case "fractionCompleted":
            guard let progress = object as? Progress else {
                return
            }
            DispatchQueue.main.async { [weak self] in
                self?.onDownloadProgress?(progress.fractionCompleted)
            }
        case "isCancelled":
            cancel()
        default:
            break
        }
    }
    else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    }
}

iOS 13 update

OperationQueue has now a progress property.

For example, the UIProgressView can observe that property and update the progress value automatically using the observedProgress. Full example in https://nshipster.com/ios-13/#track-the-progress-of-enqueued-operations.