NSURLSession and stream upload in background

Question

I have some problems with using NSURLSession to upload photos from Asset Library to the server.

At first NSURLSession doesn't support streaming upload. I got an exception when trying to using that:

@property (nonatomic, strong) NSURLSession *uploadSession;

...

_uploadSession = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration
                backgroundSessionConfiguration:kUploadBackgroundURLSessionIdentifier] delegate:self delegateQueue:nil];

...

NSURLSessionUploadTask *task = [self.uploadSession uploadTaskWithStreamedRequest:URLRequest];

This is an exception:

Terminating app due to uncaught exception 'NSGenericException', reason: 'Upload tasks in background sessions must be from a file'

That's really strange because Apple's manual doesn't contain any information about using only uploadTaskWithRequest:fromFile: for background session. What if I would like to upload really huge video file from Asset Library? Should I save it previously to my tmp directory?

Looks like the only reason is to use uploadTaskWithRequest:fromFile: anyway, right? But then I have a question how server get to know what's part of the file is uploading right now if uploading process was interrupted and started to upload next part in background?

Should I manage something for that? Previously I used Content-Range for that in URL request if I wanted to continue upload part of the file which was started previously. Now I can't do that - I have to create an URL Request before creating upload task and looks like NSURLSession have to do something like that automatically for me?

Does anyone do something like that already? Thanks

When I read the docs it was pretty clear you had to supply a file for background. Not sure of your question - if the file is only partially uploaded, and your service tells you how much of it was received, then you can trim your file and resend the unsent part, no. I have no experience doing this but would seem workable. — David H, Nov 14 '13 at 12:56
Yes, everything looks workable, but there's a problem that if I would like to upload a huge video from Asset Library I have to copy it (if I will have enough space) to my application folder, then start to copy, then from time to time check what is uploaded already, trim the file, and start new request. But Apple's docs tells that I have to create one task for each file. — gN0Me, Nov 14 '13 at 19:11
I've no experience with the asset library - what is the framework or class that you would use to retrieve the video? — David H, Nov 14 '13 at 19:25
I use exact asset library for that :) actually any photo or video from ALAssetsLibrary has path like asset-library:// but it is impossible to use it as an URL in uploadTaskWithRequest:fromFile: method. So looks like the only solution is to copy it inside the result block of assets library method assetForURL:resultBlock:failureBlock: — gN0Me, Nov 15 '13 at 06:19
Did you try to coerce the URL into a filePath -there are many interesting conversions in NSURL. Add another comment - if impossible Ill enter a bug as there should be. — David H, Nov 15 '13 at 12:24
Also did you try just passing the URL directly to the background upload task? The docs say it will fail but then it might not... — David H, Nov 15 '13 at 12:31
Streamed uploads aren't supported by background session configurations. In my experience you have to use an AVAssetExportSession to export the file to a location you specify, and upload from there. Side note: if the docs say not to do something, not a great idea to do it in a production environment, even if it seems to work. — Alfie Hanssen, Jan 14 '15 at 20:24

score 1 · Answer 1 · edited Nov 29 '13 at 16:20

Convert to NSData and copy and write in app folder

ALAsset *asset = [cameraRollUploadImages objectAtIndex:startCount];
ALAssetRepresentation *representation = [asset defaultRepresentation];

// create a buffer to hold the data for the asset's image
uint8_t *buffer = (Byte *)malloc(representation.size);// copy the data from the asset into the buffer
NSUInteger length = [representation getBytes:buffer 
                                  fromOffset:0 
                                      length:representation.size 
                                       error:nil];

// convert the buffer into a NSData object, free the buffer after
NSData *image = [[NSData alloc] initWithBytesNoCopy:buffer 
                                             length:representation.size
                                       freeWhenDone:YES];

Yeah, that's exactly what we're doing. But there's a problem if user has a huge video file in AssetsLibrary — gN0Me, Nov 17 '13 at 16:51

Jay · Answer 2 · 2014-01-27T11:29:07.170

Right now, there is no way otherthan saving the picture to local file system or temp directory.

Following code make sure your data won't be lost with exif tags. (ALAsset => NSData)

ALAssetRepresentation *assetRepresentation = [(ALAsset *)assetBeingUploaded defaultRepresentation];
uint8_t *buffer = (uint8_t *)malloc(sizeof(uint8_t)*[assetRepresentation size]);
NSUInteger buffered = 0;
if (buffer != NULL)
buffered = [assetRepresentation getBytes:buffer fromOffset:0.0 length:assetRepresentation.size error:nil];
self.imageBeingUploaded = [NSData dataWithBytesNoCopy:buffer length:buffered freeWhenDone:YES];

The upload task in background session doesn't support completion handler. We should go for.,

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL;

I doubt how do we get the response headers or body in case if we are using background session & uploadtask with request using file?

score 0 · Answer 3 · answered Nov 27 '14 at 08:21

A clean workaround will be to create a NSOperation which will copy the file from the asset library to your temporary folder using NSStream, so you won't get a crash in case of a huge file, when the operation completes you schedule a upload of that temporary file, when the upload finishes you delete it.

In my case, i need to send the file in multipart format so the creation of the temporary file is necessary but i encounter a problem in uploading large files, more then 2 Gb, example movies over 20 minutes.

score 0 · Answer 4 · answered May 13 '15 at 13:05

0

You can't upload NSData in background you need to upload file format. You can create it by directory path

answered May 13 '15 at 13:05

Devil

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NSURLSession and stream upload in background

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