I create a timer,through open the GPS keep App alive every 3 mintues,and then upload data.Then I found the timer was suspended,but the app was alive.Please someone tell me?
You need to become acquainted with Background Execution, here are few useful links I've found so far:
https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html
https://blog.newrelic.com/2016/01/13/ios9-background-execution/
https://www.raywenderlich.com/143128/background-modes-tutorial-getting-started
I'm trying to reach the file which I was downloading earlier by NSURLSession. It seems I can't read the location of the file, even though I'm doing it before delegate method ends (as the file is temporary).
Still, I'm getting nil when trying to access the data under location returned from NSURLSession delegate and error 257.
The code goes as following:
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSError *movingError = nil;
NSData *fileData = [NSData dataWithContentsOfFile:location.path options:0 error:&movingError]; // is nil
NSLog(#"%#", movingError); // is error 257
}
What's wrong with this code..? I saw similar questions NSURLSessionDownloadTask - downloads but ends with error and iPhone - copying a file from resources to Documents gives an error but these completely doesn't apply to my case.
-- edit --
I've created a new project and pasted the very same code. It works... So:
1) In my project I'm receiving error 257, probably some configuration of the project is invalid or the fact I'm using backgroundTasks somewhere else in the app messes things up
2) Same as in 1 happens if I put the source files of this download to the external framework linked in by Carthage
3) On demo project I created (copy-pasted files used in 1 & 2) everything works corretly.
If someone has an idea what can cause the fact it isn't working - would be awesome.
Late to the discussion, but I've seen the same/similar issue and investigated.
With my investigation, what you are seeing is expected (should I say “it's observed”?).
But since I'm not 100% sure how you are using URL session, I put down below what I'm trying to do, followed by my observations (important ones only).
Also, I created a sample project and put it here as downLoader.zip. You can play and check how URL session background download works. But It's better to read thru below before trying to play, though this is fairly a long note.
A. what I'm trying to do
I'm developing kind a map app, where I need to download 1000+ of small size (0.5-150kB, mostly ~20kB) PNG files at a time. Total download size of them is ~50MB, and it takes a couple of minutes to download all of them. I think that keeping users to my app just waiting for download is bad design, so I made the app to use URL Session’s background download.
I have to admit, however, that Apple’s doc says that “Background sessions are optimized for transferring a small number of large resources that can be resumed as necessary.” So, the way I’m using background session is totally opposite. But, anyway...
B. what I've observed
Below, I’ve listed my observations and my guess of why. I should say guess since they are not documented.
(1) observation: sometimes, files that should come with didFinishDownlaodingTo don’t exist.
Temporary files are put at:
/var/mobile/Containers/Data/Application/randomHexString/Library/Caches/com.apple.nsurlsessiond/Downloads/yourName.yourApp. The randomHexString changes from app’s run to run. When the files don’t exist, the randomHexString that came with didFinishDownloadingTo is the one from the “previous” session. Now, “previous” here means the session at app’s previous run !, which clearly doesn't exist at the current session at current run.
There's another scenario for non-existent file, which is that the randomHexString is ok, but the files don't exist. This seems to happen after session cancel is requested (invalidateAndCancel) and before session becomes invalid (didBecomeInvalidWithError).
Guess: This happens especially on the development phase since we terminate the app while downloading, manually or by debugger or just by bug. It seems once the download requests are handed and accepted to the OS, the OS handles our requests even after the app quits. Note we can’t know if the OS has definitely accepted the requests or not. Even after the return from URLSessionDownloadTask:resume(), sometimes files don’t exit at the next launch and sometimes they do.
Workaround: If files don’t exist, just ignore them. In a while, “this time’s” files should come.
(2) observation: sometimes, the duplicated (same) files come with didFinishDownloadingTo.
My app converts downloaded PNG files to other format. W/in didFinishDownlaodingTo, I move temp files (== OS designated) to another my app’s directory, then spawn thread (GCD) to convert the format and delete downloaded temp files. So, the other thread (didFinishDownlaodingTo) to overwrite is an issue.
Workaround: I make list of URLSessionTask:taskIdentifier and w/in didFinishDownlaodingTo, check the duplication by retrieving the taskID list to ignore duplicated files.
(3) observation: even after the user terminated the app, OS relaunches the app again.
After the user terminated the app from task switcher, quite often, the OS relaunches the app w/ application:handleEventsForBackgroundURLSession:completionHandler.
Note the sequence of relaunch is that didFinishLaunchingWithOptions comes first as regular launch, then handleEventsForBackgroundURLSession comes next.
From user’s POV, when he/she terminated the app, that’s it, done! It looks strange even after they terminated the app, it relaunches in itself and notifies something to them. It's like a zombie.
