Im trying to upload images from the Photo app through Action Extension. I use NSURLSession to upload it in background. Here is the code i use.
var configName = "com.myapp.uploadImage"
var config = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier(configName)
config.sharedContainerIdentifier = "group.myApp.sample"
var session = NSURLSession(configuration: config)
let task = session.dataTaskWithRequest(request)
task.resume()
self.extensionContext!.completeRequestReturningItems(self.extensionContext!.inputItems, completionHandler: nil)
It works fine.
The question is when i upload an image and dismiss the view once and then again try uploading a second image while the initial process is still running in the background, the initial NSURLSession isn't completed. Only the second process gets completed. In short, the second session overcomes the first session.
I tried using NSOperationQueue. But action extension once dismissed and opened again for the second session, it just creates a new NSOperationQueue and hence the problem still persists.
Any suggestion will be helpful. Thanks in advance.
Make sure you don't attempt to instantiate a second background session with the same identifier while the first is still running. Save your background session so you can use it later.
As the "Background Session Considerations" of the URL Loading System Programming Guide: Using NSURLSession says:
Note: You must create exactly one session per identifier (specified when you create the configuration object). The behavior of multiple sessions sharing the same identifier is undefined.
Note, this document also mentions that one "must" specify and implement the delegate. (If nothing else, how will you know about failure if you don't do that?) The example provided in the Performing Uploads and Downloads section of the App Extension Programming Guide specifies the delegate, too.
Also, has your main app's app delegate implemented the handleEventsForBackgroundURLSession method? You have to capture the completionHandler and call it when the NSURLSessionDelegate method URLSessionDidFinishEventsForBackgroundURLSession is called.
Finally, I notice that you're using data task. The NSURLSession documentation is specific that one should not be using data tasks with background sessions, only upload/download tasks. I always assumed that was just so you don't try to use didReceiveData delegate method, but I might try using upload task just in case there's some other issue associated with data tasks with background sessions.
Related
I am trying to build a sequential download manager where the user can initiate up to 1000 download tasks at once but only 2 will actually download and the rest will be put on hold, until one of the two finishes so that another one of the 998 can start.
Because there is no way to add a downloadTask then have that task wait for others to complete then automatically start, I have built a download queue myself and will only create new downloadTask when the old one completes.
I have read the documentations and am well aware of the mechanism of iOS's handling of background download event, and what methods I need to implement in order to handle them correctly. However, I am unable to find anything on whether it is safe and reliable to start a new downloadTask when the old one completes IN BACKGROUND.
The Main Question:
When iOS relaunches my app in background to inform me that my download tasks are finished, can I reliably create a new downloadTask and add it to the current session? If so, when that task finishes as well, will the system relaunch my app AGAIN to tell me it's finished? If so, then I can create an infinite loop of adding new tasks when the old backgroundTask finish.
Code Example
Would the following code reliable to get the entire download queue downloaded(say, 1000 items) if I download ONLY one item at a time and meanwhile the app stays entirely in background?
extension MyClass: URLSessionDelegate {
func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
// This is called when background tasks are done
// Let's add a new background tasks WHILE app has just been
// relaunched IN BACKGROUND
if let nextURL = myURLQueue.removeFirst() {
session.downloadTask(with: URLRequest(url: nextURL))
}
DispatchQueue.main.async {
if let appDelegate = UIApplication.shared.delegate as? AppDelegate,
let completionHandler = appDelegate.backgroundSessionCompletionHandler {
appDelegate.backgroundSessionCompletionHandler = nil
completionHandler()
}
}
}
}
You are creating the URLSessionDownloadTask, but never starting it with resume().
FWIW, even if you fix the above, you should recognize that requests that are submitted while the app is running in background mode will always be submitted in discretionary mode. As the docs for isDiscretionary say:
For transfers started while your app is in the background, the system always starts transfers at its discretion—in other words, the system assumes this property is true and ignores any value you specified.
Also, recognize that having the system fire up your app every time a request is done in order to submit the next request is both inefficient (because your app will constantly fire up and go back into background) and slow (because concurrent requests reduce latency effects). It’s generally better to just submit all of the requests up front, while the app is active, and let the background session manage how many it will allow to run concurrently.
I have some misunderstanding in using NSURLSession framework, that's why I decided to write small app from scratch without AFFramework/Alamofire.
