Re: Background uploads with NSURLSession

Do you recommend me to use NSURLSessionUploadTask or another class?

NSURLSessionUploadTask

What do I have to do to continue all these uploads in background?

What do I have to do to continue all these uploads when app is not running?

Dump all the uploads into an NSURLSession background session.

For all of the first 3 points, do you recommend me to wrap NSURLSession objects in NSOperation ones in order to use an NSOperationQueue? Or iOS would use its own internal queue to properly handle NSURLSession objects?

That's hard to say.  In general I'm a big fan of NSOperation but it's a tricky fit for NSURLSession background sessions because of the way that the background session interacts with your app's lifecycle.  Specifically, a common sequence is this:

1. your app starts a bunch of background transfers

2. the user moves it into the background

3. the system suspends your app

4. the transfers continue in the background

5. the system terminates your app

6. the transfers continue in the background

7. the transfers complete

8. the system relaunches your app

If you're using an NSOperation to track each transfer, you have to reconstitute those operations when you're relaunched at step 8.  Of course, you have to reconstitute something at step 8, it's just that reconstituting an NSOperation may be trickier than reconstituting some less complex object.

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Quinn "The Eskimo!"
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"

Hi,

This thread has helped me a lot but i am still seeing some strange behavior when I try to do  background upload tasks The problem I am seeing occurs only for large file uploads (300-500 MBs). What I am observing is that  iOS sometimes does NOT call didCompleteWithError even after my server sends a response.  In the client logs I can see that the didSendBodyData method gets called several times. Which is expected, however it does not seem to stop even after sending all the bytes.  I have also noticed that sometimes the taskId changes or the totalBytesSent gets reset. Which does not make any sense since I have not queued up another upload task. I only called resume once.

Also note: This behavior happens when the app is in the foreground. I have not attempted testing background uploads yet.

Everythin works as expected the file is small.(~50 Mbs). The didRecieveData method gets called along with the didCompleteWithError method.

Below are some code snippets of the background task:

//FileOperationsDelegate.mm

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
   didSendBodyData:(int64_t)bytesSent
    totalBytesSent:(int64_t)totalBytesSent
totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend
{
    NSNumber* taskId = [NSNumber numberWithUnsignedInteger:task.taskIdentifier];

    AwLog(@"DidSendBodyData for taskID: %@  BytesSent: %lld TotalBytesSent: %lld ExpectedToSend: %lld\n", taskId, bytesSent, totalBytesSent, totalBytesExpectedToSend);

    if (updateFileProgressCallback != NULL)
    {
        updateFileProgressCallback(bytesSent, totalBytesSent);
    }
}

Here is by backgroundSession.

//FileOperationsDelegate.mm

- (NSURLSession *)backgroundSession
{

    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
       NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier: KEY_BACKGROUND_SESSSION_ID];

        configuration.discretionary = YES;
        configuration.sessionSendsLaunchEvents = YES;
        configuration.URLCache = nil;

        bool wifiOnly = [ClientConfiguration.sharedInstance isWifiOnlyEnabled];

        if (wifiOnly)
        {
            configuration.allowsCellularAccess = false;
        }

        NSOperationQueue* delegateMainQueue = [NSOperationQueue mainQueue];
        delegateMainQueue.maxConcurrentOperationCount = 1;

        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue: delegateMainQueue];
    });
    return session;
}

Creating the FileUpload task

//FileUploader.mm

NSURLSession* backgroundSession = [FileOperationsSessionDelegate.sharedInstance backgroundSession];

                       NSMutableString *file = [[NSMutableString alloc] initWithString:absLocalPath];
                       [file insertString:@"file://" atIndex:0];

                       NSURL* fileToBeUploaded = [NSURL URLWithString:file];
                       NSURLSessionUploadTask* uploadTask = [backgroundSession uploadTaskWithRequest:urlRequest fromFile:fileToBeUploaded];

                       uploadTask.countOfBytesClientExpectsToSend = self.uploadSizeBytes;
                       uploadTask.countOfBytesClientExpectsToReceive = 450;


 ConnectionCallback uploadCompleteCallback = ^(NSURLResponse *  response, NSData * data,  NSError *  error) {
// A callback to complete the file upload. It will update the UI and notify the user 
// on how the upload went. 
}

  BytesUploadedCallback updateFileProgressCallback = ^(long bytesWritten, long totalBytesWritten)
// A callback that will update the progress bar.
                       };


                       [FileOperationsSessionDelegate.sharedInstance setUploadCallback:uploadCompleteCallback];
                       [FileOperationsSessionDelegate.sharedInstance setConnectionCallback:updateFileProgressCallback];;


                       [uploadTask resume];

Any help with this will be greatly apperiated.

Edit: I have attempted using a non background session and I observe the same behavior.

Thanks : )

Specs:

iOS version 12.3

Device: iPad Pro 11

Xcode:  10.1