Guess: The Apple’s document says “If the user terminates your app, the system cancels any pending tasks”. The definition of “pending tasks” is not clearly stated but I understand this is from iOS POV and not user’s or program developer’s. As guessed in (1), iOS seemed to have accepted the download requests, and they are not “pending tasks” anymore.
Workaround: I create a “flagFile” after all of download requests are handed to iOS w/ URLSessionDownloadTask:resume(), of which file means that “download is ongoing”. When users terminate the app, at UIApplicationDelegate:applicationWillTerminate, I delete the flag file. Or, if all of download requests not handed to iOS, there's no flag file. Then at app’s relaunch at UIApplicationDelegate:handleEventsForBackgroundURLSession, I check if we have the flag file. If missing, then I can assume that users terminated. Two choices here. Choice-1: I will not recreate URL session. What happens next is that iOS will terminate my app in about 20 seconds. I have no idea if this (== not creating the URL session) is a legal operation but it works. Users can launch w/in this 20 sec, so I put some more code to handle that scenario. Choice-2: I create URL session. What happens next is that iOS calls delegate methods didFinishDownlaodingTo/didCompleteWithError, followed by urlSessionDidFinishEvents. If I don't do anything here, the process (app) keeps alive indefinitely w/o any notification to users: nothing in task switcher. This is nothing more than waste of memory. The option here is to fire local notification and let users know of my app, so that they can go back to my app and can terminate (Again!), though my app clearly appears as a zombie. One issue for both choices: applicationWillTerminate may not be called in certain situation (though I've yet confirmed). At this case, flag file is left as regular ops and show zombie to users. So, the flag file method is just mitigation to the issue, but I think it works most of the time for my app.
Note the app is relaunched sometimes when it's killed by xcode debugger or killed by OS w/ bug (SEGFALUT).
(4) observation: after the app is terminated (by user, etc.) then relaunched by OS, the app is occasionally in active state (UIApplication.shared.applicationState is .active).
I want to notify the user on the download completion by local notification, but since it's active, local notification doesn't fire. So, I need to use UIAlertController instead. Therefore, I can't provide consistent user experience, and should look strange for users: most of the time local notification and very occasionally UIAlert. Note when app started in active state, it appears in task switcher.
Guess: totally no idea how this can happen. One good(?) thing is this happens only occasionally.
Workaround: seems none.
(5) observation: handleEventsForBackgroundURLSession/urlSessionDidFinishEvents is called just once.
I start the download after started background task (application.beginBackgroundTask). Then in expiration handler of beginBackgroundTask, I call endBackgroundTask. I don't know exactly why, but after endBackgroundTask, my process is still given lots of process time, so I can keep requesting download. This might be because download files keep coming w/ didFinishDownlaodingTo. To be a good citizen, I suspend to request further download, and fire local notification to user to put the app to foreground. Now, once I suspend the request, in 4-5 seconds, OS determines the URL session is over and handleEventsForBackgroundURLSession then urlSessionDidFinishEvents are called. This is one-off event. When the user put the app in foreground to resume download, then put it again in background, no handleEventsForBackgroundURLSession/urlSessionDidFinishEvents will come anymore. What's unclear to me is the definition of session's start and end. It seems the session starts at first URLSessionTask.resume(), then ends w/ timeout, which seems to be determined by URLSessionConfiguration.timeoutIntervalForRequest. However, setting large number here (1000 sec) doesn't affect anything, it's always 4-5 seconds.
Guess: no idea
Workaround: don't relay on urlSessionDidFinishEvents while the app is alive. Relay only when app relaunched by OS and at initial handleEventsForBackgroundURLSession/urlSessionDidFinishEvents.
===============
Below, I listed about the sample project (downLoader.zip). You can verify all of above with the sample.
The app has a list of download files. The number of files is 1921, and 56MB in total. They are 256x256 PNG map tile files that are located in a server managed by Geo Spatial Information Authority of Japan (GSI). After downloaded, they are moved to Library/Cache/download. If your device is jail-broken, you can view them w/ Filza.
crash itself to test relaunch
emulate background task expiration
logging to file since debugger doesn't work after relaunch by OS. The file is in Documents, can be moved to PC.
Do play with the real device. Simulator doesn't relaunch the app.
Project built w/ Xcode 8.3.3 and tested w/ iphone6+/9.3.3 and iphone7+/10.3.1
To see if app relaunched w/ xcode, go to Debug/Attach to Process menu and see if downLoader is listed.
===============
I think URL session background download behaves trickily, especially at relaunch. We need to consider at least the observations I listed above, or the app users will get confused.