I have an API that requires following steps to upload file:
POST file data
Get response (JSON)
Post some json fields to api/save
I have a background session with such config:
let configuration = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("myBackground")
let session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: nil)
I've implemented 2 methods:
func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveData data: NSData)
where I aggregate all data
and
func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?)
where I tranform this data to response object. This response object if VERY important for me.
Everything works fine, while app is in foreground, but I have problems in background.
Case 1
App crashed right after I've started to upload data. According to WWDC I need to implement
func application(application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: () -> Void)
and call this handler in didCompleteWithError method. But before calling this method I need to call api/save with data from upload response.
How can I get this data?
Case 2
Mostly similar case. User stops app while upload is in progress. Than loads app in few seconds, while session works with my task. Now session calls didReceiveData, but of course, some of data is missing. What should I do in such case? How to restore response data?
You don't mention implementing URLSessionDidFinishEventsForBackgroundURLSession (a NSURLSessionDelegate method). You really want to implement that, too. The basic process is:
In app delegate's handleEventsForBackgroundURLSession, you should:
start the background NSURLSession (which will start receiving delegate method calls associated with all of the uploads); and
save the completion handler (but do not call it, yet).
Then, in URLSessionDidFinishEventsForBackgroundURLSession (when you're done processing all of the responses), you call the completion handler you saved in handleEventsForBackgroundURLSession. (Make sure to dispatch that to the main queue.)
If you're doing all of that, when the background session is restarted, the didReceiveData calls will come in with the responses to your various uploads.
I just did a quick test, uploading five 20mb images and immediately terminating the app. Then, even though the app wasn't running, I watched the five files slowly show up on my server (obviously handled by the daemon process). When all five were done, by app was transparently restarted in the background, the handleEventsForBackgroundURLSession was called (which restarted the session), it let all of the didReceiveData calls quickly get called, and when that was done, URLSessionDidFinishEventsForBackgroundURLSession was called and my app only then called the saved completion handler.
In terms of why this isn't working for you, there's not enough to diagnose the problem. Possibilities include:
Maybe you terminated the app inappropriately. You can't kill the app by double tapping the home button and terminating the app there; you have to let it naturally terminate on it's own, or for diagnostic/testing purposes, I force it to terminate by calling exit(0) in code.
Maybe you didn't restart the session when handleEventsForBackgroundURLSession was called.
Maybe you called the supplied completion handler too soon (i.e. before URLSessionDidFinishEventsForBackgroundURLSession was called).
It's hard to say, but I suspect that there's something buried inside your implementation that isn't quite right and it's hard to say what it is on the basis of the information provided (assuming it isn't one of the above points). Unfortunately, debugging this background sessions is vexingly complicated because when the app terminates, it is no longer attached to the debugger, so you can't easily debug what happens after the app is restarted automatically by iOS). Personally, I either NSLog messages and just watch the device console (as well as watching what appears on the server), or I build some persistent logging mechanism into the app itself.
For testing Background session code it is recommended to test on a real device. When writing an app that uses NSURLSession’s background session support, it’s easy to get confused by three non-obvious artifacts of the development process:
When you run your app from Xcode, Xcode installs the app in a new container, meaning that the path to your app changes. This can confuse NSURLSession’s background session support.
Note: This problem was fixed in iOS 9; if you encounter a problem with NSURLSession not handling a container path change in iOS 9 or later, please file a bug.
Xcode’s debugging prevents the system from suspending your app. So, if you run your app from Xcode, or you attach to the process some time after launch, and then move your app into the background, your app will continue executing in situations where the system would otherwise have suspended it.
Similarly, the iOS Simulator does not accurately simulate app suspend and resume; this has worked in the past but it does not work in the iOS 8 or iOS 9 simulators.
Source: Apple Developer Forum
I want to upload a file to a server using NSURLSession.
Cases Are:
1. It should resume uploading a file to the server from where it stopped because of app crash.
2. It should handle background upload as well.
Try AFNetworking Library to upload image asynchronously.You can find a brief example in this thread.
You should use background NSURLSession. If your app crashed or user left the app while upload was in progress, with a background NSURLSession the upload would continue seamlessly in the background. When the upload is done, your app will be notified of this via the delegate (and if your app wasn't alive at the time the download finished, it will be started in a background mode, at which point you can do whatever cleanup you need).
So create NSURLSessionConfiguration with backgroundSessionConfigurationWithIdentifier, and then instantiate a NSURLSession with that configuration.
There are a few caveats:
You cannot use completion handler pattern. You have to use delegate-based implementation.
