iOS 7 NSURLSession Download multiple files in Background - ios7

Question

I want to download a List of files using NSUrlSession.
I have a variable for counting the successful downloads #property (nonatomic) int downloadsSuccessfulCounter;. While the files are being downloaded I disable the Download Button. When the counter is equal to the download list size, I enable the button again and set the counter to 0. I do this in the method:
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
...
[[NSOperationQueue mainQueue] addOperationWithBlock:^ {
downloadsSuccessfulCounter++;
if(downloadsSuccessfulCounter == self.downloadList.count) {
NSLog(#"All downloads finished");
[self.syncButton setEnabled:YES];
downloadsSuccessfulCounter = 0;
}
}];
}
Everything is working fine, but when I open again the ViewController I get the message A background URLSession with identifier com.myApp already exists!. The counter is not set to 0 and the UI elements (UIButtons, UILabels) are not responding.
I guess the problem is because the NSURLSession is still open but I'm not really sure about how it works.
I have tried all the tutorials, but 99% of them are only for downloading 1 file, not more than 1...
Any ideas?
Here is my code:
...
#property (nonatomic, strong) NSURLSession *session;
...
- (void)viewDidLoad {
[super viewDidLoad];
appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
self.downloadList = [[NSMutableArray alloc] init];
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:#"com.myApp"];
sessionConfiguration.HTTPMaximumConnectionsPerHost = 5;
self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
}
When I press the Download ButtonI call this method (
I have a Downloadable object which contains a NSURLSessionDownloadTask):
-(void)startDownload {
for (int i=0; i<[self.downloadList count]; i++) {
Downloadable *d = [self.downloadList objectAtIndex:i];
if (!d.isDownloading) {
if (d.taskIdentifier == -1) {
d.downloadTask = [self.session downloadTaskWithURL:[NSURL URLWithString:d.downloadSource]];
}else {
d.downloadTask = [self.session downloadTaskWithResumeData:fdi.taskResumeData];
}
d.taskIdentifier = d.downloadTask.taskIdentifier;
[d.downloadTask resume];
d.isDownloading = YES;
}
}
}
When the app is in Background:
-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([downloadTasks count] == 0) {
if (appDelegate.backgroundTransferCompletionHandler != nil) {
void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;
appDelegate.backgroundTransferCompletionHandler = nil;
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
completionHandler();
UILocalNotification *localNotification = [[UILocalNotification alloc] init];
localNotification.alertBody = #"All files downloaded";
[[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];
}];
}
}
}];
}

Answer 1

So, as I mentioned in my comments, the issue is that each File requires a unique NSURLSession, and each NSURLSession requires a NSURLSessionConfiguration with a unique identifier.
I think that you were close - and probably more proper than me in certain aspects...
You just need to create a structure to pass unique IDs into unique Configurations, to populate unique Sessions (say that 10x fast).
Here's what I did:
/*
* Retrieves the List of Files to Download
* Also uses the size of that list to instantiate items
* In my case, I load a character returned text file with the names of the files that I want to download
*/
- (void) getMediaList {
NSString *list = #"http://myserver/media_list.txt";
NSURLSession *session = [NSURLSession sharedSession]; // <-- BASIC session
[[session dataTaskWithURL:[NSURL URLWithString:list]
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSString *stringFromData = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
// Populate Arrays
REMOTE_MEDIA_FILE_PATHS = [stringFromData componentsSeparatedByString:#"\n"];
[self instantiateURLSessions:[REMOTE_MEDIA_FILE_PATHS count]];
// Start First File
[self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter]; // this variable is 0 at the start
}]
resume];
}
/*
* This sets Arrays of Configurations and Sessions to the proper size
* It also gives a unique ID to each one
*/
- (void) instantiateURLSessions : (int) size {
NSMutableArray *configurations = [NSMutableArray array];
NSMutableArray *sessions = [NSMutableArray array];
for (int i = 0; i < size; i++) {
NSString *index = [NSString stringWithFormat:#"%i", i];
NSString *UniqueIdentifier = #"MyAppBackgroundSessionIdentifier_";
UniqueIdentifier = [UniqueIdentifier stringByAppendingString:index];
[configurations addObject: [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:UniqueIdentifier]];
[sessions addObject:[NSURLSession sessionWithConfiguration: [configurations objectAtIndex:i] delegate: self delegateQueue: [NSOperationQueue mainQueue]]];
}
NSURL_BACKGROUND_CONFIGURATIONS = [NSArray arrayWithArray:configurations];
NSURL_BACKGROUND_SESSIONS = [NSArray arrayWithArray:sessions];
}
/*
* This sets up the Download task for each file, based off of the index of the array
* It also concatenates the path to the actual file
*/
- (void) getFile : (NSString*) file :(int) index {
NSString *fullPathToFile = REMOTE_MEDIA_PATH; // Path To Server With Files
fullPathToFile = [fullPathToFile stringByAppendingString:file];
NSURL *url = [NSURL URLWithString:fullPathToFile];
NSURLSessionDownloadTask *downloadTask = [[NSURL_BACKGROUND_SESSIONS objectAtIndex:index ] downloadTaskWithURL: url];
[downloadTask resume];
}
/*
* Finally, in my delegate method, upon the completion of the download (after the file is moved from the temp data), I check if I am done and if not call the getFiles method again with the updated counter for the index
*/
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
// Get the documents directory URL
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:LOCAL_MEDIA_PATH];
NSURL *customDirectory = [NSURL fileURLWithPath:dataPath];
// Get the file name and create a destination URL
NSString *sendingFileName = [downloadTask.originalRequest.URL lastPathComponent];
NSURL *destinationUrl = [customDirectory URLByAppendingPathComponent:sendingFileName];
// Move the file
NSError *error = nil;
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager moveItemAtURL:location toURL:destinationUrl error: &error]) {
// List
[self listCustomDirectory];
if(downloadCounter < [REMOTE_MEDIA_FILE_PATHS count] -1) {
// Increment Counter
downloadCounter++;
// Start Next File
[self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter];
}
else {
// FINISH YOUR OPERATION / NOTIFY USER / ETC
}
}
else {
NSLog(#"Damn. Error %#", error);
// Do Something Intelligent Here
}
}