I found same error code 257 from online users feedback.In the error scene, the location always refers to a strange bundle identifier:com.sdyd.SDMobileMixSmart. And I guess this's an apple system bug.
I have implemented NSURLSession for downloading fairly large files from our servers. Now as long as I'm working in foreground or background and go back to the app the transactions are working and getting finished.
But if I force-quit the app using the the multitasking screen and re-open the app again. the downloading process is not getting finished although as I understood from the docs, it should, here what the docs state:
If an iOS app is terminated by the system and relaunched, the app can use the same identifier to create a new configuration object and session and retrieve the status of transfers that were in progress at the time of termination. This behavior applies only for normal termination of the app by the system. If the user terminates the app from the multitasking screen, the system cancels all of the session’s background transfers. In addition, the system does not automatically relaunch apps that were force quit by the user. The user must explicitly relaunch the app before transfers can begin again.
Meaning if I relaunch the app the transaction before the force-quit should started again, or are they? Is there an additional operation I need to commit in order this to work?
UPDATE: I stumbled on this project:
https://github.com/Heikowi/HWIFileDownload#force-quit
That states:
Force Quit
After the app has been killed by the user, downloads do not continue in the background. On iOS 7 (and later) resume data is passed back.
Meaning there is a way to receive resume data even if the application was killed by the user in the background. Only the project is written in Objective-C and I can't understand what are they doing to achieve this.
After a force-quit the:
NSURLSessionTaskDelegate - (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
delegate method will be called when the app is restarted. If the download task can be resumed the error object will contain the resume data:
[error.userInfo objectForKey:NSURLSessionDownloadTaskResumeData].
Using this data you can resume the download process by creating a NSURLSessionDownloadTask with:
(NSURLSessionDownloadTask *)downloadTaskWithResumeData:(NSData*)resumeData.
More info about that can be found in Life Cycle of a URL Session with Custom Delegates, step 13.
I think that after your application did force-quit you should start all over again (.
If the user terminates your app, the system cancels any pending tasks.
and
When all of the tasks associated with a background session are complete, the system relaunches a terminated app (assuming that the sessionSendsLaunchEvents property was set to YES and that the user did not force quit the app)
https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html
-> use URLSession background Sessions download doesn't stop at all....you don't have to explicitly code for resuming the download or something..
https://developer.apple.com/reference/foundation/urlsession
check for background session in this link...if you're not able get the still...comment me and i would help in detail.
I have a application running in background and I need to know if device is sleeping in order to start a sincronisation process, but I didn't find information about this.
Does anyone know if it is posible and how do it?
Thanks.
You cannot know if the device is asleep because you have no control over the OS.
You can, otherwise, use the App Delegate method:
- (void)applicationWillResignActive:(UIApplication *)application
{
//your code goes here
}
if you want to wait till your app goes to background
I believe you can't do this using public API. The only thing which you can check whether your application is active or in background (using AppDelegate callbacks). And as Luke pointed out in comments, checking whether device "falls asleep" isn't iOS best design practice.
There are some private API's to do what you want, you can look at following questions:
Is there a way to check if the iOS device is locked/unlocked?
Detect screen on/off from iOS service
However, you should be aware that your app won't be accepted in AppStore in such case.
Here is the situation:
I am picking a large video using imagepicker. Obviously the picker will take a bit of time to compress the video. So to ease user experience I have enabled background task for it.
Now here comes the issue:
If user choose a video and and tap the home button, application goes to background and continue compressing video for next 600 secs. And the background task expires. In the expiration handler I have stopped my background task.
Now if the user is resuming app after the background expiration
- (void)applicationDidBecomeActive:(UIApplication *)application
is not being invoked. Can anyone explain me why this happens?
When the background tasks expires, your app will really be closed! So it's not becoming active again, it's launching.
You should handle stuff in your expiration handler or/and when your background task ends successfully. Both situations, you need to set the background_task as invalidated.
If your app goes to background while converting the video, and then user open it again BEFORE the task end or the background task expires, then you should see the app calling applicationDidBecomeActive.
I assume you know it, but maybe you are missing the multitask properties in your Info.plist file, so your app isn't accepting background tasks the way you expect.
-(void) applicationDidBecomeActive:(UIApplication)application
This method Only called when the app's sate is changed from inactive state to active state.
Is it possible to know whether video picking finished? if it's possible then just store it. and when the user comes again to the app. just fire the functionality you required.
Could you try to add log statement to method applicationDidFinishLaunching? May be the app terminates or crashes before a user opens it.
Also, I think correct way is to save current parsing context when app receives signal like applicationDidFinishLaunching and when app starts resume parsing. Because a user can close the app manually.