You have to implement handleEventsForBackgroundURLSession in the app delegate, capturing the completionHandler it passes you and also instantiate the background session again. Likewise, in your NSURLSession delegate methods, you have to implement URLSessionDidFinishEventsForBackgroundURLSession, which will call the saved completion handler.
For more information, see Background Task Considerations in URL Session Programming Guide, see a section of the same name (but different text) in the NSURLSession class reference, or see the WWDC 2013 What's New in Foundation Networking, where Apple first introduced us to background sessions.
I do not understand the following and hope some does :-)
I am downloading a file from an url (10 minutes downloadtime), which works fine, when the download completes.
BUT:
When I simulate a crash or internet-interruption after a minute and restart the app again, it behaves strange to me.
In this case when restarting my app to download again with same sessionid, it seems that 2 download tasks are working in parallel. I recognizue this with a jumping progressbar from 10% to 0% and back. One, which starts from scratch and one, which I guess continious the old transfer. (not sure). I can restart again and then there is one more in the queue.
Can someone confirm this behaviour and does someone know how I can:
- continue only the interrupted download task (preferred :-) )
- or how can I start from scratch only.
Here my code for downloading, which works fine without any interruption.
func download_file(sURL: String, sToLocation: String) {
println("start downloading ...");
println("sURL : " + sURL + " sToLocation : " + sToLocation);
bytesDownloaded=0;
var delegate = self;
delegate.storePath=sToLocation;
delegate.progressView=progressView;
struct SessionProperties {
static let identifier : String! = "url_session_background_download"
}
var configuration = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier(SessionProperties.identifier)
var backgroundSession = NSURLSession(configuration: configuration, delegate: delegate, delegateQueue: nil)
//myURLSession = NSURLSession(configuration: configuration, delegate: delegate, delegateQueue: nil)
var url = NSURLRequest(URL: NSURL(string: sURL)!)
var downloadTask = backgroundSession.downloadTaskWithRequest(url)
//downloadTask.cancel()
downloadTask.resume()
}
Update
When I am using a different sessionid, the Download starts from scratch. The previously started download task runs still in the background. So I am wondering why can't I resume the previously download task by using the old sessionid without starting a new download in parallel ?
The whole point of using a download task is so that downloads can continue even if your app isn't running or if it crashes. You don't need to resume the download. It is actually happening in a separate background daemon. You just need to re-create the session using the same ID.
After you re-create the session, any existing downloads are automatically associated with the new session, and your delegate methods are called whenever the task completes.
There's also a little bit of magic that you need to implement for handling background launches if your app isn't running when the download finishes. See URL Session Programming Guide for details.
In addition to what dgatwood said, remember to have to have your app delegate implement application:handleEventsForBackgroundURLSession:completionHandler:, which is called if the download finishes and your app isn't running at the time. When this method is called (again, only if download finishes when your app wasn't running), you should (a) save the completionHandler; (b) instantiate the background session with the same identifier; (c) let your NSURLSessionDownloadDelegate method be called (at which point which didFinishDownloadingToURL: is called) and when (d) URLSessionDidFinishEventsForBackgroundURLSession is called, if you have a completionHandler saved from when handleEventsForBackgroundURLSession was called, then this is the appropriate time to call this saved completionHandler.
The basic idea is as follows: If your app wasn't already running when the download finishes, the OS seamlessly starts your app in the background (unbeknownst to the end user), providing a reference to this completionHandler, you then have the app do all that it needs to do to move this downloaded file to its new location, and when it's all done, you call the saved completionHandler to let the OS know that you're all done handling the downloaded file in the background execution and the app can safely be suspended again (i.e. to avoid keeping the app running in the background, adversely affecting the UX and battery for the user).
Obviously, if the app happens to be running and has the NSURLSession already instantiated, you won't see these background related events taking place. If the app was running when the download finishes, it behaves much like a foreground NSURLSession and the above is not called upon. But you need that logic in case your app wasn't running when the download finishes.
See Downloading Content in the Background section of The App Programming Guide for iOS, which describes this process. Also refer to WWDC 2013 video in What’s New in Foundation Networking (it's covered later in the video).
Perhaps I have been reading the wrong stuff, but one thing that all of the literatures that I have been reading seem to agree on is that: iOS does not allow background threads to run for longer than ten minutes. That seems to violate one of the greatest principles of app development: the internet should be invisible to your users. So here is a scenario.
