Continue download on background / other screen on my ios app - ios

Question

I have a download function working perfectly, on click of my button, the file is being download correctly.
What I am trying to achieve, If I put the App in the background or change screen while it's downloading the file, I'd like the download to be continued. Currently if the App is in the background ,the download stop and user need to restart it from zero.
How is this possible to make ?? Below is my download function, I tried to add a background download without success.
func createDownloadTask() {
let downloadRequest = URLRequest(url: URL(string: "\(posts[selectedIndexPath].link)")!)
let session = Foundation.URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: OperationQueue.main)
downloadTask = session.downloadTask(with: downloadRequest)
downloadTask!.resume()
}
and
#IBAction func startDownload(_ sender: UIButton) {
let urlString = "\(posts[selectedIndexPath].link)"
DispatchQueue.global(qos: .default).async(execute: {
print("downloadVideo");
let url=NSURL(string: urlString);
let urlData=NSData(contentsOf: url! as URL);
if((urlData) != nil)
{
let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
let fileName = urlString as NSString;
let filePath="\(documentsPath)/\(fileName.lastPathComponent)";
DispatchQueue.main.async(execute: { () -> Void in
print(filePath)
urlData?.write(toFile: filePath, atomically: true);
print("video Saved to document directory of app");
self.downloadButton.alpha = 0;
self.progressView.alpha = 0;
self.cardboardButton.alpha = 1;
self.videoButton.alpha = 1;
self.removefile.alpha = 1;
})
}
})
}

Answer 1

You need to use UIBackgroundTaskIdentifier. You can start some task in background with beginBackgroundTask see:
https://developer.apple.com/reference/uikit/uiapplication/1623031-beginbackgroundtask.
System will give you some time and you can finish your task. If the time expire you will receive handler.See example that I use to upload a image
static var backgroundUpload:UIBackgroundTaskIdentifier?
static func praiseCollegue(praiseCollegueId:Int,image:UIImageView,praisedBy:Int,praiseText:String,praiseCategory:Int,successBlock: (()->())?,errorBlock: (()->())?){
backgroundUpload = UIApplication.shared.beginBackgroundTask(expirationHandler: {
if let task = self.backgroundUpload{
UIApplication.shared.endBackgroundTask(task)
}
})
let url = NSURL(string: "someUrl")
let request = NSMutableURLRequest(url: url! as URL)
request.httpMethod = "POST"
request.setValue(InfoManager.sharedInstance.tokenInfo?.accessToken, forHTTPHeaderField: "AccessToken")
let session = URLSession.shared
let task = session.dataTask(with: request as URLRequest) {
(
data, response, error) in
if let responseData = data{
do {
if let dict = try JSONSerialization.jsonObject(with: responseData, options: []) as? [String: AnyObject]{
if let errorCode = dict["errorCode"] as? Int{
if(errorCode == 1){
successBlock?()
if let task = self.backgroundUpload{
UIApplication.shared.endBackgroundTask(task)
}
}
else{
errorBlock?()
if let task = self.backgroundUpload{
UIApplication.shared.endBackgroundTask(task)
}
}
}
}
}
}
}
task.resume()
}