Download Progress stopped on Screen lock in iOS - xamarin

Question

I have developed an Xamarin forms application. Provided download option to download file in our application. I have clicked download file and download progress show in app itself. If lock the iphone while download is in progress and unlock it again download stopped. How can I process download even locked phone?. This occurs only in iOS and works properly in Android.
I have used webclient DownloadFileTaskAsync process to download a file and maintain progress value in it.
WebClient client = new WebClient();
client.DownloadFileTaskAsync(new Uri(DownloadUrl), FileName);
Updated Query:
I have implemented the back-grounding process into my source. I have used "Creating Background-Safe Tasks" concept for my download process. I can download more than one file, so put this process in Task itself previously. Now, I have used BeginBackgroundTask in my download process but download process not carried to UI, even BeginBackgroundTask code doesn't hit while debug the code.
Below function put in native and called this function from forms when click download button.
public async Task DownloadFile(string DownloadUrl, string FileName)
{
var taskID = UIApplication.SharedApplication.BeginBackgroundTask(async() =>
{
WebClient client = new WebClient();
client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
client.DownloadFileTaskAsync(new Uri(DownloadUrl), FileName);
});
UIApplication.SharedApplication.EndBackgroundTask(taskID);
}
Also registered the app into Background fetch registration categories and now also progress not carried out while lock screen or minimize the app.
Am I missing any process in background techniques? Could you please help me to resolve this or suggest some simple sample as my scenario?
Please help me on this to resolve it.
Regards,
Cheran

Answer 1

You will need to use NSURLSession instead and create a background session for it. You can read how to do this in the official Xamarin Documentation.
You might get away with using WebClient in a background session, but only for a very short time span by using the pattern described in the "Handle iOS Background Limits" docs:
Task.Factory.StartNew(() =>
{
//expirationHandler only called if background time allowed exceeded
var taskId = UIApplication.SharedApplication.BeginBackgroundTask(() => {
Console.WriteLine("Exhausted time");
UIApplication.SharedApplication.EndBackgroundTask(taskId);
});
while(myFlag == true)
{
Console.WriteLine(UIApplication.SharedApplication.TimeRemaining);
myFlag = SomeCalculationNeedsMoreTime();
}
//Only called if loop terminated due to myFlag and not expiration of time
UIApplication.SharedApplication.EndBackgroundTask(taskId);
});
Where you would replace the while with your WebClient code.
However, as mentioned you really need to use Background Transfers for this to work properly.