A user is going through a tunnel or flying on an airplane, which causes no or unreliable network. At that instant, the user pulls out my email app, composes an email, and hits the send button.
Question: How do I the developer make sure that the email is sent when network becomes available? Of course I am using email as a general example, but in reality I am dealing with a very much simple http situation where my app needs to send a POST to my server.
Side Note: on android, I use Path’s priority job queue, which allows me to set it and forget it (i.e. as soon as there is network it sends my email).
another Side Note: I have been trying to use NSOperationQueue with AFNetworking, but does not do it.
What you want to achieve can be done using a background NSURLSession. While AFNetworking is based on NSURLSession I’m not quite sure if it can be used with a background session that runs while your app doesn’t. But you don’t really need this, NSURLSession is quite easy to use as is.
As a first step you need to create a session configuration for the background session:
let config = URLSessionConfiguration.background(withIdentifier: "de.5sw.test")
config.isDiscretionary = true
config.waitsForConnectivity = true
The isDiscretionary property allows the system to decide when to perform the data transfer. waitsForConnectivity (available since iOS 11) makes the system wait if there is no internet connection instead of failing immediately.
With that configuration object you can create your URL session. The important part is to specify a delegate as the closure-based callbacks get lost when the app is terminated.
let session = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main)
To perform your upload you ask the session to create an upload task and then resume it. For the upload task you first create your URLRequest that specifies the URL and all needed headers. The actual data you want to upload needs to be written to a file. If you provide it as a Data or stream object it cannot be uploaded after your app terminates.
let task = session.uploadTask(with: request, fromFile: fileUrl)
task.resume()
To get notified of success or failure of your upload you need to implement the URLSessionDataDelegate method urlSession(_:task:didCompleteWithError:). If error is nil the transfer was successful.
The final piece that is missing is to handle the events that happened while your app was not running. To do this you implement the method application(_:handleEventsForBackgroundURLSession:completionHandler:) in your app delegate. When the system decides that you need to handles some events for background transfers it launches your app in the background and calls this method.
In there you need first store the completion handler and then recreate your URLSession with the same configuration you used before. This then calls it’s delegate for the events you need to handle as usual. Once it is done with the events it calls the delegate method urlSessionDidFinishEvents(forBackgroundURLSession:). From there you need to call the completion handler that was passed to your app delegate.
The session configuration provides some more options:
timeoutIntervalForResource: How long the system should try to perform your upload. Default is 7 days.
sessionSendsLaunchEvents: If false the app will not be launched to handle events. They will be handled when the user opens the app manually. Defaults is true.
Here is a small sample project that shows how everything fits together: https://github.com/5sw/BackgroundUploadDemo
Your app needs to store the data internally and then you either need something which will cause the app to run in the background (but you shouldn't necessarily add something specially if you don't already have a reason to be doing it) or to wait until the user next brings the app to the foreground - then you can check for a network connection and make the call.
Note that e-mail is very different to a POST, because you can pass an e-mail off to the system mail app to send for you but you can't do exactly the same thing with a POST.
Consider looking also at NSURLSessionUploadTask if you can use it.
In three words: you don't.
And that's actually a good thing. I certainly do not want to have to think and speculate about my last 20 apps, if they are still running in the background, using memory and battery and bandwidth. Furthermore, they would be killed if more memory is needed. How would the user be able to predict if it completed its task successfully? He can't, and need to open the app anyhow to check.
As for the email example, I'd go with showing the email as "pending" (i.e. not sent), until it transferred correctly. Make it obvious to the user that he has to come back later to fulfill the job.
While every developer thinks that his app has an extremely good reason for backgrounding, reality is, for the user in 99% it's just a pain. Can you say "task manager"? ;-)
I wrote a pod that does pretty much this - https://cocoapods.org/pods/OfflineRequestManager. You'd have to do some work listening to delegate callbacks if you want to monitor whether the request is in a pending or completed/failed state, but we've been using it to ensure that requests go out in poor or no connectivity scenarios.
The simplest use case would look something like the following, though most actual cases (saving to disk, specific request data, etc.) will have a few more hoops to jump through:
import OfflineRequestManager
class SimpleRequest: OfflineRequest {
func perform(completion: #escaping (Error?) -> Void) {
doMyNetworkRequest(withCompletion: { response, error in
handleResponse(response)
completion(error)
})
}
}
///////
OfflineRequestManager.defaultManager(queueRequest: SimpleRequest())